• HLMD 4353 (specimen no. 3 of Harrassowitz (1922)), syntype, a poorly-preserved shell (Harrassowitz 1922, pl. 2.2; Suppl. material 1: fig. S1.4); • HLMD 4283 (specimen no. 4 of Harrassowitz (1922)), syntype, hyoplastra, hypoplastra and xiphiplastra (Harrassowitz 1922, pl. 6.4; Suppl. material 1: fig. S1.3); • HLMD 1461 (specimen no. 6 of Harrassowitz (1922)), syntype, nearly complete skeleton embedded in a concretion (Harrassowitz (1922), pl. 6.1; Fig. 14).

This list comprises all specimens from Messel Pit that are, to our knowledge, available in public collections and that can be undoubtedly referred to Allaeochelys crassesculpta, but we suspect there are many more. For figures, please consult Table 1 and Suppl. material 1. BSPG 1982 I 74, a nearly complete skeleton; FHM 45, a nearly complete skeleton; FHM 46, a nearly complete juvenile skeleton; HLMD Be158, a nearly complete skeleton; HLMD Be159, a nearly complete skeleton; HLMD Be169, a juvenile skeleton; HLMD Be217, a crushed and folded skeleton; HLMD Be 235, a nearly complete skeleton; HLMD Me3173, a partial skeleton; HLMD Me3377, a fragmentary skeleton; HLMD Me4000, a partial carapace (syntype of Allaeochelys gracilis, specimen no. 5 of Harrassowitz (1922)); HLMD Me4282 (syntype of Allaeochelys gracilis, specimen no. 1 of Harrassowitz (1922)), a nearly complete skeleton (Harrassowitz 1922; pl. 1.3, pl. 4.2, pl. 4.4, pl. 6.5, pl. 6.6); HLMD Me5001, a partial carapace; HLMD Me7427, a nearly complete, but folded shell; HLMD Me7428, posterior half of carapace; HLMD Me7429, a nearly complete carapace; HLMD Me7592, a nearly complete skeleton; HLMD Me7593, a nearly complete pair of skeletons; HLMD Me7599, a partial skeleton; HLMD Me7892, a partial skeleton; HLMD Me9722, a nearly complete skeleton; HLMD Me9795, a nearly complete skeleton; HLMD Me10375, a partial skeleton; HLMD Me13397, a fragmentary skeleton; HLMD Me13432, a costal; HLMD Me13436, a nearly complete skeleton; HLMD Me13484, a nearly complete skeleton; HLMD Me13721, a nearly complete skeleton; HLMD Me14919, a nearly complete, folded skeleton; HLMD Me14920, a nearly complete skeleton; HLMD Me14997, a nearly complete skeleton; HLMD Me14998, a nearly complete skeleton; HLMD Me14999, a nearly complete skeleton; HLMD Me15012, a nearly complete skeleton; HLMD Me15273, a partial skeleton; HLMD Me15372, a nearly complete skeleton; HLMD Me15563, a partial skeleton; HLMD Me15660, a fragmentary skeleton; HLMD Me15956, a nearly complete skeleton; HLMD Me42827, a nearly complete skeleton; IRSNB MeR001, a nearly complete skeleton; IRSNB MeR002, a nearly complete skeleton; IRSNB MeR006, a nearly complete skeleton; IRSNB MeR007, a juvenile skeleton; IRSNB MeR010, a nearly complete pair of skeletons; IRSNB MeR011, a nearly complete skeleton; IRSNB MeR012, a nearly complete, but crushed cranium; IRSNB MeR014, a nearly complete skeleton; IRSNB MeR015, a juvenile skeleton; IRSNB MeR018, a nearly complete skeleton; IRSNB MeR020, a nearly complete skeleton; IRSNB MeR022, a nearly complete skeleton; IRSNB MeR024, a nearly complete skeleton; IRSNB MeR025, a juvenile skeleton; IRSNB MeR029, a nearly complete skeleton; IRSNB MeR030, a nearly complete skeleton; IRSNB MeR031, a nearly complete skeleton; IRSNB MeR032, a nearly complete skeleton; IRSNB MeR034, a nearly complete skeleton; SMF Me373, a nearly complete skeleton; SMF Me436, a partial skeleton; SMF Me441, a partial skeleton; SMF Me514, a nearly complete skeleton; SMF Me611, a nearly complete skeleton; SMF Me718, a partial shell; SMF Me938, partial posterior part of carapace; SMF Me1444, a nearly complete skeleton; SMF Me1794, a complete shell; SMF Me1808, a partial skeleton; SMF Me1809, a nearly complete skeleton; SMF Me2009, a partial skeleton; SMF Me2010, a partial skeleton; SMF Me2034, a partial pair of skeletons; SMF Me2449, a nearly complete skeleton; SMF Me2606, a partial skeleton; SMF Me3004, an isolated and crushed cranium; SMF Me3018, a nearly complete skeleton; SMF Me3566, a nearly complete pair of skeletons; SMF Me3775, a partial skeleton; SMF Me3779, a partial skeleton; SMF Me3783, a partial skeleton; SMF Me3787, a partial skeleton; SMF Me11381, a juvenile skeleton; SMF Me11418, a partial skeleton; SMF Me11573, a partial skeleton; SMNK Pal986, a nearly complete skeleton; SMNK Pal45053, a fragmentary skeleton.

Messel Pit, near the village of Messel, State of Hesse, Germany (Harrassowitz 1922: fig. 6); MP11, early Lutetian (ca. 47 Ma), middle Eocene (Joyce et al. 2012).

Allaeochelys crassesculpta can be diagnosed as a representative of Carettochelyidae by a bone surface ornamentation consisting of thick ridges separated by equally-sized grooves, presence of a quadrate fossa, presence of a midline keel on the dorsal surface of the carapace, a single suprapygal, presence of a lip formed on the visceral side of the posterior peripherals and pygal, plastral kinesis, a triangular entoplastron, and reduction of plastral scutes. Allaeochelys crassesculpta can be diagnosed as a member of Carettochelyinae by the presence of a maxilla-quadratojugal contact below the jugal, absence of a contact between the quadratojugal and squamosal, presence of elongate tubercula basioccipitale, complete loss of carapace and plastral scutes, and an equilateral entoplastron. Allaeochelys crassesculpta can be differentiated from other carettochelyines by having a straight labial margin of the maxilla, an anteroposteriorly short postorbital, a moderately deep quadrate fossa, a large antrum postoticum, a foramen posterius canalis carotici interni located close to the parabasisphenoid-pterygoid suture at about mid-length of the parabasisphenoid, a pterygoid fossa ventrally open and not covered by an enfolded ridge of the pterygoid, a supraoccipital excluded from the skull roof, and a flat ventral surface of the basioccipital.

Cranium

General comments. SMF Me3004 is a crushed, flattened cranium (Fig. 1). After segmentation and reconstruction, it appears that none of the bones is completely preserved.

All reconstructed bones are incomplete. The prefrontals, frontals, premaxilla, maxillae, jugals, quadrates, and parabasisphenoid are fairly well-preserved, whereas the vomer, pterygoids, and opisthotics are highly incomplete (Fig. 2). As the cranium is heavily compressed dorsoventrally, the secondary wall of the braincase is the area that suffered most. In the µCT dataset, we were not able to identify any pieces in that area as belonging to either bone forming the secondary braincase wall of other carettochelyids and most of the material that can visually be attributed to bone and not matrix was either too crushed or too small to be identified with confidence. These fragments were, thus, omitted from the segmentations and reconstructions. Consequently, most anatomical structures located in that area, such as the foramen nervi trigemini or cavum labyrinthicum, are either incomplete or missing completely. Likewise, the epipterygoids, which were likely present by reference to other pan-trionychians, could not be segmented and described as they were completely crushed and destroyed.

SMF Me3004 preserves the typical surface ornamentation of carettochelyids, which is made of thick ridges separated by equally-sized grooves that are mostly visible on the prefrontals, frontals, and parietals (Figs 2, 3). The surface ornamentation of the cranium, however, is less pronounced than on the shell. As in all carettochelyids (Waite 1905; Walther 1922; Havlik et al. 2014; Joyce 2014; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024a, 2024b, 2025), the cranium has a deep upper temporal emargination, a posteriorly expanded supraoccipital crest that ventrally forms a broad horizontal plate, an incisura columella auris that is fully enclosed by the quadrate, a quadrate fossa, a low mandibular condyle, and a contact between the palatines and parabasisphenoid that prevents the pterygoids from contacting one another medially (Figs 2, 3).

Nasal. Nasals are absent in Allaeochelys crassesculpta, as in all carettochelyids (Waite 1905; Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024a, 2025).

Prefrontal. The prefrontals of SMF Me3004 are generally well-preserved and only damaged ventrally at the level of their descending process, of which only two small pieces remain preserved on the right side (Figs 2A–E, 3A–E). The prefrontal contacts the maxilla ventrolaterally along its ascending process and the frontal posteriorly (Figs 2A–E, 3A–E). The dorsal plate of the prefrontal forms the dorsal margin of the apertura narium externa. The rim of the apertura narium externa, which is otherwise also formed by the maxilla and premaxilla, is thin (Figs 2E, 3E), as in most carettochelyids (Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024a), except for Allaeochelys delheidi (YR, pers. obs. on NHMUK PAL PV OR 38953) and Allaeochelys meylani (Rollot et al. 2025) in which the bones forming the aperture are greatly thickened. The prefrontal also forms the anterodorsal margin of the orbit and roofs the fossa nasalis. The dorsal plate of the prefrontal is rectangular and longer than wide, as in Anosteira pulchra (Joyce et al. 2018) and Carettochelys insculpta (Rollot et al. 2024b), which contrasts with the prefrontal of Allaeochelys libyca (Havlik et al. 2014; Rollot et al. 2024a) and Allaeochelys meylani (Rollot et al. 2025), where the dorsal plate is about as long as wide. The descending process of the prefrontal of SMF Me3004 is almost completely lacking on both sides (Fig. 3C, D). Only two small pieces are preserved on the right side. The larger of these two pieces corresponds to the end of the posterior process of the descending process of the prefrontal, which contacts the vomer ventromedially. The dorsal surface of this element is smooth, suggesting that a contact with the crista cranii of the frontal is absent. This is confirmed by specimens HLMD Me7592 and HLMD Me14997, in which most of the posterior process is preserved. Allaeochelys crassesculpta thus lacks an ophthalmic foramen, but possesses an ophthalmic slit instead, as in Anosteira pulchra (Joyce et al. 2018) and Carettochelys insculpta (Rollot et al. 2024b).

Frontal. The two frontals of SMF Me3004 are nearly complete, albeit broken into several pieces that had to be reassembled for the reconstruction of the cranium (Figs 2A, 3A). The frontal contacts the prefrontal anteriorly, the postorbital posterolaterally, the parietal posteriorly, and its counterpart medially (Figs 2A, 2C, 2D, 3A, 3C–E, 4). The frontal slightly protrudes anteromedially between the prefrontals by means of a short process. The process is particularly well-developed on the left frontal, which contacts the two prefrontals, whereas the right frontal process only contacts the right prefrontal. The frontal forms the dorsal margin of the orbit margin (Figs 2A, 2D, 3A, 3C–E, 4). The contribution of the frontal to the orbit margin is large as in Anosteira maomingensis (Danilov et al. 2017a), Anosteira pulchra (Joyce et al. 2018), and Carettochelys niahensis (White et al. 2023) and contrasts with the moderate contribution seen in Allaeochelys libyca (Havlik et al. 2014) and the small contribution seen in Allaeochelys meylani (Rollot et al. 2025). In the extant Carettochelys insculpta, the contribution of the frontal to the orbit margin appears to vary from small to moderate, but these differences might be linked to ontogeny, as small specimens have a moderate contribution, whereas large specimens exhibit a small one (Waite 1905; Walther 1922; Rollot et al. 2024b). Ventrally, the frontal of SMF Me3004 forms a robust and well-developed crista cranii (Fig. 4). The crista cranii posteriorly forms an articular facet on both frontals, showing that a contact was present with the descending process of the parietal along the entire height of the crista, likely constricting the foramen interorbitale dorsally. Ventral contact of the crista cranii with the dorsal aspect of the descending process of the prefrontal is absent, as explained above (see Prefrontal; Fig. 4). Additional evidence to postulate the absence of this foramen is visible on the left crista cranii of SMF Me3004, as the ventral edge of the crista cranii does not form an articular facet, in contrast to carettochelyids that exhibit this structure (Fig. 4). In Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys niahensis (White et al. 2023), the crista cranii forms a broad, flat central plateau that is developed as a well-defined articular surface for articulation with the prefrontal. This is clearly absent in SMF Me3004. These observations are backed up by the morphology of the crista cranii of HLMD Me7592, which is identical to that of SMF Me3004. The crista cranii forms the lateral wall of the sulcus olfactorius, which is notably broad (Fig. 4) as in Allaeochelys libyca (Rollot et al. 2024a), Carettochelys niahensis (White et al. 2023) and Carettochelys insculpta (Rollot et al. 2024b). Posteriorly, along the lateral surface of the crista cranii, the frontal of SMF Me3004 forms a low, curved ridge that extends anterodorsolaterally and forms the anterior and medial margin of a relatively deep and large fossa, whose posterior half is formed by the parietal (Fig. 4A).

Parietal. The dorsal plate of both parietals is completely preserved in SMF Me3004, but the processus inferior parietalis is almost completely lacking on both elements (Figs 2A, 2C, 2D, 3A, 3C–E). The length of each parietal clearly exceeds the combined width of the two elements (Fig. 3A). On the skull roof, the dorsal plate of the parietal contacts the frontal anteriorly, the postorbital anterolaterally, the supraoccipital posteriorly, and its counterpart medially (Figs 2A, 3A). The dorsal plate of the parietal also forms the anteromedial and medial margin of the deep upper temporal emargination, as in all carettochelyids (Waite 1905; Walther 1922; Havlik et al. 2014; Joyce 2014; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2025). The processus inferior parietalis is only preserved on both sides by a few isolated pieces that belong to the most dorsal aspect of the process, just below the dorsal plate of the parietal (Fig. 3C). It is clear that the processus inferior parietalis had an elongate contact laterally with the prootic, as in other carettochelyids (Walther 1922; Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2025). However, a contact between the parietal and opisthotic appears to have been absent, prevented by a clear contact between the prootic and supraoccipital, as in Allaeochelys libyca (Havlik et al. 2014) and Allaeochelys meylani (Rollot et al. 2025), but contrasting Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018), in which a small contact is present. In Carettochelys insculpta, the contact is variably present or absent (Walther 1922; Joyce 2014; Rollot et al. 2024b). In SMF Me3004, the processus inferior parietalis is anteriorly continuous with the tall crista cranii of the frontal, to which it is tightly sutured (see Frontal above). Anteroventrolaterally, the parietal bears a ridge that extends anterolaterally from the processus inferior parietalis and forms the posterolateral rim of a large fossa formed by the frontal and parietal in the posterodorsomedial corner of the orbit. The ridge extends further on the inner, medial surface of the postorbital, forming a broad bony wall posterior to the orbit (Fig. 3E). The ridge likely extended to the jugal, as the latter bone appears to be medially expanded, but the complete extent of the posterior orbit wall cannot be fully determined because of damage to the jugal. A similar, mediolaterally expanded wall posterior to the orbit is also present in Allaeochelys meylani (Rollot et al. 2025) and maybe Carettochelys niahensis (White et al. 2023) and is reminiscent of the septum orbitotemporale seen in pleurodires and trionychids (Gaffney et al. 2006). In the extant Carettochelys insculpta, a similar ridge is present, but a mediolateral thickened postorbital wall is not apparent (Rollot et al. 2024b). Anterolaterally, the braincase is greatly enlarged and contained broad cerebral hemispheres, as in Allaeochelys libyca (Havlik et al. 2014), Allaeochelys meylani (Rollot et al. 2025), Carettochelys niahensis (White et al. 2023), and Carettochelys insculpta (Rollot et al. 2024b).

