The record of paleopathologies in Mesozoic marine reptiles is relatively scarce in the literature compared to that on other lineages such as Dinosauria. In the case of Sauropterygia, these pathologies generally correspond to avascular necrosis, vertebral lesions or anomalies, or bite marks, mostly reported in Jurassic and Cretaceous plesiosaurs. The documented pathologies in Triassic sauropterygians are very limited. Among the Triassic eosauropterygians, Simosauridae is a relatively poorly known lineage, being restricted to the western margin of the Tethys Sea, recorded from Europe and the Middle East. To date, no pathologies have been reported for Simosauridae. In this study, we describe several abnormal vertebral centra corresponding to the most posterior dorsal region of the holotype of the Spanish simosaurid Paludidraco multidentatus. These centra exhibit paired and symmetrical bulks located on their articular facets. The bulks cannot be differentiated externally or internally from their surrounding healthy bone tissue, and there are no signs of reactive new bone formation, fractures, or remodeling. These pathologic structures have never been reported for any other Sauropterygia or marine reptiles, having only been rarely documented in some dinosaurs. A differential diagnosis rules out several possible pathological origins for these structures, suggesting that long-term biomechanical stress or a congenital disorder may be the potential causes.

Eosauropterygia, Europe, late Triassic, paleopathology, vertebral lesion

Literature on paleopathologies in Mesozoic marine reptiles is relatively scarce (Surmik et al. 2022), compared to that on other lineages, such as Dinosauria (e.g., Rothschild et al. 2023 and references therein). The record includes avascular necrosis (e.g., Rothschild and Martin 1987; Rothschild and Storrs 2003; Farke 2007; Rothschild 2009; Rothschild et al. 2012a, 2012b; Rothschild and Naples 2013; Mitidieri 2021), bite marks (e.g., Müller et al. 2005; Rothschild et al. 2005; Pardo-Pérez et al. 2018; Mitidieri 2021), infections (e.g., Rothschild 2009; Rothschild et al. 2012b; Pardo-Pérez et al. 2019, 2020; Mitidieri 2021), injuries or fractures (e.g., Rothschild 2009; Rothschild et al. 2012b; Pardo-Pérez et al. 2018, 2019, 2020; Mitidieri 2021), osteoarthritis (e.g., Rothschild 2009; Fraser and Furrer 2013), spondylosis (e.g., Rothschild 2009; Sassoon 2019), tumors (e.g., Rothschild et al. 2012b; Mitidieri 2021), Schmorl nodes (e.g., Rothschild 2009; Rothschild et al. 2012b; Mitidieri 2021) and congenital malformations (e.g., Lydekker 1889; Witzmann 2007; Rothschild 2009; Rothschild et al. 2012b; Sassoon 2019). In contrast, evidence of pathologies in the lineage of aquatic reptile Sauropterygia is very limited (Schmeisser McKean 2012; Talevi et al. 2021), mostly corresponding to avascular necrosis (e.g, Rothschild and Storrs 2003; Surmik et al. 2017), vertebral lesions or anomalies (e.g, Hopley 2001; Witzmann 2007; Roberts et al. 2017; Sassoon 2019), or bite marks (e.g., Buchy, 2007; Holland, 2018; Rothschild et al. 2018). Most of the pathologies reported in Sauropterygia affect Jurassic and Cretaceous plesiosaurs (e.g., Mudge 1878; Lydekker 1889; Witzmann 2007; Wilhelm and O’Keefe 2010; Ketchum and Benson 2011; Kubo et al. 2012; Schmeisser McKean 2012; Smith 2013; Sassoon 2019; Solonin et al. 2021; Talevi et al. 2021), whereas those in Triassic sauropterygians are scarcer (e.g., Diedrich 2014; Maisch 2014; Surmik et al. 2017, 2018, 2022). Simosauridae is a poorly known lineage among the Triassic eosauropterygians considering both the fossils found and their diversity and paleobiogeographic distribution. Thus, it is recognized as restricted to the western margin of the Tethys Sea, being exclusively known by two defined species: Simosaurus gaillardoti Meyer, 1842, from the Ladinian of France and Germany (Rieppel 1994), and Paludidraco multidentatus de Miguel Chaves, Ortega, Pérez-García 2018a, from the Carnian of Spain (García-Ávila et al. 2021). Most identified simosaurid material at species level in Europe corresponds to S. gaillardoti, which has been extensively studied in both classical and more recent works (see de Miguel Chaves et al. 2018b, 2020 and references therein). Paludidraco multidentatus was recently named based on the relatively complete skeleton of the holotypic individual, including the skull and numerous postcranial elements, and the skull of a paratypic individual (de Miguel Chaves et al. 2018a). Additional well-preserved and partially articulated skeletons, currently under preparation, have also been identified in the type locality, the El Atance fossil site (see García-Ávila et al. 2021). To date, no pathologies have been reported for Simosauridae.