Postorbital. The postorbitals of SMF Me-3004 are overall well-preserved on both sides, with damage mostly affecting the ventral part of the postorbital wall, which is partially missing (Figs 2A, 2C–E, 3A, 3C–E). The postorbital is a small bone that forms the posterior margin of the orbit and the anterior margin of the upper temporal emargination (Figs 2A, 3A, 3C–E). The postorbital is anteroposteriorly short (Fig. 3A, C, D), as seen in Anosteira pulchra (Joyce et al. 2018), but which contrasts with the elongate postorbital seen in Anosteira maomingensis (Danilov et al. 2017a), Allaeochelys libyca (Havlik et al. 2014), Allaeochelys meylani (Rollot et al. 2025), Carettochelys niahensis (White et al. 2023), and Carettochelys insculpta (Walther 1922; Rollot et al. 2024b). On the external skull surface of SMF Me3004, the postorbital contacts the frontal anteromedially, the parietal posteromedially, the jugal ventrally, and the quadratojugal posteroventrally (Figs 2A, 2C–E, 3A, 3C–E). Medially, the postorbital is greatly expanded, forming a broad wall posterior to the orbit (Fig. 3E), as in Allaeochelys meylani (Rollot et al. 2025) and, likely, Carettochelys niahensis (White et al. 2023). An anteroventral contact with the maxilla is prevented in SMF Me3004 by the contribution of the jugal to the orbit margin (Fig. 3C–E), as in all carettochelyids (Waite 1905; Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024b, 2025).

Jugal. The jugals of SMF Me3004 show good preservation on both sides, with damage only affecting their medial process (Figs 2B–E, 3C–E). The jugal is a small bone that forms the posteroventral margin of the orbit and the anterior margin of the lower temporal emargination (Fig. 3C–E). The contribution of the jugal to the orbit margin is broad (Fig. 3C–E), as in all other carettochelyids (Waite 1905; Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024b), except Allaeochelys meylani (Rollot et al. 2025), where this contribution is greatly reduced. On the external skull surface, the jugal contacts the maxilla anteroventrally, the quadratojugal posteriorly and posteroventrally, and the postorbital dorsally (Figs 2C–E, 3C–E). The jugal is prevented from contributing to the upper temporal emargination by a short contact between the postorbital and quadratojugal posterodorsally (Fig. 3D). The jugal is also prevented from contributing to the cheek emargination by a contact between the maxilla and quadratojugal ventrally (Fig. 3C, D), as in Allaeochelys meylani (Rollot et al. 2025), Carettochelys niahensis (White et al. 2023), and Carettochelys insculpta (Waite 1905; Walther 1922; Rollot et al. 2024b), but not Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018). Within the fossa orbitalis, the jugal anteriorly contacts the maxilla and deeply protrudes within the latter, giving the suture a V-shaped appearance. The medial process of the jugal is damaged, but the preserved portion shows that the process was well-developed and contributed to the broad, but likely thin, postorbital wall (Fig. 3E), as in Allaeochelys meylani (Rollot et al. 2025) and likely Carettochelys niahensis (White et al. 2023). Posteromedially, the jugal of SMF Me3004 likely contacted the pterygoid, as both bones are extremely close to one another in both the original and reassembled 3D reconstructions. A medial contact with the palatine is less certain, as the jugal and palatine appear to be prevented from contacting one another by a contact between the maxilla and pterygoid, but the great degree of incompleteness of the palatines anterolaterally makes this determination uncertain.

Quadratojugal. The quadratojugals of SMF Me3004 are both damaged (Figs 2B–D, 3B–D). The right element is fairly complete, but lacks its central portion and its posteroventral process, whereas the left element only consists of the most anterior part and the posterodorsal process. The quadratojugal is a small, plate-like bone that forms the anterolateral margin of the upper temporal emargination and the dorsal and posterior margins of the reduced cheek emargination (Fig. 3C, D). The quadratojugal contacts the jugal anteriorly, the postorbital anterodorsally, the maxilla anteroventrally, and the quadrate posteriorly along a tall, concave suture (Figs 2B–D, 3B–D). The contact between the quadratojugal and postorbital is only slightly shorter than that between the jugal and postorbital (Fig. 3D), which contrasts with the condition of Anosteira maomingensis (Danilov et al. 2017a) and Allaeochelys meylani (Rollot et al. 2025), in which it is greatly reduced. Posteriorly, the quadratojugal of SMF Me3004 contributes to the anterior margin of the cavum tympani (Fig. 3C, D), as in Anosteira maomingensis (Danilov et al. 2017a), Anosteira pulchra (Joyce et al. 2018), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Walther 1922; Rollot et al. 2024b), but contrary to Allaeochelys meylani (Rollot et al. 2025). Posterodorsally, the quadratojugal of SMF Me3004 forms an elongate process that extends over the anterodorsal margin of the cavum tympani (Figs 2C, 2D, 3C, 3D). Contact between the quadratojugal and squamosal is absent as in Allaeochelys libyca (Rollot et al. 2024a), Allaeochelys meylani (Rollot et al. 2025), and Carettochelys insculpta (Walther 1922; Rollot et al. 2024b), which contrasts with Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018), in which a quadratojugal-squamosal contact is present.

Squamosal. The squamosals of SMF Me3004 are damaged (Figs 2A–D, 2F, 3A–D, 3F). The right element is highly incomplete and the few, disarticulated elements that remain preserved only allow reconstructing portions of the anterior third of the element, which contacts the quadrate anteroventrally just posterodorsal to the cavum tympani (Figs 2D, 3D, 3F). The left squamosal is more complete, but lacks a substantial part of its central aspect, which prevents direct assessment of the presence of an antrum postoticum (Figs 2C, 3C, 3F; but see Quadrate). In HLMD Me14997 and SMF Me3566 (female individual), however, this portion of the right squamosal and right quadrate and squamosal, respectively, are preserved and show the presence of a large antrum postoticum. A large antrum postoticum is otherwise present in Kizylkumemys schultzi (Nessov 1977) and Anosteira pulchra (contra Joyce et al. (2018); pers. obs. by YR on the μCT data of FMNH PR 966), while a shallow antrum postoticum is present in Anosteira maomingensis (Danilov et al. 2017a) and Allaeochelys libyca (Havlik et al. 2014) and the antrum is absent in adult Carettochelys insculpta, but present in juveniles (Rollot et al. 2024b). The squamosal of SMF Me3004 forms the posterolateral margin of the upper temporal emargination (Figs 2A, 3A). The only preserved contact of the squamosal is with the quadrate anteroventrally (Fig. 3C, D). A contact with the quadratojugal is absent (Figs 2A, 2C, 3A, 3C), as in Allaeochelys libyca (Rollot et al. 2024a), Allaeochelys meylani (Rollot et al. 2025), and Carettochelys insculpta (Walther 1922; Rollot et al. 2024b), but not Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018), where a quadratojugal-squamosal contact is present. The squamosal of SMF Me3004 does not contribute to the margin of the cavum tympani. This contribution is also lacking in Allaeochelys libyca (Rollot et al. 2024a) and Carettochelys insculpta (Rollot et al. 2024b), but is present in Anosteira maomingensis (Danilov et al. 2017a), Anosteira pulchra (Joyce et al. 2018) and, likely, Allaeochelys meylani (Rollot et al. 2025). The posterior portion of the squamosal of SMF Me3004 forms a mediolaterally thin, but anteroposteriorly elongate posterior process (Figs 2C, 2F, 3C, 3F).

Premaxilla. The premaxilla of SMF Me3004 is well-preserved and only lacks small portions of its most posterior aspects (Figs 2B, 2E, 3B, 3E, 5). The premaxilla is a single, small bone that forms the anteroventral tip of the snout (Figs 2B, 2E, 3B, 3E). The premaxilla forms the ventral margin of the apertura narium externa, which is thin (Figs 2E, 3E), as in most carettochelyids (Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024b), except for Allaeochelys delheidi (YR, pers. obs. on NHMUK PAL PV OR 38953) and Allaeochelys meylani (Rollot et al. 2025), which have greatly thickened bones forming the aperture. The premaxilla of SMF Me3004 floors the anterior part of the fossa nasalis and likely formed the anterior margin of the foramen intermaxillaris. The only preserved contact of the premaxilla is with the maxilla laterally (Figs 3E, 5), but comparisons with other carettochelyids suggest that no other contacts were present (Walther 1922; Danilov et al. 2017a; Rollot et al. 2024b, 2025). There are two pairs of foramina in the premaxilla. The first pair of foramina pierces the premaxilla along its dorsal surface and each foramen is located at about one-third of the bone’s width (Fig. 5A, B). Each foramen leads into a very short canal that extends anteromedially and anastomoses with its counterpart. At about the same level, a canal branches off from each side of the anastomosis and extends ventrally to join the anteroventral surface of the premaxilla, where the premaxilla forms a second pair of foramina (Fig. 5C). These foramina and canals are reminiscent of the premaxillary canal of other turtles, in which the premaxillary canal serves for the passage of a branch of the anterior nasal artery (Tada et al. 2025). A closer look at µCT data and 3D models of Carettochelys insculpta (Rollot et al. 2024b) demonstrated that similar foramina and canals are also present in the latter taxon.

Maxilla. The maxillae of SMF Me3004 are dislocated, but almost completely preserved and could, thus, be reconstructed with ease (Figs 2B–E, 3B–E). The maxillae are only slightly damaged along the anterior part of the ascending process and along the suture with the premaxilla (Figs 2B–E, 3B–E). The maxilla forms the anteroventral portion of the snout, the lateral margin of the apertura narium externa, the lateral wall of the fossa nasalis, the anterior and ventral margin of the orbit, the anteroventral margin of the reduced cheek emargination, and the lateral margin of the foramen intermaxillaris (Figs 2B–E, 3B–E). The rim of the external nares, which is formed by the prefrontal, maxilla, and premaxilla, is thin as in most carettochelyids (Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; White et al. 2023; Rollot et al. 2024b), except in Allaeochelys delheidi (YR, pers. obs. on NHMUK PAL PV OR 38953) and Allaeochelys meylani (Rollot et al. 2025), in which the rim is greatly thickened. On the external skull surface of SMF Me3004, the maxilla contacts the premaxilla anteromedially, the prefrontal dorsally along an elongate suture for all the length of its ascending process, the jugal posteriorly, and the quadratojugal posteriorly just below the jugal (Figs 2C–E, 3C–E). A maxilla-quadratojugal contact is also present in Allaeochelys meylani (Rollot et al. 2025), Carettochelys niahensis (White et al. 2023), and Carettochelys insculpta (Waite 1905; Walther 1922; Rollot et al. 2024b), but not in Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018). The labial margin of SMF Me3004 is nearly straight (Fig. 3C, D), as in Anosteira pulchra (Joyce et al. 2018), which contrasts with the more sinuous margin seen in Allaeochelys meylani (Rollot et al. 2025) and Carettochelys insculpta (Waite 1905; Walther 1922; Rollot et al. 2024b). Within the fossa nasalis of SMF Me3004, just medial to the anteroventral corner of the orbit, the maxilla forms a relatively large foramen alveolare superius. The foramen alveolare superius leads into a large, unnamed canal that extends anteroventrolaterally within the maxilla and merges with the canalis alveolaris superior. In the floor of the fossa orbitalis, the maxilla forms at about mid-length the foramen supramaxillare, which leads into the large canalis infraorbitalis. The canalis infraorbitalis extends anteriorly through the maxilla and merges with the canalis alveolaris superior. The canalis alveolaris superior seems to form in the most posterior part of the maxilla by the successive merging of multiple small canals that connect to the lateral and triturating surfaces of the maxilla. The posterior portion of the canalis alveolaris superior is fairly small and the canal extends anteriorly through the maxilla to merge with the canalis infraorbitalis slightly anterolateral to the level of the foramen supramaxillare. The resulting canal, which retains the name canalis alveolaris superior (Albrecht 1967; Gaffney 1972), is extremely large and extends anteriorly through the maxilla, while giving off multiple small branches. At the level of the anterior wall of the orbit, the canalis alveolaris superior merges with the unnamed canal that extends from the foramen alveolaris superior and the resulting canal continues to extend anteriorly through the maxilla. The path of the canal is difficult to follow, as it gives off numerous small to large canals along its way that connect to the lateral, medial, and triturating surfaces of the maxilla. One canal, however, extends anteriorly straight through the maxilla and exits the cranium by means of a foramen formed by the maxilla, along the ventrolateral margin of the external nares. The location of this foramen is reminiscent of the maxillary foramen described in Allaeochelys meylani (Rollot et al. 2025) and Carettochelys insculpta (Rollot et al. 2024b), which transmits a branch of the superior alveolar artery that may be linked to proboscis blood supply (Rollot et al. 2025). In SMF Me3004, just posterodorsal to the foramen alveolare superius, the edge of the maxilla is developed as a thin ridge that likely forms the anteroventral margin of the foramen orbito-nasale. Within the fossa orbitalis, the maxilla contacts the jugal posteriorly along a V-shaped suture, the pterygoid posteromedially, and palatine medially. In ventral view, the triturating surface is moderately broad and progressively expands mediolaterally towards the posterior (Fig. 3B). The labial margin is nearly straight and anteriorly continuous with the anteroventral edge of the premaxilla (Fig. 3B). The morphology of this area is, therefore, similar to that of Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018), but greatly contrasts with that of Allaeochelys meylani (Rollot et al. 2025) and Carettochelys insculpta (Rollot et al. 2024b). In the latter two taxa, the labial ridge forms a short process medially that separates the triturating surface posteriorly from a half-cup fossa anteriorly formed by the maxilla and premaxilla. This fossa is large in Allaeochelys meylani (Rollot et al. 2025), small in Carettochelys insculpta (Rollot et al. 2024b), and completely absent in Anosteira maomingensis (Danilov et al. 2017a), Anosteira pulchra (Joyce et al. 2018), and Allaeochelys crassesculpta. In SMF Me3004, the maxilla forms an elongate articular facet posteromedially for about one third of its length, which corresponds to a suture shared with the palatine medially and pterygoid posteromedially and posteriorly. As the articular surface is uninterrupted for its entire length, we postulate that the maxilla had no contribution to the foramen palatinum posterius, as some part of the maxilla would otherwise be smooth and highlight the position of the foramen palatinum posterius. Bony contributions to the foramen palatinum posterius are variable among carettochelyids and the foramen is formed by the maxilla, palatine, and pterygoid in Anosteira maomingensis (Danilov et al. 2017a), by the maxilla and palatine in Allaeochelys meylani (Rollot et al. 2025), and only by the palatine in Anosteira pulchra (Joyce et al. 2018) and adult Carettochelys insculpta (Rollot et al. 2024b).

Palatine. The palatines are overall well-preserved in SMF Me3004, but the anterolateral aspect of both elements is damaged and dislocated (Figs 2B, 3B). The palatine contacts the vomer anteromedially, the maxilla anterolaterally, the pterygoid posterolaterally, the parabasisphenoid posteriorly, and its counterpart medially (Figs 2B, 3B). The posterior contact of the palatine with the parabasisphenoid prevents the pterygoids from contacting one another along the midline, as in all carettochelyids (Waite 1905; Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2024b). Dorsally, at about mid-length, the palatine forms a tall ascending process (Fig. 6), as in all trionychians (Meylan 1987; Joyce 2014). The posterior half of the ascending process is much lower than the anterior half. This difference is only minimally due to breakage, as µCT scans of SMF Me3004 show that most of the dorsal aspect of the preserved portion corresponds to an articular facet. This facet likely articulated with the parietal and epipterygoid, but the corresponding portions of the latter two bones are not preserved and we are not able to identify the actual limits of the contacts. Anteriorly, on its dorsal surface, the palatine forms a low, but relatively broad ridge that extends posteriorly and posterolaterally and that forms the lateral wall of a shallow groove (Fig. 6A, B). This groove extends posteriorly and posterolaterally and connects to a large foramen located at the medial base of the ascending process of the palatine. The foramen leads into a canal that extends posteriorly for a short distance and connects to other canals that cross the ascending process of the palatine. Various foramina linked to this network of canals are visible on the lateral surface of the ascending process of the palatine, just posterior to its tallest extent (Fig. 6C). At about two-thirds of the anteroposterior length of the dorsal groove, the palatine forms a small fossa, which posteriorly ends in a small foramen. This foramen leads into a canal that extends posteriorly through the palatine. The location of the foramen and path of the canal are identical to that of the foramen anterius canalis nervi vidiani and canalis nervus vidianus in Carettochelys insculpta (Rollot et al. 2021, 2024b) and we, therefore, identify the foramen and canal as such in SMF Me3004 (Fig. 6B). Anterolaterally, the left palatine preserves a small piece of which the two longest sides have a smooth surface. Given the orientation of this bony piece, its location in the original specimen, and the morphology of this area in other carettochelyids (Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024b), we interpret this element as being the anterolateral end of the palatine, with the smooth side facing anteromedially corresponding to the posterior or posterolateral margin of the foramen orbito-nasale and the smooth side facing posterolaterally corresponding to the anteromedial margin of the foramen palatinum posterius (Fig. 3B).

Vomer. The vomer of SMF Me3004 lacks its anterior and anterolateral ends, is eroded on its ventral surface, and broken into four pieces (Figs 2B, 3B). The vomer contacts the prefrontal anterolaterally and the palatine posterolaterally and posteriorly. Although the anterior and anterolateral sides of the vomer are damaged, the premaxilla and maxilla are sufficiently complete to ascertain that there is no contact of the vomer with the latter two bones, as in other carettochelyids (Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024b). Dorsolaterally, the vomer forms low processes for articulation with the descending process of the prefrontal (Fig. 3B). Dorsally, the vomer forms a narrow, but deep sulcus vomeri that extends anteroposteriorly for all the length of the element. Ventrally, along its posterior end, the vomer forms an elongate fossa that is open posteriorly and delineated by a thin, curved ridge anteriorly and laterally, just at the level of contact with the palatine (Figs 2B, 3B).

Pterygoid. The pterygoids are moderately damaged in SMF Me3004 (Figs 2B, 3B). The left element is the most complete of the two, preserving the anterior end of the anterior process and most of the central and posterior portions of the pterygoid, although substantial parts of its dorsal surface are eroded (Figs 2B, 3B, 7). Most of the right element, however, is missing, as it only preserves the anterior tip of the anterior process and damaged portions of the posteromedial part of the bone (Figs 2B, 3B). The anterior half of the pterygoid is developed as an elongate process that contacts the maxilla anteriorly, the palatine medially, and the parabasisphenoid posteromedially (Fig. 3B). Anterolaterally, the anterior process forms a small and low, but elongate processus pterygoideus (Fig. 7D), as in Carettochelys insculpta (Rollot et al. 2024b). Contact of the pterygoid with its counterpart is prevented by a connection between the palatine and parabasisphenoid (Figs 2B, 3B), as in all carettochelyids (Waite 1905; Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2024b). The posterior half of the pterygoid contacts the parabasisphenoid medially, the basioccipital-exoccipital complex posteromedially, and the quadrate laterally (Figs 3B, 7). The dorsal contacts of the pterygoid are not preserved, as the central part of the cranium is heavily damaged due to its compression. The pterygoid preserves several structures of interest on its ventral surface. The pterygoid forms the foramen posterius canalis carotici interni, which is likely entirely formed by the pterygoid, but very closely located to the parabasisphenoid, at about two-thirds of the anteroposterior length of the pterygoid-parabasisphenoid suture (Fig. 7B, C). The position of the foramen posterius canalis carotici interni is, therefore, reminiscent of that seen in Kizylkumemys schultzi (Danilov and Obraztsova 2015) and Anosteira pulchra (Joyce et al. 2018), but differs from other carettochelyids, in which the foramen is located either more posteriorly or posterolaterally far from the pterygoid-parabasisphenoid suture (Waite 1905; Walther 1922; Havlik et al. 2014; Danilov et al. 2017a; Rollot et al. 2024a, 2024b, 2025). Given the position of the foramen posterius canalis carotici interni in SMF Me3004, the canalis caroticus internus likely extended anteriorly along the pterygoid-parabasisphenoid suture, but damage to that area precludes the description of the internal carotid artery system. Posteriorly, starting from the foramen posterius canalis carotici interni, the pterygoid forms a well-defined groove that extends posterolaterally on its ventral surface and ends close to the posterior rim of the pterygoid, which highlights the posterior path of the internal carotid artery (Figs 2B, 3B, 7B, 7C). Lateral to this groove, the pterygoid forms a deep pterygoid fossa (Figs 2B, 3B, 7B, 7C), as in Anosteira maomingensis (Danilov et al. 2017a), Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Havlik et al. 2014; Rollot et al. 2024a), and Carettochelys insculpta (Waite 1905; Walther 1922), but not Anosteira pulchra (Joyce et al. 2018). An enfolded ridge, formed by the pterygoid that ventrally covers the pterygoid fossa when present, is absent in SMF Me3004 (Figs 2B, 3B, 7B, 7C), as is the case in Kizylkumemys schultzi (Nessov 1987; Danilov and Obraztsova 2015), Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018), but not in Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Walther 1922; Joyce 2014). Posteromedially, the pterygoid contributes to the elongate tuberculum basioccipitale by forming its ventrolateral part (Figs 2B, 3B, 7). Elongate tubercula basioccipitale are also present in Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Havlik et al. 2014; Rollot et al. 2024a), and Carettochelys insculpta (Waite 1905; Walther 1922; Rollot et al. 2024b), but not in Anosteira pulchra (Joyce et al. 2018), where the tubercula are short. Laterally, the pterygoid has a tall suture with the quadrate that extends vertically just medial to the mandibular condyle (Fig. 7C, D). Dorsally, most of the surface of the pterygoid is eroded and damaged, preserving only a few features of interest. The dorsal surface of the posterior half of the pterygoid is intact overall and forms a low ridge at about mid-length that crosses most of the preserved element mediolaterally (Fig. 7A). This ridge is visible on both sides of the specimen and located at the same level, slightly posterior to the level of the suture between the parabasisphenoid and basioccipital-exoccipital complex. In Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Rollot et al. 2024b), the pterygoid is slightly raised at the same level, forming either a plateau or a ridge that corresponds to the articulation facet with the processus interfenestralis of the opisthotic. Considering the morphology exhibited by these carettochelyids, we interpret the dorsal pterygoid ridge of SMF Me3004 as representing at least a portion of the articulation facet with the processus interfenestralis of the opisthotic. The area posterior to that ridge, therefore, corresponds to the floor of the recessus scalae tympani (Fig. 7A). The posterodorsal surface of the pterygoid is rough, posterior to the recessus scalae tympani. A close look at the µCT dataset shows that this surface appears to correspond to a suture, which is confirmed to be present by specimen HLMD Me7592, in which this area is partially preserved on the right side with a pterygoid-opisthotic contact (Fig. 7A, C, D). By reference to other carettochelyids, this opisthotic-pterygoid contact corresponds to the ventral contact of the secondary process of the opisthotic with the pterygoid, which restricts the fenestra postotica medially (Danilov et al. 2017a; Rollot et al. 2024a, 2024b, 2025). Just lateral to this inferred articulation facet and contact, the dorsal surface of the pterygoid is smooth and concave, forming a short groove that extends anteriorly just lateral to the recessus scalae tympani (Fig. 7A). In Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Rollot et al. 2024b), this part of the pterygoid corresponds to the ventral margin of the fenestra postotica, which is greatly reduced in carettochelyids because of the posterior contact between the pterygoid and opisthotic. We, therefore, interpret this portion of the pterygoid in SMF Me3004 as representing the ventral margin of the fenestra postotica and the short anterior groove corresponds to the posterior floor of the cavum acustico-jugulare (Fig. 7A, C, D). In HLMD Me7592, the pterygoid forms a foramen that is located lateral to the foramen jugulare posterius, just lateral to its suture with the basioccipital-exoccipital complex and medial to its suture with the opisthotic. The foramen leads into a short canal that extends anterolaterally within the pterygoid and basioccipital-exoccipital complex and joins the recessus scalae tympani anteriorly, just lateral to the foramen that leads into the canal that posteriorly connects to the foramen jugulare posterius. The location of these structures is identical to the position of the foramen and canal for the passage of the glossopharyngeal nerve (CN IX; foramen oropharyngeale, sensuEvers and Al Iawati (2024)) seen in Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Rollot et al. 2024b) and we, therefore, identify them as such in HLMD Me7592. Nevertheless, some variation in the bony contributions to these structures seems to be apparent. In HLMD Me7592, the foramen oropharyngeale is likely only formed by the opisthotic, although very closely located to the pterygoid-opisthotic suture, as observed in adult Carettochelys insculpta (Rollot et al. 2024b), whereas the foramen is formed by the opisthotic and pterygoid in Allaeochelys meylani (Rollot et al. 2025) and Allaeochelys libyca (Rollot et al. 2024a).

Epipterygoid. The epipterygoids are not preserved in SMF Me3004 and no insights can be obtained for other specimens.

Quadrate. The quadrates of SMF Me3004 are broken into several pieces, but most of their morphology remains preserved (Figs 2, 3, 8). The sole exception is the posterodorsal part of both elements, at the level of the antrum postoticum, which is largely missing. In lateral view, the quadrate contacts the quadratojugal along a tall, C-shaped suture and the squamosal posterodorsally (Figs 2C, 2D, 3C, 3D). The quadrate forms most of the cavum tympani, with the exception of the anterior margin, which is formed by the quadratojugal, as in Anosteira maomingensis (Danilov et al. 2017a), Anosteira pulchra (Joyce et al. 2018), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Walther 1922; Rollot et al. 2024b), but not Allaeochelys meylani (Rollot et al. 2025). The quadrate usually forms the incisura columella auris, which in carettochelyids is completely enclosed by the latter bone (Waite 1905; Walther 1922; Havlik et al. 2014; Joyce 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2024b, 2025), but the area is not preserved in SMF Me3004. In specimen SMF Me3566, however, the incisura columella auris is perfectly visible and completely enclosed by the quadrate. The quadrate of SMF Me3004 forms a moderately deep precolumellar fossa in the posteroventromedial corner of the cavum tympani (Figs 3D, 8A). A similar fossa, albeit shallower, is also present in Allaeochelys meylani (Rollot et al. 2025), but such a depression is absent in Allaeochelys libyca (Rollot et al. 2024a) and Carettochelys insculpta (Rollot et al. 2024b). Notably deep precolumellar fossae are also known in various pleurodires (Gaffney et al. 2006), but their function remains uncertain (Foth et al. 2019). According to Harrassowitz (1922), the antrum postoticum is absent in Allaeochelys crassesculpta, but several specimens studied herein suggest otherwise. Indeed, a large antrum postoticum is clearly visible in the µCL scans of HLMD Me14997 and in the female individual of specimen SMF Me3566. In SMF Me3004, this area is damaged but still preserves enough of the quadrate to draw the same conclusion. Dorsal to the precolumellar fossa of the cavum tympani, the dorsal surface of the preserved quadrate forms a smooth and slightly concave surface that appears to have an anteromedial-posterolateral orientation (Figs 3C, 8A). Just anteroventral to this surface, the quadrate forms a smooth, horizontal ridge, that weakly extends above the precolumellar fossa. The morphology of this area, particularly the smooth dorsal surface of the quadrate and the low ridge anteroventral to it, is reminiscent of that of SMF Me3566 and other carettochelyids that possess an antrum postoticum, such as Anosteira pulchra (Joyce et al. 2018) and Allaeochelys libyca (Rollot et al. 2024a), but also juvenile Carettochelys insculpta (Rollot et al. 2024b). In dorsal view, the quadrate of SMF Me3004 contacts the quadratojugal anterolaterally, the prootic medially, the opisthotic posteromedially and the squamosal posterolaterally (Figs 2A, 3A). At about two-thirds of the anteroposterior length of the prootic-quadrate suture, the right quadrate preserves the most dorsolateral aspect of the canalis stapedio-temporalis, showing that the quadrate forms the lateral wall of that canal and the lateral margin of the foramen stapedio-temporale, as in other carettochelyids (Waite 1905; Walther 1922; Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2025). The quadrate also contributes to the processus trochlearis oticum (Figs 2B, 3B, 8B), although it only forms the most lateral part of the process, which is otherwise mostly formed by the prootic (Figs 2A, 2B, 3A, 3B). The posterior surface of the quadrate is excavated and forms a fossa (Figs 2F, 3F, 8C), as in all carettochelyids (Walther 1922; Havlik et al. 2014; Joyce 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2024b, 2025). The fossa in SMF Me3004 is only moderately deep, as in Anosteira maomingensis (Danilov et al. 2017a) and Anosteira pulchra (Joyce et al. 2018), which contrasts with the deeper fossa found in Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Havlik et al. 2014; Rollot et al. 2024a), and Carettochelys insculpta (Joyce 2014; Rollot et al. 2024b). In SMF Me3004, the posterolateral edge of the quadrate protrudes posteriorly, forming a relatively broad process that folds slightly over the quadrate fossa (Figs 2F, 3F, 8C), as seen to a variable extent in other carettochelyids. This process or enfolded ridge is slightly more prominent and well-defined in SMF Me3004, Allaeochelys libyca (Rollot et al. 2024a) and Carettochelys insculpta (Rollot et al. 2024b) than in Allaeochelys meylani, in which it laterally delineates an additional shallow fossa (Rollot et al. 2025). In SMF Me3004, the medial surface of the quadrate is slightly concave and forms a shallow groove dorsomedially and just dorsal to the medial suture with the pterygoid. This feature is visible on both sides at the same level and, by reference to other carettochelyids (Havlik et al. 2014; Joyce 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2025), can be confidently identified as the lateral margin and lateral wall of the fenestra postotica and cavum acustico-jugulare, respectively (Fig. 8C). Ventrally, the quadrate forms a low and ventrally oriented mandibular condyle (Figs 2B, 3B, 8B, 8C), as in other carettochelyids (Waite 1905; Walther 1922; Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2025). The mandibular condyle consists of two facets, with the lateral one being about three times larger than the medial (Figs 2B, 3B). The two facets are separated by a wide and deep sulcus (Figs 2B, 3B, 8C), as seen in Allaeochelys libyca (Rollot et al. 2024a), which contrasts with the deep, but narrower sulcus seen in Allaeochelys meylani (Rollot et al. 2025) and the wide, but shallower sulcus seen in Carettochelys insculpta (Rollot et al. 2024b). On the anterior surface of the quadrate of SMF Me3004, just anterodorsal to the mandibular condyle, the quadrate forms a small foramen that leads into a canal that extends dorsally within the latter bone. The path of this canal is difficult to follow as it blends into porous bone, but the position of the foramen is reminiscent of the anterior quadrate foramen seen in Allaeochelys libyca (Rollot et al. 2024a) and Carettochelys insculpta (Rollot et al. 2024b). The quadrate of carettochelyids usually contributes to various other internal structures, such as the canalis cavernosus and foramen arteriomandibulare, but these areas are neither preserved in SMF Me3004 nor any other specimen for which we acquired tomographic data.