This study identifies and describes several anomalous dorsal vertebral centra of the Paludidraco multidentatus holotype (MUPA-ATZ0101), from the Carnian (Late Triassic) of El Atance fossil site (Guadalajara, Central Spain). In addition, a differential diagnosis is provided, and the possible etiology for these vertebral pathological conditions is discussed.

This analysis focuses on four anomalous disarticulated centra of the Paludidraco multidentatus holotype (MUPA-ATZ0101), corresponding to the most posterior dorsal region: MUPA-ATZ0101-21 (Figs 1A, B1–B6, 2A1–A4), MUPA-ATZ0101-22 (Figs 1A, C1–C6, 2B1, B2), MUPA-ATZ0101-25 (Figs 1A, D1–D6, 2C1–C3, 3A–K), and MUPA-ATZ0101-23 (Figs 1A, E1–E6, 2D1–D6, 4A–L). For each of these elements, the measurements of the centrum height (cH), length (cL), and width (cW), and both the maximum height (mH) and width (mW) of the anomalous structures were taken. The anomalous structures, referred to as “bulks”, were numbered from 1 to 11 (Bulk ID in Table 1) for detailed description (see Suppl. material 1). In addition, a non-anomalous disarticulated vertebral centrum of the posterior dorsal region of the same individual, with collection number MUPA-ATZ0101-13 (Figs 1A, 5A–K), has been considered for the external and internal macroscopic comparative studies. Despite the skeleton of MUPA-ATZ0101 is relatively articulated, the anomalous centra were not found in their original anatomical position. They were partially disarticulated, although close to each other, and located near the sacral region (Fig. 1A). Hence, their exact position within the vertebral series, as well as that relative to each other, and their anterior and posterior articular facets cannot be accurately identified.

Figure 1. 

Position and detail of the pathological vertebrae corresponding to the holotype of Paludidraco multidentatus (MUPA-ATZ0101), from the Carnian (Late Triassic) of El Atance (Guadalajara, Central Spain). A. MUPA-ATZ0101 as originally found in the field and before the disarticulation of several elements, including the centra studied in this work, which are represented in color; B–E. the pathological vertebral centra after removal from the block and preparation: MUPA-ATZ0101-21 (B1–B6); MUPA-ATZ0101-22 (C1–C6); MUPA-ATZ0101-25 (D1–D6); MUPA-ATZ0101-23 (E1–E6). The elements are shown in: ?cranial (B1, C1, D1, E1), ?caudal (B2, C2, D2, E2), lateral (B3, B6, C3, C6, D3, D6, E3, E6), dorsal (B4, C4, D4, E4), and ventral (B5, C5, D5, E5) views. Scale bar: 20 mm.