Prootic. The prootics are incomplete in SMF Me3004 (Figs 2A, 3A). Both elements preserve most of their dorsal aspect, with that of the left prootic being almost complete and the most anterior part of the ventral process. Within the upper temporal fossa, the prootic contacts the parietal medially, the quadrate laterally, the supraoccipital posteromedially, and the opisthotic posteriorly, which clearly prevents the quadrate from contacting the supraoccipital (Figs 2A, 3A). The prootic forms most of the processus trochlearis oticum, which is developed as a relatively thin lip of bone that is strongly curved ventrally. Posterolaterally, at about two-thirds of the anteroposterior length of the prootic-quadrate suture, the prootic forms the medial margin of the left foramen stapedio-temporale, which is only visible on the left side of the specimen. Anteroventrally, the small portion of the prootic that remains preserved is anteriorly concave with a smooth, intact surface. This aspect is visible on both sides of the specimen and, by reference to the same area in other carettochelyids (Rollot et al. 2024a, 2024b, 2025), can be identified as the posterior margin of the foramen nervi trigemini sensu stricto (sensuRollot et al. (2024a)). The preserved parts of the prootic show that it forms the anterior half of the cavum labyrinthicum and anterior half of the anterior semicircular canal. The fossa acustico-facialis, canalis nervus facialis, fenestra ovalis, and hiatus acusticus are not preserved.

Opisthotic. The opisthotic of SMF Me3004 is heavily damaged on both sides (Figs 2A, 2F, 3A, 3F). The left element only preserves the most anterior and dorsal aspects, while the right element, to which two large pieces can be attributed, preserves its anterodorsal and dorsolateral portions. The preserved parts of the opisthotic contact the supraoccipital medially, the prootic anteriorly, which prevents the supraoccipital to contact the quadrate, and the quadrate laterally (Figs 2A, 2F, 3A, 3F). The clear contact between the prootic and supraoccipital just anterior to the opisthotic also prevents the latter from contacting the parietal (Figs 2A, 3A). Internally, the opisthotic forms the posterior margin of the lateral semicircular canal, which is not fully enclosed by bone anteriorly, and the posterior margin of the posterior semicircular canal. On the right side, the most anterior portion of the paroccipital process is preserved. Ventrolaterally, the surface of the opisthotic is slightly concave and forms a very shallow groove. With reference to other carettochelyids (Rollot et al. 2024a, 2024b, 2025), this lightly concave surface likely corresponds to the medial margin of the fenestra postotica and posteromedial wall of the cavum acustico-jugulare. The ventromedial surface of the paroccipital process forms a broad, slightly concave surface, which corresponds to the roof of the recessus scalae tympani. As both opisthotics are heavily damaged, we cannot evaluate the fenestra ovalis, hiatus acusticus, fenestra perilymphatica, foramen oropharyngeale, processus interfenestralis, and canals and foramina for the glossopharyngeal nerve (CN IX). A ventral contact of the processus interfenestralis with the pterygoid can nevertheless be inferred by the presence of a raised ridge on the dorsal surface of the pterygoid and that extends mediolaterally, which is in the same position as the raised surface of the pterygoid that contacts the processus interfenestralis in other carettochelyids (see Pterygoid above; Rollot et al. (2024a, 2024b, 2025)). An additional contact of the opisthotic with the pterygoid, which corresponds to the ventral contact of the secondary process of the opisthotic with the pterygoid, is present as in other carettochelyids, based on SMF Me3004 and HLMD Me7592 (see Pterygoid above; Danilov et al. (2017a); Rollot et al. (2024a, 2024b, 2025)).

Supraoccipital. The supraoccipital is overall well-preserved in SMF Me3004 and only lacks portions of the crista supraoccipitalis and its most anteroventral aspects (Figs 2A–D, 2F, 3A–D, 3F). Within the upper temporal fossa, the supraoccipital contacts the parietal anteriorly, the prootic anterolaterally, and the opisthotic posterolaterally (Figs 2A, 3A). A posteroventral contact with the basioccipital-exoccipital complex is documented by HLMD Me14997, where the supraoccipital forms the dorsal margin of the foramen magnum and roofs the braincase posteriorly. The supraoccipital does not contribute to the foramen stapedio-temporale and canalis stapedio-temporalis, as is the case in other carettochelyids (Waite 1905; Walther 1922; Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2025). The supraoccipital is also excluded from the skull roof (Figs 2A, 3A), which appears to be a unique feature among carettochelyids, although this contribution is generally small in members of the clade (Walther 1922; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024b). Posteriorly, the supraoccipital forms an elongate crista supraoccipitalis, which is ventrally expanded into a broad, flat shelf (Figs 2A, 2B, 2F, 3A, 3B, 3F). The crista supraoccipitalis and associated flat shelf represent more than half of the length of the supraoccipital and posteriorly extend far beyond the level of the squamosal processes and appear to be slightly longer than that of Carettochelys insculpta (Walther 1922; Rollot et al. 2024b) and similar to the condition seen in Anosteira maomingensis (Danilov et al. 2017a). In SMF Me3004, the supraoccipital also roofs the braincase posteriorly and forms the posterior half of the anterior semicircular canal and anterior half of the posterior semicircular canal. Ventromedially, the surface of the supraoccipital perfectly preserves the dorsal aspect of the anterior and posterior semicircular canals. The area between the two canals, which corresponds to the dorsal imprint of the common crus, is nearly flat and horizontally continuous with the path of the anterior and posterior semicircular canals. This morphology suggests that the dorsal embayment of the common crus was very low, if not completely absent, as seen in Allaeochelys meylani (Rollot et al. 2025).

Basioccipital-exoccipital complex. The basioccipital and exoccipitals are fused in SMF Me3004 (Figs 2A–D, 2F, 3A–D, 3F, 9), and we, therefore, describe these bones as a single osteological unit. Remnants of the basioccipital-exoccipital suture are visible in HLMD Me7592, but only internally in the area of the foramen jugulare posterius. In SMF Me3004, the basioccipital-exoccipital complex is overall well-preserved, with damage only affecting its most anterior, anterolateral, and dorsal aspects. The basioccipital-exoccipital complex forms the posterolateral wall and posteroventral floor of the braincase and the lateral and ventral margins of the foramen magnum (Figs 3F, 9A). The basioccipital-exoccipital complex contacts the parabasisphenoid anteriorly, the pterygoid anterolaterally and laterally, along the elongate tubercula basioccipitale, and the supraoccipital dorsally (visible in HLMD Me14997; Figs 2A, 2B, 2F, 3A, 3B, 3F). Additional contact with the opisthotic was likely present, but is not preserved. Posterolaterally, the basioccipital-exoccipital complex forms most of the elongate tubercula basioccipitale, the ventrolateral aspect of it being formed by the pterygoid (Figs 2A, 2B, 3A, 3B, 7, 9). Elongate tubercula basioccipitale are also present in Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Havlik et al. 2014; Rollot et al. 2024a), and Carettochelys insculpta (Waite 1905; Walther 1922; Rollot et al. 2024b), which contrasts with the short tubercula seen in Anosteira pulchra (Joyce et al. 2018). The basioccipital-exoccipital complex of SMF Me3004 also forms the posterior wall of the recessus scalae tympani (Fig. 9A). In the posterolateral corner of the recess, the basioccipital-exoccipital complex forms a foramen that leads into a canal, which extends posterolaterally through the bone. The canal joins the external cranial surface via a relatively large foramen jugulare posterius just medial to the suture with the pterygoid (Fig. 9B). The foramina and canal are particularly well-preserved on the right side of SMF Me3004. Within the braincase, ventrally, the basioccipital-exoccipital complex forms two foramina for the hypoglossal nerve (CN XI), the anterior foramen being the smallest of the two (Fig. 9A). Each foramen leads into a separate canal that extends posteroventrolaterally. The anterior and smallest canal joins the posterior surface of the cranium by means of a posterior hypoglossal nerve foramen located just ventromedial to the foramen jugulare posterius (Fig. 9B). The posterior and largest canal also joins the posterior surface of the cranium, but its posterior hypoglossal nerve foramen is located more medially, closer to the occipital condyle (Fig. 9B). Some variation is apparent, as the two posterior hypoglossal nerve foramina are closer to one another on the left side of the specimen. Two posterior foramina for the hypoglossal nerve are also present in Anosteira pulchra (Joyce et al. 2018), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Walther 1922; Rollot et al. 2024b), contra the single foramen seen in Anosteira maomingensis (Danilov et al. 2017a) and Allaeochelys meylani (Rollot et al. 2025). In HLMD Me7592, the foramen and canalis oropharyngeale are preserved and the basioccipital-exoccipital complex forms the anterolateral margin of the canal. In SMF Me3004, on the posterior surface of the basioccipital-exoccipital complex, it forms a relatively low, but broad neurapophyseal ridge (Fig. 9B). The neurapophyseal ridge is visible just lateral to the foramen magnum, where it extends ventrolaterally for about half its preserved portion, then bends and extends laterally towards the anterior base of the tubercula basioccipitale, separating the foramen jugulare posterius from the posterolateral hypoglossal nerve foramen. The neurapophyseal ridge laterally ends at the most posterior level of the hypoglossal nerve foramina. The neurapophyseal ridge separates the posterior surface of the basioccipital-exoccipital complex into two distinct fossae, located ventromedial and dorsolateral from the ridge, as also seen in Allaeochelys meylani (Rollot et al. 2025), Allaeochelys libyca (Rollot et al. 2024a), and Carettochelys insculpta (Rollot et al. 2024b). In SMF Me3004, the central part of the ventral surface of the basioccipital-exoccipital complex is flat (Figs 2B, 3B), contrasting with the morphology of this area in other carettochelyids, in which the basioccipital forms a semicircular depression (Waite 1905; Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2024a, 2025).

Parabasisphenoid. The parabasisphenoid of SMF Me3004 is overall well-preserved with most of the damage affecting the rostrum basisphenoidale, the left anterolateral aspect, and the right clinoid process (Figs 2B, 3B, 10). Although the parabasisphenoid is nearly complete, we are not able to assess its dorsal contacts and contributions to anatomical structures as the central part of the cranium is completely crushed. The parabasisphenoid contacts the palatine anteriorly, the pterygoid laterally, and the basioccipital-exoccipital complex posteriorly (Figs 2B, 3B, 10). The anterior contact of the parabasisphenoid with the palatine prevents the pterygoid from contacting its counterpart medially (Figs 2B, 3B), as in all carettochelyids (Waite 1905; Walther 1922; Havlik et al. 2014; Danilov et al. 2017a; Joyce et al. 2018; Rollot et al. 2025). Laterally, the parabasisphenoid forms a tall, concave, and smooth surface. Given the position of the foramen posterius canalis carotici interni, which is located just lateral to the parabasisphenoid-pterygoid suture, we can confidently infer that this concave surface corresponds to the medial margin of the canalis caroticus internus and canalis caroticus basisphenoidalis (Fig. 10C). The path of the canalis caroticus internus is preserved in HLMD Me7592, in which it extends anteromedially along the pterygoid-parabasisphenoid suture. Just anterior to the foramen posterius canalis carotici interni, the canalis caroticus internus laterally connects to the canalis pro ramo nervi vidiani, which serves as a passage for the vidian nerve from the geniculate ganglion to the canalis caroticus internus. Slightly anterior to that point, a small canal seems to split from the canalis caroticus internus laterally. This split may correspond to the bifurcation between the canalis caroticus internus and canalis nervus vidianus, but the area is not sufficiently preserved to assess the identity of this canal with confidence. On the right side of HLMD Me3566, a similar arrangement of canals is present, with the possible canalis nervus vidianus bifurcating from the canalis caroticus internus along the pterygoid-parabasisphenoid suture, then laterally merging with an unidentified canal coming from the posterior and anteriorly extending through the pterygoid and, very likely, the palatine. Furthermore, in HLMD Me3566, still on the right side, no additional canal seems to split from the canalis caroticus internus nor extend anteriorly along the pterygoid-parabasisphenoid suture and we, therefore, postulate that the canalis caroticus lateralis is absent in Allaeochelys crassesculpta, as in Allaeochelys libyca (Rollot et al. 2024a) and Carettochelys insculpta (Rollot et al. 2021, 2024b). In HLMD Me7592, the canalis caroticus internus anteriorly becomes the canalis caroticus basisphenoidalis, which extends through the parabasisphenoid and joins the sella turcica by means of the foramen anterius canalis carotici basisphenoidalis. Some variation is apparent in HLMD Me7592, as the right internal carotid canal is about three to four times larger than its left counterpart. In SMF Me3004, the parabasisphenoid anteriorly forms the broad rostrum basisphenoidale, which is mostly missing. The rostrum basisphenoidale likely extended over the posterodorsal surface of the palatine, as the latter surface is developed as an articular facet, as seen in Carettochelys insculpta (Rollot et al. 2024b). Anterodorsally, the parabasisphenoid of SMF Me3004 forms a relatively thin dorsum sellae that projects anteriorly over the sella turcica (Fig. 10C). On the left side, the clinoid process appears to be completely preserved and consists of a short process that extends anterolaterally (Fig. 10A, C), as in Carettochelys insculpta (Rollot et al. 2024b). In SMF Me3004, the dorsal surface of the parabasisphenoid is concave. The foramina and canals associated with the path of the abducens nerve (CN VI) are apparent in HLMD Me7592. In the latter specimen, the parabasisphenoid anteriorly forms the foramen posterius canalis nervi abducentis along its dorsal surface. The latter foramen leads into the canalis nervus abducentis, which extends anteriorly within the parabasisphenoid, and anteriorly exits the parabasisphenoid by means of the foramen anterius canalis nervi abducentis, which appears to be located somewhat ventral to the base of the clinoid process, as in Carettochelys insculpta (Rollot et al. 2024b). Based on the right side of the specimen, the foramen anterius canalis nervi abducentis seems to be exclusively formed by the parabasisphenoid, but as it is located immediately medial to the lateral articular facet of the parabasisphenoid, a minor contribution of the pterygoid and/or prootic may be present. In SMF Me3004, the posterior and posterolateral surfaces of the parabasisphenoid appear to be nearly intact and the shape of the parabasisphenoid is, therefore, likely very similar to that of other carettochelyids (Havlik et al. 2014; Rollot et al. 2024a, 2024b). This area may have had a similar morphology to that of Carettochelys insculpta (Rollot et al. 2024b), which exhibits a ventrally enlarged cavum labyrinthicum, but the lack of preservation of the surrounding bones hinders certainty regarding this observation.