The software ImageJ was used to calculate the percentage of bulk occupation (Bo), corresponding to the ratio between the total surface occupied by the anomaly (whether it corresponds to one or several bulks) in an articular facet (Ba) and by the total surface of the facet (Fa), multiplied by 100 (Table 1; see Suppl. material 1 for measurements). For the internal macroanatomical study, MUPA-ATZ0101-13, MUPA-ATZ0101-25, and MUPA-ATZ0101-23 were selected for CT scanning as representatives of the three morphological centrum stages identified in MUPA-ATZ0101: centra with non-anomalous facets, centra with one anomalous facet, and centra with both anomalous facets, respectively. The specimens were CT-scanned at the Servicio de Técnicas no Destructivas, of the MNCN. This process was done through a high-resolution scanner Nikon XT H-160 with the following settings: voltage 160 kV (for all specimens); current 280 µA (for MUPA-ATZ0101-13), 260 µA (for MUPA-ATZ0101-23), 229 µA (for MUPA-ATZ0101-25); 3015 number of projections (for all of them); 1533 images (for MUPA-ATZ0101-13), and 1536 (for MUPA-ATZ0101-23, MUPA-ATZ0101-25); and voxel size 31 µm (for MUPA-ATZ0101-13), 27 µm (for MUPA-ATZ0101-23), and 28.5 µm (for MUPA-ATZ0101-25). The DICOM files obtained from the CT-scan were processed using Avizo 7.1 (VSG) to obtain the 3D models and the bone sections.

For the etiology assessment, the anomalous centra were subjected to a differential diagnosis procedure. This is an essential technique in paleopathology (Rothschild and Martin 2006; Grauer 2022; Rothschild et al. 2023) which involves identifying diseases with similar osteological features and determining those that explain the observed lesions. The discussion was based on the comparison of both medical and veterinary bone pathology literature, especially that referring to extinct and extant reptiles.

Paludidraco multidentatus displays amphicoelous vertebral centra (de Miguel Chaves et al. 2018a, 2020) and articular facets lacking bulks, which is the common condition seen in basal eosauropterygians (see Rieppel 2000). All the observed vertebral centra of the holotype of P. multidentatus (MUPA-ATZ0101) lack bulks in their articular facets (see Cabezuelo-Hernández et al. 2024), except for the four elements analyzed in this study, which display paired and symmetrically placed bulks, in one (MUPA-ATZ0101-22, MUPA-ATZ0101-25) or both (MUPA-ATZ0101-21, MUPA-ATZ0101-23) of their articular facets. Occasionally, vertebral centra displaying bulks in their facets are present in other reptile lineages as a single roundish structure that can or cannot be associated with a notochordal pit. However, these structures are considered anatomical features rather than pathological conditions (e.g., Arbour and Curry 2013, fig. 2b; Burns et al. 2015; Matsumoto et al. 2019). Within Sauropterygia, the presence of a notochordal pit, accompanied or not by a single bulk [i.e., a central mammilla sensu Storrs (1999)], is exclusive of some Plesiosauria (Brown 1981; Storrs 1999; Kear 2002; Lazo and Cichowolski 2003; Bell et al. 2014; Cau and Fanti 2015; Miedema et al. 2019; Zverkov et al. 2024). None of the observed vertebral centra of MUPA-ATZ0101 or any other individual of P. multidentatus (including three other partial skeletons and isolated vertebral centra from the type locality) presents a notochordal pit, which is also absent in other basal sauropterygians (Rieppel 2000). The presence of paired bulks in MUPA-ATZ0101 is restricted to the most posterior dorsal vertebral region, near the sacrum (Fig. 1A). These bulks have not been observed in any other centra of MUPA-ATZ0101 or in similarly placed centra of the other individuals of P. multidentatus so far identified (ACH pers. obs.; see de Miguel Chaves et al. 2020, fig. 3e-f). Therefore, the presence of these bulks is identified here as an anomaly of the individual MUPA-ATZ0101 and are identified as of pathological origin (see Discussion below). Anomalies affecting vertebral centra of Sauropterygia are relatively scarce in the literature. Most of them have been interpreted as pathological (see Suppl. material 2). Schmorl nodes and osteophytes are the most frequently documented vertebral pathologies in this lineage, having been exclusively reported for Plesiosauria (see Suppl. material 2). Among Triassic sauropterygians, the only report of a pathology affecting the vertebrae corresponds to that of the eosauropterygian Proneusticosaurus silesiacus Volz, 1902, being recognized as a tuberculosis-like infection (Surmik et al. 2018).

Pathologies affecting the vertebral centrum may have multiple origins, including: tumoral (neoplasms), infectious (e.g., osteomyelitis, discitis), metabolic (e.g., Calcium Pyrophosphate Deposition Disease), traumatic (e.g., fractures, luxation/subluxation, long-term biomechanical stress), degenerative (e.g., Intervertebral Disc Disease, osteoarthritis, osteoporosis), or congenital (e.g., hemivertebra, block vertebrae) (Manfrè and Van Goethem 2020; Grauer 2022; Rothschild et al. 2023). The absence of irregular or disproportionate bone overgrowth, distinct pathological inner bone from that non-pathological, and lytic inner lesions, allows to disregard a tumoral origin for the bulks in MUPA-ATZ0101 (Manfrè and Van Goethem 2020; Anné et al. 2022; Rothschild et al. 2023). An infectious origin for the bulks in MUPA-ATZ0101 is not supported by the absence of necrotic bone tissue, sinuses or sequestrae, or reactive new bone formation (Anné et al. 2022; Grauer 2022; Rothschild et al. 2023). Metabolic diseases, such as the Calcium Pyrophosphate Deposition Disease (CPDD) or gout can also produce bulks in the centrum facets (Anné et al. 2022; Rothschild et al. 2023). Nevertheless, these bulks are deposits with well delimited internal margins and a different inner structure compared to the adjacent tissue (Anné et al. 2022; Rothschild et al. 2023), unlike the condition observed in the centra of MUPA-ATZ0101. The lack of fractures associated with callus formation in the vertebral centra of MUPA-ATZ0101 allows to disregard traumatic fractures (Anné et al. 2022). The identification of the microfractures observed in both the pathological and the non-pathological centra as pre-mortem or post-mortem is challenging as no associated bone reaction or malunions are observed (Grauer 2022; Rothschild et al. 2023). However, the multifocal nature of the pathology as well-circumscribed lesions in an anatomic and a subanatomic location eliminates a luxation/subluxation origin (Anné et al. 2022). Hence, a single traumatic event is not supported here as the origination of the bulks in the P. multidentatus holotype. Nevertheless, long-term biomechanical stress causes cannot be eliminated (see Discussion below).

Common causes of vertebral centrum lesions related to advanced ontogenetic stages include osteoporosis, Intervertebral Disc Disease (IDD) and arthritis (Pérez-Hernández et al. 2015). Based on anatomical comparisons with other available specimens of P. multidentatus, MUPA-ATZ0101 is tentatively considered as an adult or sub-adult individual considering its size, which is approximately 19% smaller than the largest individual hitherto known for this taxon, considering scapular and ischial lengths, interclavicular width, and the ossification of its bony elements (e.g., significantly more expanded distal region of dorsal ribs in relation to younger individuals, but less expanded compared to the larger ones). Despite the relative bone mass decrease observed in the centrum MUPA-ATZ0101-25, and the documentation of centrum facet bulks in osteoporotic human vertebrae (possibly caused by vascular ischemic damage leading to necrosis and edema) (Muratore et al. 2018; Vordemvenne et al. 2020), osteoporosis as the cause of the bulks in MUPA-ATZ0101 is disregarded based on the absence of: significant bone mass decrease between the pathological (Figs 3A–K, 4A–L) and the non-pathological centra (Fig. 5A–K) (Grauer 2022; Rothschild et al. 2023); osteoporotic fractures (Muratore et al. 2018; Vordemvenne et al. 2020; Grauer 2022); or necrosis (Muratore et al. 2018).