Mandible

General comments. SMNK Pal45053 is a nearly complete mandible that is slightly crushed (Fig. 11). Most of the damage that affects the mandible is located around the fossa Meckelii. The mandibular bones are generally well-preserved, except for the prearticular and surangular, where the anterior half is damaged on both sides. The mandible generally closely resembles that of Carettochelys insculpta (Evers et al. 2022; Rollot et al. 2024b) and only minor differences can be identified (see below).

Dentary. The dentaries are fused in SMNK Pal45053 along a slightly expanded symphysis (Fig. 11A, B). The dentary contacts the coronoid posterodorsally, the surangular posteriorly, and the angular posteroventrally (Fig. 11). The dentary forms a low labial ridge, which is best visible along the anterior half of the element (Fig. 11G–J). The posterior half of the triturating surface of the dentary is slightly concave (Fig. 11D), forming a similar depression to that observed in Carettochelys insculpta (Rollot et al. 2024b). Some variation is apparent, as this depression is only formed by the dentary in SMNK Pal45053, but by the dentary and coronoid in adult Carettochelys insculpta (Rollot et al. 2024b). In Anosteira maomingensis, the morphology of the mandible is different, as the triturating surface is subdivided into a flattened posterior expanded region, which is anteromedially delimited by a low, longitudinal ridge (Tong et al. 2010). A lingual ridge is absent in SMNK Pal45053 (Fig. 11H, J). In lateral view, along its posterior half, the dentary forms a deep adductor fossa (Fig. 11G, I), which likely served as the insertion area for the M. zygomaticomandibularis as seen in Carettochelys insculpta (Werneburg 2011; Rollot et al. 2024b). In SMNK Pal45053, the adductor fossa is anterolaterally and ventrolaterally delineated by a lateral dentary ridge (sensuEvers et al. (2022)). Along the anterior margin of the fossa, the dentary forms the foramen dentofaciale majus. In medial view, the dentary forms a groove that extends anteroposteriorly for most of its length and which is the anterior continuation of the fossa Meckelii (Fig. 11H, J). Just dorsal to this groove and ventral to the suture between the dentary and coronoid, the dentary forms the foramen alveolare inferius.

Surangular. The surangular of SMNK Pal45053 forms most of the posterolateral aspect of the mandible (Fig. 11). The surangular contacts the dentary anteriorly and anteroventrally, the prearticular anteromedially, the articular medially and posteromedially, and the angular ventrally (Fig. 11). The anterodorsal process is almost completely missing and contact with the coronoid is, therefore, not preserved. Such a contact was nevertheless likely present given the arrangement of these two bones and by reference to Carettochelys insculpta, in which it is present (Evers et al. 2022; Rollot et al. 2024b). In lateral view, posterior to the adductor fossa and ventral to the anterior aspect of the ectocondylar flange, the surangular forms the foramen auriculotemporalis (Fig. 11G). Although the area of the fossa Meckelii is severely damaged, a small portion of the surangular remains preserved, showing that it formed the posterolateral wall of the fossa. Anterodorsomedially, the surangular forms a large dorsal surangular foramen (sensuEvers et al. (2022); Fig. 11A), which is located just lateral to the suture with the articular and appears to be larger than that of Carettochelys insculpta (Rollot et al. 2024b). Posterodorsally, the surangular forms a laterally expanded ectocondylar flange, which forms slightly more than half of the mandibular articular surface (Fig. 11C). Posterior to the mandibular articular surface, the surangular borders the retroarticular process of the articular laterally (Fig. 11C, F).

Coronoid. The two coronoids are almost completely preserved in SMNK Pal45053 (Fig. 11A–D, F–J). Only their most posteroventromedial aspect is slightly damaged. The coronoid contacts the dentary anteroventrally and the prearticular posteroventromedially (Fig. 11A–D, F–J). Contact with the surangular was likely present posteroventrolaterally, but damage to the latter bone prevents assessment (see Surangular above). The coronoid forms a process anteromedially that extends over the dentary medially and forms the posteromedial corner of the triturating surface (Fig. 11C, D, F). The coronoid also forms the most dorsal aspect of the adductor fossa (Fig. 11G, I). The coronoid process is tall and slightly recurved posteriorly (Fig. 11G–J), as in Carettochelys insculpta (Evers et al. 2022; Rollot et al. 2024b), but unlike Anosteira maomingensis, where the anterior margin of the coronoid is vertically oriented (Tong et al. 2010). A weak coronoid notch is present in SMNK Pal45053 (Fig. 11G–J), again as in Carettochelys insculpta (Evers et al. 2022; Rollot et al. 2024b). On its lateral surface, the coronoid of SMNK Pal45053 forms a shallow depression, similar to that of Carettochelys insculpta (Evers et al. 2022; Rollot et al. 2024b), albeit larger than in the latter as it expands on almost all the lateral surface of the element (Fig. 11G, I).

Prearticular. The prearticular is heavily damaged on both sides in SMNK Pal45053 (Fig. 11A–F, H, J). The preserved bony contacts of the prearticular are with the surangular anteroventrally, the angular ventrally, and the articular laterally (Fig. 11A–F, H, J). On the right side, several small and thin pieces of bone were segmented and attributed to the prearticular, based on their morphology and position on the medial and posteromedial side of the mandible. The most anterodorsal of these plates slightly contacts the coronoid anterodorsally and likely represents the most anterior portion of the anterodorsal process of the prearticular (Fig. 11H), as seen in Carettochelys insculpta (Evers et al. 2022; Rollot et al. 2024b). The small plates of bone on the right side of SMNK Pal45053, identified as belonging to the prearticular, thus also form the medial wall of the fossa Meckelii and the medial margin of its dorsal foramen. A posterior chorda tympani foramen seems to be preserved on the left prearticular and is located just medial to the prearticular-articular suture and ventral to the anterior extent of the ectocondylar flange of the articular (Fig. 11J).

Angular. The angular is nearly completely preserved on both sides in SMNK Pal45053 (Fig. 11A, B, E, F, G–J). The angular is small and elongate and forms the posteroventromedial aspect of the mandible (Fig. 11E). The anterior half of the angular is developed as a mediolaterally thin sheet of bone, whereas the posterior half of the element is mediolaterally more expanded and almost as wide as tall. The angular contacts the dentary anteriorly and anterolaterally, the surangular laterally and posterolaterally, the prearticular dorsomedially, and the articular dorsally and posterodorsally (Fig. 11A, B, E, F, G–J).

Splenial. The splenials are absent in SMNK Pal45053, as in Carettochelys insculpta (Evers et al. 2022; Rollot et al. 2024b) and, likely, Anosteira maomingensis (Tong et al. 2010).

Articular. The articular is nicely preserved on both sides of SMNK Pal45053 and only shows minor signs of damage surficially along the retroarticular process (Fig. 11). The articular contacts the prearticular anteromedially, the angular ventrally and ventromedially, and the surangular laterally (Fig. 11). As the articular is dorsally expanded and tightly sutured to the surangular and prearticular along the entire height of the latter two bones, it forms a tall wall that structures the posterior margin of the fossa Meckelii, which is posteriorly completely enclosed by bone as in adult Carettochelys insculpta (Rollot et al. 2024b). The articular of SMNK Pal45053 also forms the medial portion of the mandibular articular surface, which represents slightly less than half of the entire surface, most of it being shaped by the surangular (Fig. 11C, F). Posteriorly, the articular forms the retroarticular process (Fig. 11C, F, H, J). Between the mandibular articular surface and distal part of the retroarticular process, the articular forms a smooth and slightly concave surface, which corresponds to the articular notch (Fig. 11H, J).

Shell

General comments. Allaeochelys crassesculpta is known from more than a hundred often complete and articulated specimens. As mentioned above (see Material and methods), we here focus on the material housed at HLMD and SMF, with the addition of one nicely preserved specimen from the SMNK.

As most specimens are flattened, carapacial bones are typically slightly displaced, often preventing accurate assessment of the exact contacts between bones, in particular between the costals and peripherals. A few specimens, however, at least partially preserve the original morphology of the shell and we explicitly refer to those in the following sections when needed. We otherwise describe the shell morphology of Allaeochelys crassesculpta without specific reference to individual specimens because of the great number of specimens studied herein (Figs 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29). Likewise, concerning intraspecific variation, reference to single specimens is only made when such variation can confidently be identified among our sample.

The shell of Allaeochelys crassesculpta is covered by the typical surface ornamentation of carettochelyids consisting of thick ridges separated by equally-sized grooves. As in all carettochelyids, the carapace has a midline keel, a nuchal, ten peripherals, a single suprapygal, and a pygal (Waite 1905; Walther 1922; Havlik et al. 2014; Joyce 2014). The plastron of Allaeochelys crassesculpta resembles that of other carettochelyids by having a triangular entoplastron, large epiplastra, and plastral kinesis (Havlik et al. 2014; Joyce 2014). The carapace of Allaeochelys crassesculpta notably differs from that of Kizylkumemys spp. and Anosteira spp. by lacking carapacial scutes (Leidy 1873; Clark 1932; Zangerl 1947; Chow and Liu 1955; Nessov 1977; Tong et al. 2005) and from that of Carettochelys insculpta by having an uninterrupted neural series (Walther 1922; Joyce 2014). The plastron of Allaeochelys crassesculpta differs from the earliest branching representatives Kizylkumemys schultzi and Anosteira spp. by having a larger posterior plastral lobe (Leidy 1873; Gilmore 1931; Clark 1932; Zangerl 1947; Chow and Liu 1955; Nessov 1977), which is similar in width to that of other Allaeochelys species (Broin 1977; Carbot-Chanona et al. 2020; Rollot et al. 2025), but narrower than that of Carettochelys insculpta (Waite 1905; Walther 1922; Joyce 2014). The plastron of Allaeochelys crassesculpta also differs from Kizylkumemys schultzi by lacking plastral scutes (Nessov 1977), a similar condition to that observed in other carettochelyids (Leidy 1873; Gilmore 1931; Clark 1932; Zangerl 1947; Chow and Liu 1955; Young and Chow 1962; Broin 1977; Hutchison et al. 2004; Carbot-Chanona et al. 2020; Rollot et al. 2025).

Nuchal. The nuchal forms the anterior margin of the shell. The shape of the anterior margin shows minor variation and is either completely straight (HLMD Me1461, Fig. 14; HLMD Me13721, Fig. 18; SMF Me611, Fig. 21; SMF Me1794, Fig. 22; SMNK Pal986, Fig. 29) or slightly concave (HLMD Be158, Fig. 12; HLMD Be159, Fig. 13; SMF Me3783, Fig. 27), but this may be a taphonomic artefact. A straight anterior carapace margin is also observed in Kizylkumemys schultzi (Nessov 1977), Kizylkumemys khoratensis (Tong et al. 2005), and Carettochelys insculpta (Waite 1905; Walther 1922), but differs from Anosteira mongoliensis (Gilmore 1931), Anosteira ornata (Leidy 1873) and, perhaps, Anosteira pulchra (Clark 1932), where a pronounced anterior emargination can be observed. The nuchal of Allaeochelys crassesculpta is wider than long and contacts peripheral I laterally, costal I posterolaterally, and neural I posteriorly. On its ventral surface, the nuchal forms paired nuchal processes, which are particularly visible in specimen SMF Me2449 (Fig. 23).

Neurals. Among our sample of Allaeochelys crassesculpta, the neural series is usually damaged, incomplete, or hidden from view in many specimens, but a few individuals preserve the entire intact series (HLMD Be159, Fig. 13; HLMD Me1461, Fig. 14; HLMD Me7429, Fig. 17; SMF Me611, Fig. 21; and SMF Me11418, Fig. 28), showing that Allaeochelys crassesculpta has an uninterrupted series of seven neurals. An uninterrupted series of neurals is observed in all carettochelyids (Leidy 1873; Clark 1932; Zangerl 1947; Broin 1977), except Carettochelys insculpta, in which the neural series is interrupted by midline contacts between pairs of costals (Waite 1905; Walther 1922; Joyce 2014). In Allaeochelys crassesculpta, the first neural is four-sided, whereas neurals II to VI are hexagonal, with short anterolateral and long posterolateral sides. Neural VII is hexagonal, too, albeit much smaller than the other neurals and about as wide as long. Neurals I to III are elongate and much longer than wide. Neural V and perhaps sometimes neural IV (e.g., HLMD Be158, Fig. 12; and SMNK Pal986, Fig. 29) appear to be somewhat slightly wider than neurals I–III. It is nevertheless clear that a progressive decrease in length of the neurals can be observed from neural V to VII. Neural I contacts the nuchal anteriorly, costal I laterally, and neural II posteriorly. Neurals II to VII each contact two pairs of costals: costals I–II, II–III, III–IV, IV–V, V–VI, and VI–VII, respectively. The outlines and contacts of the neurals appear to be nearly identical in most carettochelyids (Leidy 1873; Zangerl 1947; Broin 1977; Nessov 1977; Tong et al. 2005), but three deviations to the common neural arrangement can be observed. In Anosteira pulchra, neurals II and III are four-sided and only contact one pair of costals, costals II and III, respectively (Clark 1932). In Allaeochelys liliae, neural VII is a fairly robust element that is relatively larger than in other carettochelyids and also much longer than wide (Carbot-Chanona et al. 2020). Finally, in Carettochelys insculpta, the neural series is interrupted by midline contacts of several pairs of costals (Waite 1905; Walther 1922; Joyce 2014). The carapace of Allaeochelys crassesculpta forms a midline keel to which several neurals may contribute. There appears to be variation to the neural on which the keel anteriorly commences, but caution is required when interpreting this feature, as the shells studied herein show different amounts of crushing and because the keel of carettochelyids generally forms gradually. In our sample of Allaeochelys crassesculpta, the keel seems to commence anteriorly at either neural II (HLMD Me1461, Fig. 14; HLMD Me7593, Suppl. material 1: fig. S1.8), neural III (HLMD Me4282, Fig. 15), neural IV (SMF Me3783, Fig. 27; SMNK Pal986, Fig. 29), neural V (HLMD Me7429, Fig. 17; HLMD Me15012, Fig. 19; SMF Me11418, Fig. 28), or even more posteriorly along costal pair VII or VIII (SMF Me611, Fig. 21; SMF Me1794, Fig. 22).

Costals. Eight pairs of costals are present in the carapace of Allaeochelys crassesculpta. Costals I and II are recurved towards the anterior, costal III is nearly straight, and costals IV to VIII are recurved towards the posterior, with the degree of curvature progressively increasing from costal IV to VIII. The mediolateral width of costals IV to VIII also progressively decreases towards the back of the carapace, with costal IV being the widest element and costal VIII the narrowest. Costal I is the largest costal and contacts the nuchal anteromedially, peripheral I anterolaterally, peripheral II laterally, peripheral III posterolaterally, neural I medially, neural II posteromedially, and costal II posteriorly. Costals II to VI each contact two pairs of neurals: neurals II–III, III–IV, IV–V, V–VI, and VI–VII, respectively. Costal VII contacts neural VII anteromedially and its counterpart medially, while costal VIII medially only contacts its counterpart. Laterally, the contacts of the costals with the peripherals are difficult to assess due to crushing and flattening of most specimens that slightly displaced most carapacial bones. This prevents us from unequivocally determining morphological variation, as apparent differences in contacts might be the result of crushing, not intraspecific variation. We nevertheless tentatively identify the most likely contacts between costals and peripherals by reference to single specimens (sometimes even on one side only) that show the least crushing and displacement in the areas of interest. The lateral aspect of costal II is best preserved on the left side of SMF Me3018, where it contacts peripherals III and IV (Fig. 24). For costal III, genuine lateral contacts are preserved in SMF Me3018 and SMF Me11418, in which it contacts peripheral V for most of its length and peripheral VI posterolaterally for a very short distance (Figs 24, 28). A brief contact with peripheral IV might have occurred anterolaterally, but remains hypothetical because of crushing. For costal IV, contacts are best preserved in SMF Me1794 and SMF Me3018, which show an elongate suture of this bone joining with peripheral VI laterally and, perhaps, a short contact with peripheral VII posterolaterally (Figs 22, 24). HLMD Me7429 and SMF Me1794 best preserve the lateral aspect of costal V, which mostly contacts peripheral VII laterally and, likely, also peripheral VIII posterolaterally (Figs 17, 22). The morphology of costal VI is particularly well-preserved in numerous specimens, including HLMD Be158 (Fig. 12), HLMD Me7429 (Fig. 17), SMF Me1794 (Fig. 22), SMF Me3783 (Fig. 27), and SMF Me11418 (Fig. 28), in which the bone mostly contacts peripheral VIII and also has a short contact posterolaterally with peripheral IX. For costal VII, contacts are best visible in HLMD Be158 (Fig. 12), HLMD Me1461 (Fig. 14), HLMD Me7429 (Fig. 17), HLMD Me14920 (Suppl. material 1: fig. S1.21), and HLMD Me15012 (Fig. 19), in which costal VII only contacts peripheral IX. However, in SMF Me1794, an additional contact with peripheral X is visible posterolaterally on both sides, which appears to reflect the actual bony arrangement in the area rather than being the result of crushing, highlighting possible intraspecific variation for this feature (Fig. 22). Although many specimens seem to nicely preserve costal VIII, a close look reveals that minor damage or displacement prevents us from being certain of the lateral contacts of costal VIII. In specimens HLMD Me7428 (Fig. 16), HLMD Me7429 (Fig. 17), and HLMD Me14920 (Suppl. material 1: fig. S1.21), costal VIII mostly contacts peripheral X, but also has an extremely short contact with peripheral IX posterolaterally.

Peripherals. Ten pairs of peripherals contribute to the carapacial rim in Allaeochelys crassesculpta. As the contacts between the peripherals and costals are difficult to identify and were discussed in detail above (see Costals), we here only list the most certain contacts of each peripheral with the neighboring costals. According to our observations, the peripherals have the following contacts: peripheral I with the nuchal medially, costal I posteriorly, and peripheral II posterolaterally; peripheral II with peripheral I anteromedially, costal I posteromedially, and peripheral III posterolaterally; peripheral III with peripheral II anteriorly, costal I anteromedially, costal II posteromedially, and peripheral IV posteriorly; peripheral IV with peripheral III anteriorly, costal II medially, and with peripheral V posteriorly; peripheral V with peripheral IV anteriorly, costal III medially, and peripheral VI posteriorly; peripheral VI with peripheral V anteriorly, costal III anteromedially, costal IV medially, and peripheral VII posteriorly; peripheral VII with peripheral VI anteriorly, costal V medially, and peripheral VIII posteromedially; peripheral VIII with peripheral VII anteriorly, costal VI medially, and peripheral IX posteromedially; peripheral IX with peripheral VIII anterolaterally, costal VI anteromedially, costal VII medially, and peripheral X posteromedially; peripheral X with peripheral IX anterolaterally, costal VIII anteromedially, the suprapygal medially, and the pygal posteromedially. Peripherals IV to VII contribute to the formation of the bridge, which is particularly well-preserved in specimens HLMD Me7593 (Suppl. material 1: fig. S1.8), HLMD Me9722 (Suppl. material 1: fig. S1.12), HLMD Me13484 (Suppl. material 1: fig. S1.19), HLMD Me15012 (Fig. 19), HLMD Me15956 (Fig. 20), SMF Me1794 (Fig. 22), SMF Me3018 (Fig. 24) and SMF Me3566 (Fig. 25). Peripherals IV to VII are V-shaped in cross-section and their ventromedial edge forms a series of sockets for articulation with the hyo- and hypoplastron. The bridge anteriorly starts at about one third to half of the anteroposterior length of peripheral IV and posteriorly ends along the anterior aspect of peripheral VII, which only contributes its anterior third. The extent of the bridge in juveniles seems to be similar (see below). The posterior peripheral elements (i.e. peripherals VII to X and the pygal) form a continuous lip along their visceral side, as in other carettochelyids (Leidy 1873; Dollo 1886; Nessov 1977; Joyce 2014; Rollot et al. 2025).

Suprapygal. A single suprapygal is present in Allaeochelys crassesculpta, as in other carettochelyids (Leidy 1873; Dollo 1886; Waite 1905; Walther 1922; Chow and Liu 1955; Broin 1977; Nessov 1977; Carbot-Chanona et al. 2020; Rollot et al. 2025). The suprapygal is pentagonal with short posterolateral sides, making it mostly triangular. The suprapygal is about as wide as long, as in Kizylkumemys schultzi (Nessov 1977), Allaeochelys delheidi (Dollo 1886), Allaeochelys parayrei (Broin 1977), Allaeochelys liliae (Carbot-Chanona et al. 2020), and Allaeochelys meylani (Rollot et al. 2025), which likely contrasts with Allaeochelys magnifica (Hutchison et al. 2004), where the suprapygal is slightly longer than wide. The suprapygal contacts costal VIII anteriorly and laterally, peripheral X posterolaterally, and the pygal posteriorly. A sutural articulation to the pygal only occurs along the dorsal half of the latter, as its ventral part forms a continuous lip with peripherals VII to X. The midline keel crosses the full length of the suprapygal, which is also visible in the juveniles.

Pygal. The pygal is a single element that forms the posterior margin of the carapace. It is slightly longer than wide and contacts the suprapygal anteriorly and peripheral X laterally. The pygal forms a relatively thick lip on its visceral side that is continuous with peripherals VII to X, as in other carettochelyids (Leidy 1873; Dollo 1886; Nessov 1977; Joyce 2014; Rollot et al. 2025). The keel that extends anteroposteriorly along the midline of the carapace ends on the pygal, very close to the posterior margin of the carapace, in both adult and juvenile individuals.

Epiplastron. The epiplastron of Allaeochelys crassesculpta is a relatively large element, that, along with its counterpart, forms most of the anterior plastral lobe and the anterior and anterolateral margin of the plastron. The anterior plastral lobe is similar to that of Allaeochelys parayrei (Broin 1977), broader than that of Kizylkumemys schultzi (Nessov 1977), and slightly narrower than that of Carettochelys insculpta (Waite 1905; Walther 1922; Joyce 2014). The epiplastron is notably broad as in Allaeochelys parayrei (Broin 1977) and similar to that of Carettochelys insculpta (Waite 1905; Walther 1922; Joyce 2014), but broader than in Kizylkumemys schultzi (Nessov 1977) and Anosteira manchuriana (Zangerl 1947). The epiplastron of Allaeochelys crassesculpta contacts its counterpart anteromedially, the entoplastron posteromedially, and the hyoplastron posteriorly. The midline suture of the epiplastron that is shared with its counterpart is about as long as or slightly longer than the length of the entoplastron, as in Carettochelys insculpta (Waite 1905; Walther 1922; Joyce 2014). This differs from Kizylkumemys schultzi (Nessov 1977) and Anosteira manchuriana (Zangerl 1947), in which the midline suture between the epiplastra is clearly shorter than the length of the entoplastron. As in other carettochelyids, the epiplastra form a unit with the entoplastron that is ligamentously connected to the remainder of the plastron along a sinuous contact that allows for some kinesis. For this reason, these two bones are usually found disarticulated.

Entoplastron. The entoplastron of Allaeochelys crassesculpta forms the central part of the anterior plastral lobe. The shape of the entoplastron is that of a nearly perfect equilateral triangle, with straight anterolateral sides and a subtly convex posterior side. The shape and dimensions of the entoplastron are very similar to that of Allaeochelys parayrei (Broin 1977) and Carettochelys insculpta (Waite 1905; Walther 1922; Joyce 2014), but clearly different from that of Kizylkumemys schultzi (Nessov 1977) and Anosteira manchuriana (Zangerl 1947), in which the entoplastron is an elongate element with clearly longer anterolateral sides. The entoplastron of Allaeochelys crassesculpta contacts the epiplastron anteriorly and laterally and the hyoplastron posteriorly.

Hyoplastron. The hyoplastron is relatively short anteroposteriorly and forms the anterior aspect of the plastral bridge. The medial third of the anterior margin of the hyoplastron forms a dorsoventrally expanded surface that serves as an articular facet for kinetic contact with the epiplastron anteriorly and entoplastron anteromedially. This surface is slightly convex and laterally ends in a well-defined notch. The morphology of the hyoplastron is similar to that of Allaeochelys lingnanica (Young and Chow 1962) and Allaeochelys parayrei (Broin 1977), but it is more robust and more anteroposteriorly expanded than that of Kizylkumemys schultzi (Nessov 1977), Anosteira manchuriana (Zangerl 1947), Anosteira maomingensis (Chow and Liu 1955), Anosteira mongoliensis (Gilmore 1931), Anosteira ornata (Leidy 1873) and Anosteira pulchra (Clark 1932), but less so than in Carettochelys insculpta (Waite 1905; Walther 1922; Joyce 2014). Laterally, the hyoplastron of Allaeochelys crassesculpta forms a series of short processes that articulate with the carapace, inserting into the sockets formed by peripherals IV and V. The hyoplastron also contacts its counterpart medially and the hypoplastron posteriorly.

Hypoplastron. The hypoplastron is the largest element of the plastron and forms the posterior part of the plastral bridge and the anterior half of the posterior plastral lobe. Laterally, the hypoplastron forms short processes that articulate with the carapace and insert into sockets formed by peripherals V, VI, and VII. The hypoplastron otherwise contacts the hyoplastron anteriorly, the xiphiplastron posteriorly, and its counterpart medially. The shape of the suture between the hypo- and xiphiplastron varies in Allaeochelys crassesculpta and can be either straight or wavy. This difference was already noted by Joyce et al. (2012) and interpreted as sexual dimorphism, with males having the akinetic, wavy suture and females the straight suture, which likely allowed for limited kinesis during oviposition (Fig. 26). In addition, females are about 20% larger than males and have proportionally shorter tails.

Xiphiplastron. The xiphiplastron forms the most posterior extent of the plastron and the posterior half of the posterior plastral lobe. The posterior plastral lobe is relatively large and notable differences can be observed with other carettochelyids, based on the morphology of the xiphiplastron. In Allaeochelys crassesculpta, the xiphiplastron is moderately large and has straight, anteroposteriorly oriented margins for at least half of its length, similar to that of Allaeochelys parayrei (Broin 1977) and Allaeochelys liliae (Carbot-Chanona et al. 2020). This morphology contrasts with that of Kizylkumemys schultzi (Nessov 1977), in which the posterior plastral lobe and xiphiplastron are extremely narrow and that of Anosteira manchuriana (Zangerl 1947), Anosteira maomingensis (Chow and Liu 1955), and Anosteira mongoliensis (Gilmore 1931), where the posterior plastral lobe and xiphiplastron are slightly narrower and the lateral margin of the xiphiplastron is inclined posteromedially for most of its length. The morphology observed in Allaeochelys crassesculpta also differs from that of Carettochelys insculpta, which has a much broader posterior plastral lobe and xiphiplastron than any other carettochelyid (Waite 1905; Walther 1922; Joyce 2014). The xiphiplastron contacts the hypoplastron anteriorly, along a suture that can be either straight (females) or wavy (males) as described above (see Hypoplastron and Joyce et al. (2012)).

Appendicular skeleton

General comments. The left fore- and hindlimb of SMF Me611 were segmented and reconstructed and, therefore, serve as a primary base for the description of the limbs of Allaeochelys crassesculpta, unless stated otherwise by explicit reference to other specimens. The left forelimb is complete and although some damage and displacement may affect the bones individually, the general anatomy of most elements is apparent (Fig. 30). The left hindlimb, however, is incomplete, as the femur is not preserved and the tibia and fibula are damaged. Preliminary descriptions of the femur, tibia and fibula are nevertheless provided to the extent possible, with the addition of supplementary insights, based on other specimens included in our study that preserve these bones.

Humerus. The left humerus of SMF Me611 is slightly damaged and lacks most of its medial process and a substantial part of its distal end, with the capitellum and radial epicondyle mostly missing (Fig. 31A–F). Overall, the humerus of Allaeochelys crassesculpta is similar to that of Carettochelys insculpta (Walther 1922; Hermanson et al. 2024). The proximal end of the humerus of Allaeochelys crassesculpta consists of a large humeral head, which seems proportionally larger than that of Carettochelys insculpta (Hermanson et al. 2024), and a greatly reduced lateral process. Although the medial process is broken near its base in SMF Me611 (Fig. 31B, D), it is perfectly preserved in HLMD Be159 (Fig. 13C, D) and SMF Me3566 (Fig. 25A, B), showing that it is well-developed in Allaeochelys crassesculpta, similar to that of Carettochelys insculpta (Hermanson et al. 2024). In Allaeochelys crassesculpta, on the radial side of the humeral head, the humerus forms a reduced shoulder (Fig. 31B). The shoulder seems slightly better developed in Allaeochelys crassesculpta than in adult Carettochelys insculpta (Hermanson et al. 2024), as the area between the shoulder and humeral head is slightly more concave and the shoulder forms a low ridge on the capitular side of the humerus, which extends proximally for a short distance towards the humeral head, albeit not joining it. As in Carettochelys insculpta (Hermanson et al. 2024), the proximal limit of the lateral process is positioned much more distally than the proximal border of the humeral head (Fig. 31C). In Allaeochelys crassesculpta, the medial and lateral processes are separated from one another by a shallow intertubercular fossa (Fig. 31D). Just next to the lateral process, immediately distal and radial to the intertubercular fossa, the humerus forms a small, narrow, but well-defined and oval-shaped pit, which is also present in Carettochelys insculpta, where it has been interpreted as a possible secondary intertubercular fossa (Hermanson et al. 2024). A subcapitular fossa is present in Allaeochelys crassesculpta and visible on the capitular side of the humerus, within the proximal third of the element (Fig. 31B). The distal expansion of the humerus starts more proximally along the length of the element and distal ulnar-radial width is not greatly expanded relative to shaft, as in Carettochelys insculpta (Hermanson et al. 2024). The humerus of Allaeochelys crassesculpta exhibits minimal shaft curvature and is essentially straight (Fig. 31A, C), again as in Carettochelys insculpta (Hermanson et al. 2024). In Allaeochelys crassesculpta, the distal end of the humerus of SMF Me611 is damaged (Fig. 31B–D, F). The ulnar epicondyle and trochlea are almost completely preserved, but the capitellum is heavily damaged and the radial epicondyle is almost completely missing (Fig. 31B, D, F). It is nevertheless clear that the trochlea is located more proximally than the capitellum (Fig. 31D), as in Carettochelys insculpta and unlike most turtles (Walther 1922; Hermanson et al. 2024), and that an intercondylar groove is present that separates the trochlea from the capitellum (Fig. 31D, F). Despite damage in the area, it seems that a shallow fossa or depression is present between the distal end of the ulnar epicondyle and ulnar side of the trochlea, which contrasts with the morphology of the humerus in Carettochelys insculpta, where this area is straight (Hermanson et al. 2024). The ectepicondylar foramen is not preserved in SMF Me611 of Allaeochelys crassesculpta, but undoubtedly visible in HLMD Me15012 (Fig. 19C, D) and SMF Me1794 (Fig. 22C, D), in which it appears to be in a similar position as that of Carettochelys insculpta (Hermanson et al. 2024), although it is perhaps slightly smaller. The humerus of Allaeochelys crassesculpta also appears to form a very shallow supraepicondylar fossa, which is visible on the intertubercular fossa proximal to the trochlea.

Radius. The proximal end of the radius is subcircular with a slightly oval outline (Fig. 31M). It is only slightly larger mediolaterally than it is at midshaft, but is expanded to a greater degree dorsoventrally. The proximal articulation surface is oval and concave, serving for articulation with the capitellum of the humerus. Ventrolateral and just distal to the proximal end, the radius forms a tuberosity that expands distally, nearly reaching the midshaft. (Fig. 31K, L), which might correspond to the attachment site of the superior radio-ulnar ligament (Haines 1946; Gaffney 1990). In Carettochelys insculpta (specimen MAGNT R35037), the radius also forms a low ridge along its medial side, just distal to the proximal end, which extends for about one third of the length of the radius. Such a ridge is not visible in the radius of Allaeochelys crassesculpta specimen SMF Me611, but this could also be a preservation artefact. Likewise, dorsolaterally at about mid-shaft, the radius of Carettochelys insculpta forms a very low and short ridge that is not visible in specimen SMF Me611 of Allaeochelys crassesculpta. The ventral aspect of the distal half of the radius is developed as a relatively low, but broad ridge (Fig. 31K, L), on which two low bumps can be identified. The most proximal bump is located at about the level of midshaft, while the most distal one is visible just proximal to the distal end of the radius. In Carettochelys insculpta (specimen MAGNT R35037), no bumps are visible and the ventral aspect of the radius is more ventrally expanded and forms a mediolaterally flattened shelf that extends distally to the sigmoid process. The distal part of the radius of Allaeochelys crassesculpta broadens mediolaterally, but remains only weakly expanded dorsoventrally (Fig. 31K, L, N). The medial edge is nearly straight, whereas the lateral edge forms a process with a lateral surface that is developed as a triangular tuberosity, which seems to be slightly larger in Carettochelys insculpta (specimen MAGNT R35037) than in Allaeochelys crassesculpta. Distomedially, the styloid process of the radius of Allaeochelys crassesculpta is well-developed and expanded distally (Fig. 31K). The distal articulation surface is elongate and oval, forming a shallow depression, but the articulation facets are not completely preserved (Fig. 31N). Light damage and erosion of the bony surface prevent the facets from being clearly observed or distinguished from one another. Nevertheless, we infer that two facets were present by reference to other turtles (Gaffney (1990); Sterli et al. (2007); Joyce et al. (2014); Szczygielski and Piechowski (2023)), with the most proximolateral facet corresponding to the intermedium and the most distomedial one associated with the first distal carpal.

Ulna. The proximal end of the ulna is about twice as large as the midshaft and about as broad as tall (Fig. 31G, H). The olecranon is likely located on the proximodorsal side of the proximal end, by reference to Carettochelys insculpta (specimen MAGNT R35037), but the area is damaged in Allaeochelys crassesculpta specimen SMF Me611 (Fig. 31I). The proximal articulation surface (i.e., the sigmoid notch) is subdivided into two parts: one nearly flat ventromedial facet, which is dorsoventrally elongate and forms slightly more than one third of the entire articulation surface, and one ventrolateral facet, which forms nearly two thirds of the sigmoid notch (Fig. 31I). In Carettochelys insculpta (MAGNT R35037), similar facets are present and the ventrolateral facet is gently concave. Despite damage to the ulna of Allaeochelys crassesculpta specimen SMF Me611, the preserved portion shows that the morphology of the proximal end was likely similar to that of Carettochelys insculpta. In Allaeochelys crassesculpta, the ulnar midshaft is not flattened and is approximately as broad as tall. The shape of the midshaft in cross-section is diamond-shaped, with four well-defined sides (Fig. 31G, H). The edges between sides are quite abrupt, which slightly differs from Carettochelys insculpta (MAGNT R35037), in which the midshaft is slightly more circular and the limits between faces are smoother. In Allaeochelys crassesculpta, the distal end of the ulna is mediolaterally expanded, about twice as broad as the midshaft, flattened, and has a distal margin that is sinusoidal in shape in dorsal view (Fig. 31G, H, J). In distal view, the distal articulation surface is about twice as broad as tall and has a semilunar, curved shape (Fig. 31J). The distal articulation surface was originally preserved as slightly compressed against the intermedium, ulnare, and radius, resulting in minor surface damage that obscures the articular facets and prevents their accurate description. However, the general morphology of the distal end of the ulna greatly resembles that of Carettochelys insculpta (MAGNT R35037) and we, therefore, base most anatomical insights on the latter. In Carettochelys insculpta, the distal articulation surface is subdivided into two facets. The ventromedial facet is the smallest, forming about one third of the entire articulation surface; it is circular, gently concave, and articulates with the intermedium (Fig. 31J). The lateral facet is the largest, forming about two thirds of the entire articular surface; it is triangular, approximately twice as broad as tall, with the dorsal and ventral margins being the longest, and it articulates with the ulnare (Fig. 31J). The surface of the lateral facet is quite complex. The most dorsomedial portion forms a shallow depression, while the most lateral and ventrolateral part is nearly flat and the area between the two is slightly domed. Dorsomedially, between the ventromedial and lateral facets, the ulna forms a short, blunt process that separates the two facets along their dorsal half and likely inserted between the intermedium and ulnare. It is unclear if such a process is present in Allaeochelys crassesculpta as the area is damaged in SMF Me611. In Carettochelys insculpta, the lateral side of the distal process forms a moderately deep notch, which is also visible in Allaeochelys crassesculpta, albeit slightly less deep than in the latter (Fig. 31G, H). In Carettochelys insculpta, the area distolateral to the notch is slightly expanded and, on the lateral side, forms another facet between the lateral notch and distal articulation surface. In lateral view, this facet has a straight distoventral margin, but is otherwise rounded and as broad as tall, articulating with the pisiform. In Allaeochelys crassesculpta, the morphology of the ulna suggests that a similar facet was also likely present, as the distolateral aspect of the ulna is slightly expanded just distal to the notch (Fig. 31G, H). In Carettochelys insculpta, the ventromedial aspect of the ulna, immediately proximoventral to the distal articular surface, forms an elongate, rough surface that is about three times longer than tall, likely corresponding to a tuberosity.

Carpus and manus. The carpus of Allaeochelys crassesculpta has a reduced number of elements, as in Carettochelys insculpta (Harrassowitz 1922; Walther 1922), which are the intermedium, ulnare, pisiform, and distal carpals I–V (Fig. 30A–C). Proximodistal compression of the forelimb has produced minor damage and localized surface erosion on several elements, introducing some uncertainty in the reconstruction and orientation of the distal carpals.

The intermedium is a small bone that is nearly square, except for its distomedial edge, which is developed as a short but stout process (Fig. 30B, C). The distal surface of the process forms a round and very gently concave articular surface, which likely joins with distal carpal I. The lateral half of the distal side is straight and flat and contacts distal carpal II. A point contact with the radius might have been present proximomedially, as in Carettochelys insculpta (Walther 1922), but remains hypothetical in Allaeochelys crassesculpta because of the deformation of the forelimb. The proximal side of the intermedium forms a gently concave articular surface for the ulna. The lateral side of the intermedium is subdivided into two parts: one flat distal surface that is oriented proximodistally and one flat proximal surface that is slightly inclined proximomedially. Both surfaces contact the ulnare.

The ulnare is an octagonal bone that is slightly larger than the intermedium (Fig. 30B, C). The distal half of the element is flat, albeit slightly dorsoventrally expanded along its distal end, whereas the proximal half is clearly thickened. The proximal half of the ulnare is cone-shaped in lateral view and its proximal surface forms a large, circular, and very gently concave surface that serves for the articulation with the ulna. The lateral side of the ulnare is subdivided into two surfaces: the proximal surface, which constitutes about half of the lateral side, is gently concave and oriented proximodistally, whereas the distal surface is gently convex and slightly oriented distalomedially. Both surfaces contact the medial side of the intermedium. Distally, the ulnare is also subdivided into two surfaces that are equally sized, gently concave and positioned 130° from one another. The medial surface contacts distal carpal III, while the lateral surface contacts distal carpal IV. A contact of the ulnare with distal carpal II was reported in Carettochelys insculpta (Walther 1922), but is not evident in Allaeochelys crassesculpta. The distolateral edge of the ulnare forms a small, flat surface that is almost circular. According to our reconstruction of the forelimb, this surface might represent an articular surface for distal carpal V, but there is no such contact in Carettochelys insculpta (Walther 1922). Alternatively, this surface could form a short contact laterally with the pisiform. The lateral side of the ulnare of Allaeochelys crassesculpta is much less expanded dorsoventrally than the medial side. It is developed as a curved, smooth and very gently concave surface, but does not resemble an articular surface. Point contacts with the pisiform were perhaps present proximally and/or distally, but an expanded contact — such as the broad articulation between the ulnare and distal carpals III–IV — was likely absent, as in Carettochelys insculpta (Walther 1922). This is also supported by the morphology of the pisiform, as its medial surface, which faces the ulnare, is concave.

The pisiform of Allaeochelys crassesculpta is the largest element of the carpus (Fig. 30B, C). The lateral half of the element is heavily damaged, but nevertheless shows that the pisiform is laterally greatly expanded, forming a flat and thin process, as in Carettochelys insculpta (Walther 1922). In Allaeochelys crassesculpta, only the most medial aspect of the pisiform is moderately thickened. The medial side of the pisiform forms a curved, smooth and concave surface that faces the ulnare. The distomedial edge of the pisiform forms a broad and oval-shaped articulation surface, which is about two times broader than tall and contacts distal carpal V. The most distomedial aspect of this surface might have contacted the ulnare, but such a contact remains hypothetical (see ulnare section above). The proximomedial edge of the pisiform forms a circular and very gently concave surface that serves for articulation with the ulna. This articular surface is slightly inclined distoventrolaterally. The most medial aspect of the articulation surface perhaps contacted the most proximolateral part of the ulnare, but remains hypothetical (see ulnare section above).

Five distal carpals are preserved in the carpus of Allaeochelys crassesculpta (Fig. 30B, C). The taphonomic compression of the forelimb was relatively minimal, but all distal carpals were nevertheless displaced and damaged, which complicated their identification and reconstruction. Although we provide detailed descriptions in the following section, our insights into the identification and anatomy of the distal carpals should be taken with caution because of this displacement and damage. Distal carpal I is a relatively large element, which is slightly taller than wide. Its general shape is quite irregular and each side forms surfaces that are either convex or concave. As preserved and reconstructed, distal carpal I contacts the radius proximomedially, the fused metacarpal and first phalanx of digit I distally, distal carpal II laterally and, perhaps, the intermedium proximolaterally. Distal carpal II is a relatively large and columnar element, of which a substantial portion of its ventral part is damaged and missing. The dorsal half of the element is more elongate proximodistally, whereas the ventral half is more elongate mediolaterally. The dorsal surface of distal carpal II is eroded. The proximal and lateral sides form concave surfaces, which serve for articulation with the intermedium and distal carpal III, respectively. The medial and distal sides seem to have more complex morphologies, but are also affected by damage, which hinders detailed descriptions. The distal side is perhaps subdivided into two facets, with a centrally located fossa separating the two, while the medial side might be subdivided into two nearly equally-sized facets, facing mediodorsally and medioventrally. Distal carpal III is a small element, of which we estimate about half is missing. As reconstructed and oriented, we are able to identify four sides of distal carpal III: dorsal, proximal, lateral, and distal. The dorsal side is concave and laterally and medially bordered by low ridges, which gives this ‘fossa’ a gutter-like morphology with a proximodistal orientation. This side could also, perhaps, correspond to an articular surface for contact with metacarpal III, in which case the element would be falsely oriented in our reconstruction by 90° towards the ulnare. The distal side is incomplete, but appears to have been largely flat, except for its lateralmost portion, which curves to merge with the lateral surface and forms the articulation with metacarpal III. The lateral side is incomplete too and forms a convex surface that articulates with distal carpal IV. The proximal side likely preserves only its dorsal half, which is flat and contacts the ulnare. Distal carpal IV is a columnar element, of which all sides appear to be damaged. As currently oriented, only parts of the proximal, medial and ventral surfaces seem to be preserved. The proximal surface is flat and contacts the ulnare, the medial side is flat and contacts distal carpal III, and the ventral surface is convex and smooth and faces ventrally. Distal carpal V is incomplete and was rotated laterally by about 45° to fit in our reconstruction of the forelimb. The proximal side forms a concave surface that seems to fit for articulation with metacarpal V. The most distal aspect of the medial side forms a flat surface that contacts distal carpal IV. The lateral side appears to be mostly preserved, forming a very gently concave surface that serves for articulation with the pisiform.

The manus of Allaeochelys crassesculpta has a digital formula of 2-3-3-3-3 (Fig. 30B, C), as Carettochelys insculpta (Walther 1922; Delfino et al. 2010). The first digit of Allaeochelys crassesculpta is the shortest, followed by digit II and digit V. Digits III and IV are the longest, about twice the length of digits I and II. The length of the digits relative to one another is very similar to that of Carettochelys insculpta (Walther 1922). Phalanges 2 and 3 of digits III and IV of Allaeochelys crassesculpta are notably elongate. Metacarpal I is fused to the first phalanx of digit I, again as in Carettochelys insculpta (Walther 1922). The metacarpal portion of the fused element is slightly longer than the phalangeal one. Metacarpal I is the shortest, but thickest among the metacarpals, forming a robust rod that proximally articulates with distal carpal I. Metacarpal II of Allaeochelys crassesculpta is slightly longer and more slender than metacarpal I and its proximal end is more expanded than the distal one. Metacarpal II articulates with distal carpal II proximally and the fused phalanges 1 and 2 distally. Harrassowitz (1922) noted that metacarpal II was fused with phalanges 1 and 2 of the second digit in HLMD Me1461 (his ‘Exemplar 6’; see Harrassowitz (1922), pl. 4.3), but we are not able to confirm the identification of the bones made by Harrassowitz (1922) nor establish if fusion exists between the bones of interest. In SMF Me611, it is unambiguous that only the first two phalanges are fused and that metacarpal II remains an isolated element. The same observation can be made in the two individuals preserved in SMF Me3566 (Fig. 25). However, variation in this feature seems to occur in Carettochelys insculpta and might, therefore, also be present in Allaeochelys crassesculpta. Indeed, Walther (1922) described metacarpal II of Carettochelys insculpta as being fused to the phalanges 1 and 2, although remnants of the articulation between metacarpal II and the first phalanx remain visible. Radiographs provided by Delfino et al. (2010) likely show a partial fusion between phalanges 1 and 2 of digit II and also possibly the beginning of fusion with metacarpal II, as the articulation of the latter with the first phalanx is faint in the image, which contrasts with other articulations in the manus. Finally, personal observations of KU H158563 (juvenile) show a complete fusion between metacarpal II and phalanges 1 and 2. It is, therefore, possible that similar variation occurs in Allaeochelys crassesculpta. Metacarpal III of Allaeochelys crassesculpta is longer and more slender than metacarpal II and appears, along with metacarpal IV, to be the longest. Metacarpal III articulates with distal carpal III proximally and the first phalanx of digit III distally. Metacarpal IV has a very similar morphology to that of metacarpal III and is as long as the latter. The proximal and distal ends of metacarpal IV are perhaps slightly less expanded than those of metacarpal III, but that could be the result of damage. They articulate with distal carpal IV and the first phalanx of digit IV, respectively. Metacarpal V is shorter than metacarpal III and IV, slightly more robust than the latter, and about the same length as metacarpal II. Metacarpal V is distinctly curved, with the concave surface facing medially. On its lateral side at mid-length, metacarpal V forms a low, but distinct swelling that expands for nearly one third of the proximodistal length of the element.

Digit I consists of two phalanges, of which the first one is tightly fused to metacarpal I. Phalanx 1 was the smallest bone of the first digit and is slightly shorter than phalanx 2, which is claw-like. Digit II consists of three phalanges, of which the first and second are fused. Phalanges 1 and 2 were likely equally sized, but the former was more robust. Phalanx 3 is claw-like and about as long as phalanges 2 and 3. Digit III is elongate and much longer than digits I and II. The three phalanges are clearly elongate. The longest phalanx is the second, which is slightly longer than the entire digit II. Phalanges 2 and 3 are similar in length and each is slightly longer than the fused phalanges 1 and 2 of digit II. Digit IV is elongate and as long as digit III, although the proportions of the phalanges differ from those of digit III. The first phalanx of digit IV is slightly shorter than that of digit III, whereas the second phalanx is clearly longer than that of digit III. Phalanx 3 is the shortest of the three, albeit only slightly shorter than phalanx 1 and clearly shorter than phalanx 3 of digit III. Digit V is the third longest digit and slightly longer than digit II, but clearly shorter than digits III and IV. The first phalanx of digit V is as long as metacarpal V and only slightly shorter than phalanx 2. Phalanx 2 is the longest, whereas phalanx 3 is greatly reduced and the smallest phalanx of the manus.

Femur. The left femur is not preserved in SMF Me611 (Fig. 32). Other specimens, which were not scanned, preserve femora and allow us to provide some preliminary albeit limited anatomical insights on its anatomy. A femur is preserved in HLMD Be159 (left side; Fig. 13C, D), exposing its ventral side, in SMF Me2449 (right side of male; Fig. 23C, D), where it shows its anterior side, in SMF Me3018 (Fig. 24C, D), where it shows its dorsal side on the right and its ventral side on the left, and in SMF Me3566 (Fig. 25), in which the femur is preserved on both sides in both specimens, revealing either its anterior or ventral side. The proximal portion of the femur forms the trochanter major, the trochanter minor, and the femoral head. The femoral head is not clearly visible in any specimen. It can be recognized as such on the right side of SMF Me2449, but is greatly eroded (Fig. 23C, D). The trochanter major is visible in HLMD Be159 (Fig. 13C, D), in which its proximal aspect seems damaged, and in SMF Me3566 (Fig. 25), in which it seems complete. The trochanter major is well-developed and proximally extends far beyond the proximal level of the femoral head, similar to that of Carettochelys insculpta (MAGNT R35037), but different from other turtles (Zug 1971; Walker 1973; Gaffney 1990; Sterli et al. 2007; Marzola 2019; Szczygielski and Piechowski 2023). The trochanter minor is visible in SMF Me2449 (Fig. 23C, D), in which its proximal extent is greatly reduced to the trochanter major and just reaches about mid-height of the femoral head, as in Carettochelys insculpta (MAGNT R35037). The anteroproximal edge of the trochanter minor seems to be smooth and curved, whereas that of Carettochelys insculpta (MAGNT R35037) forms a somewhat ovoid and flat platform. In ventral view, the intertrochanteric fossa is broad and C-shaped. The shaft is likely nearly straight for most of its length, as is the case in Carettochelys insculpta (MAGNT R35037), which contrasts with that of other turtles, in which it bends more strongly ventrodistally (Zug 1971; Walker 1973; Gaffney 1990; Sterli et al. 2007; Marzola 2019; Szczygielski and Piechowski 2023). The distal end of the femur is anteroposteriorly expanded. On its anterior side, the distal end forms a low and proximodistally elongate protuberance, which is visible in SMF Me2449 (Fig. 23C, D) and in Carettochelys insculpta specimen MAGNT R35037 where it likely corresponds to an anterior epicondyle. The fibular and tibial condyle can hardly be differentiated from one another because of damage, but the popliteal fossa appears to be shallow.

Tibia. Most of the left tibia is preserved in SMF Me611 but damaged (Fig. 32A–C). The proximal articulation surface is eroded and damage locally affects the more distal aspect of the proximal end on all sides. The tibia is also broken at about two thirds of its proximodistal length, where some of the bone is missing. The proximal end of the tibia is greatly expanded, and its mediolateral width is at least twice that of the shaft. The proximal articular surface is mostly eroded, which prevents assessment of its general shape, but some margins remain preserved nearly intact. The dorsal and lateral margins appear to be straight and are oriented nearly perpendicular to one another. The most medial aspect of the dorsal margin is slightly convex. The dorsomedial part of the articular surface is slightly projected proximally and forms the highest point of the tibia. The ventromedial portion of the articulation is inclined medially. Dorsally, the tibia forms a low cnemial crest that extends proximodistally from slightly distal to the proximal articular surface to beyond the midshaft. Damage to the distal part of the tibia prevents assessing the exact distal end of the cnemial crest and we are not able to identify it dorsally along the distal end of the tibia. On the ventral surface of the tibia, along its proximal end, the tibia seems to form a low and blunt, but relatively broad ridge that separates the ventral surface into two gently concave areas. The distal articulation surface is well-preserved. The central part of the surface has the shape of a shallow gutter that is oriented dorsoventrally. The lateral portion of the surface is slightly raised relative to the gutter and is also gently convex. The medial edge of the articular surface is projected distally, forming a short but relatively broad process and is greatly convex in medial view. Ventrally, along the distal aspect of the distal end, the tibia forms a ridge that extends proximodistally. This ridge subdivides the ventral surface of the tibia into two very gently concave areas and distally joins the edge of the articular surface, separating the latter into two distinct edges. The ventromedial edge of the articular surface is rounded and gently concave and slightly upturned, forming a low lip that is particularly visible in ventral view. The ventrolateral edge is slightly rounded and convex.

Fibula. Only the distal part of the fibula is preserved in SMF Me611, though it is slightly damaged (Fig. 32A–C). The distal part is triangular in shape, with nearly equally-expanded sides. The medial edge of the fibula is notably flattened. The dorsomedial and ventromedial sides of the fibula are gently concave, whereas the lateral side is flat. The distal articular surface is convex, but it is not possible to determine how many facets were present. The lateral side is almost completely straight except along its most dorsal and ventral aspects, which are gently convex. The dorsomedial edge is straight and the ventromedial edge is concave. The three sides are approximately equal in length, but a small piece of the articular surface is missing ventrolaterally and the lateral edge was perhaps the longest.

Tarsus and pes. The tarsus is not complete in SMF Me611, but likely consists of at least five elements: the astragalocalcaneum, which is not preserved, and distal tarsals I–IV, which are preserved (Fig. 32A–C). The astragalocalcaneum seems to be preserved in HLMD Me15956 (Fig. 20C, D) and SMF Me3566 (left side of female; Fig. 25A, B), but is badly crushed, precluding anatomical description. As was the case for the carpus, most elements were slightly displaced and some of them are, therefore, damaged and eroded. Thus, some uncertainty also remains here regarding the reconstruction and orientation of the distal tarsals. Harrassowitz (1922) identified only two tarsals in Allaeochelys crassesculpta, but our reconstruction of the hindlimb of SMF Me-611 shows that four distal tarsals are present, as in Carettochelys insculpta (TMM M18053). In specimen SMF Me611 of Allaeochelys crassesculpta, distal tarsal I is damaged, but appears to be a relatively broad element, clearly wider than long with a concave anterior surface that articulates with metatarsal I. The morphology of distal tarsal II is uncertain because of damage, which is quite substantial along its distal, medial, and ventral sides as currently oriented. The proximal side of distal tarsal II is deeply concave. The medial side is clearly convex, forming a bump that is punctured by a fossa distodorsally. Distal tarsal III is the smallest bone of the tarsus. Distal tarsal III is dorsoventrally quite elongate, moderately broad, but proximodistally reduced. It is medially, medioventrally and ventrally damaged. The dorsal side of distal tarsal III is convex, whereas the preserved portions of its proximal and distal sides are flat. Distal tarsal IV is the largest of the four distal tarsals, but its morphology is also uncertain as several parts of the element are eroded. Distal tarsal IV appears to be as tall as broad as long, with convex medial and lateral sides. The dorsal part of its distal side forms a triangular fossa that, perhaps, serves as a contact or articulation facet with distal tarsal III.

The pes of Allaeochelys crassesculpta consists of five metatarsals and five digits (Fig. 32B, C). The pes of SMF Me611 only preserves phalanges 1 and 2 for digits IV and V, but three phalanges are present in those digits in specimens SMF Me-2449 (female) and SMF Me-3566 (female). Thus, the digital formula is 2-3-3-3-3, as in the manus and as in the pes of Carettochelys insculpta (Walther 1922). However, it seems that all digits of the pes of Allaeochelys crassesculpta are slightly shorter than those of Carettochelys insculpta (Walther 1922). The first pes digit of Allaeochelys crassesculpta is perhaps the smallest, but the incompleteness of digit V prevents certainty regarding which of the two is smallest. Digit II is the third smallest digit and digits III and IV are the longest, but determining which is the longest is not possible as the third phalanx is missing in digit IV. Metatarsal I is fused to the first phalanx, which also occurs in Carettochelys insculpta (Walther 1922). In Allaeochelys crassesculpta, the metatarsal portion of the fused element is slightly longer than the phalangeal one and appears to be slightly thicker than metatarsals II–IV and it contacts distal tarsal I. Metatarsal II is as long as metatarsal I, albeit slightly more slender, and it articulates with distal tarsal II proximally and the first phalanx distally. Metatarsal III is slightly longer than metatarsal II and is the longest along with metatarsal IV. Metatarsal III articulates with distal carpal III proximally and phalanx 1 distally. Metatarsal IV is as long as metatarsal III and slightly curved, with its concave surface facing laterally, and it contacts distal carpal III proximomedially and metatarsal V proximolaterally. Metatarsal V is slightly damaged and its most proximal aspect is missing. Metatarsal V has a peculiar morphology as it is not elongate as the other metatarsals, but rather rectangular and blocky, bearing a distinct and rounded process distodorsally. Metatarsal V is dorsoventrally curved with a dorsal convex surface and a ventral concave surface. The lateral side of metatarsal V is dorsoventrally expanded and nearly as tall as wide, forming a gently concave surface that contacts distal tarsal IV. The medial side of metatarsal V is relatively thick, albeit clearly less expanded than the lateral surface, and forms a slightly convex surface that represents the distomedial limit of the pes. The distal side of metatarsal V is gently concave and topped by a large process that protrudes distodorsally. The distal end of this process forms a rounded surface that articulates with phalanx 1 of digit V distomedially and metatarsal IV distolaterally. Digit I consists of two phalanges and the first is fused to metatarsal I. Phalanx 1 is likely the smallest bone of the first digit as it appears slightly shorter than phalanx 2, which is claw-like. Digit II consists of three phalanges which are nearly equally long, with phalanx 2 being slightly shorter than the other two and phalanx 1 likely slightly longer than phalanx 3. Phalanx 3 is claw-like. Digit III is elongate and much longer than digits I and II. The three phalanges are clearly elongate and longer than the respective phalanges on digits I and II. The longest phalanx is the second. Although the distal end of phalanx 3 is missing, most of the element seems to be preserved and it was likely slightly shorter than phalanx 1. Digit IV is elongate, but phalanx 3 is not preserved. The first phalanx appears identical in shape and size to that of digit III, whereas the second phalanx is slightly more slender than that of digit III, which is notably visible at about mid-length and perhaps also slightly shorter. Digit V is also incompletely preserved, as phalanx 3 is completely missing and the distal end of phalanx 2 is damaged. The first phalanx of digit V is as long as that of digit IV, but is slightly thinner. The preserved portion of phalanx 2 appears to be slightly more gracile than phalanx 2 of digit IV and it was, therefore, at least slightly shorter than the latter.