Other lesions affecting the vertebral centrum facets are those related to intervertebral disc degeneration (Manfrè and Van Goethem 2020; Rothschild et al. 2023). This could be caused by degenerative diseases (i.e., IDD), metabolic disorders, mechanical stress, or congenital disorders (Shapiro et al. 2014; Pérez-Hernández et al. 2015). Disc degeneration is mostly considered age-dependent (i.e., degenerative) (Shapiro et al. 2014; Pérez-Hernández et al. 2015; Manfrè and Van Goethem 2020), often resulting in disc herniations associated with osteophytes, Schmorl nodes, centrum facet sclerosis, and/or spondyloarthritis (Hopley 2001; Pérez-Hernández et al. 2015; Manfrè and Van Goethem 2020). In the case of MUPA-ATZ0101, the development of the bulks on the centrum articular facets due to IDD seems not plausible based on the absence of Schmorl nodes or osteophytes (Manfrè and Van Goethem 2020; Anné et al. 2022), diffuse heterogeneous pitting on the surface (Anné et al. 2022 and references therein), and the lack or any sings of apparent vertebral arthritis in the specimen (but see Discussion below). Repetitive mechanical stress also plays an important role in intervertebral disc degeneration, affecting the centrum articular facets of adjacent centra, often generating osteophytes or other malformations (Shapiro et al. 2014). Alternatively, congenital disorders can lead to disc degeneration, resulting in disc weakness and eventual centrum articular facet lesions or deformities (Rothschild et al. 2023). The presence of intervertebral discs in extinct marine reptiles, including sauropterygians, is controversial. Some studies suggest the existence of a synovial joint between the vertebrae in both Plesiosauria and Mosasauria (Witzmann et al. 2016; Rothschild et al. 2020; Talevi et al. 2021). The presence of synovial joints in the vertebral column of Sauropterygians might be supported based on extant phylogenetic bracketing (Witzmann et al. 2016). Additionally, different osteological correlates may support the latter hypothesis as spines with synovial joints present smooth vertebral articular surfaces, which is the case with sauropterygians, contrary to the rugose articular vertebral surface observed in the intervertebral-disc-bearing spines of mammals (Witzmann et al. 2016). Conversely, Wintrich et al. (2020) interprets the presence of intervertebral discs in Sauropterygia. In this context, spine pathologies originate by rather different causes depending on the intervertebral joint type existing (i.e., intervertebral disc or synovial joint) (Witzmann et al. 2016; Rothschild et al. 2020). Hence, very different etiologies must be considered in the differential diagnosis if one or another intervertebral joint is present (Witzmann et al. 2016). In this sense, spine diseases afflicting both intervertebral discs and synovial joints have been considered in the present work considering the controversy on whether sauropterygians may have presented one or another intervertebral joint type.

The nature of the bulks observed in the centra of MUPA-ATZ0101 is unclear, and such structures have not been reported so far in any other marine reptile, including sauropterygians, or marine mammal. The only reported case of a similar pathology corresponds to an undetermined hadrosaurid caudal vertebra (TMP 1979.008.0096) from the Campanian (Upper Cretaceous) of Canada (Tanke and Rothschild 2014; Rothschild et al. 2023). As the pathological centra of MUPA-ATZ0101, TMP 1979.008.0096 displays paired and symmetrically placed bulks that are internally continuous with the periosteum and the underlying bone tissue (Rothschild et al. 2023). In contrast, the inner tissue of the bulks in the hadrosaurid centrum is denser compared to the rest of the centrum, but the same configuration is present in the non-pathological areas (Rothschild et al. 2023). Although previously referred to as central osteophytes (see Rothschild et al. 2023), the bulk pathophysiology for the centrum TMP 1979.008.0096 is unclear and its etiology has not been determined (Rothschild et al. 2023). Similar bulks (considering their morphology, location, and texture), but not symmetrically placed, were also reported in several caudal vertebrae of the hadrosaur Edmontosaurus annectens from the Maastrichtian (Upper Cretaceous) of United States, although their attribution to specific individuals could not be confirmed (Anné et al. 2022); and in two non-contiguous caudal vertebrae of the same individual (MCT.R.495a) of an undetermined lithostrotian sauropod from the Santonian or Campanian (Upper Cretaceous) of Brazil (Lacerda et al. 2025). The bulks in the vertebral centra of E. annectens, referred to as button-like lesions (sensu Anné et al. 2022), are subrounded protrusions located in the centrum facets, with no different texture compared to the non-pathological centrum areas, similar to those observed in MUPA-ATZ0101. Those of the lithostrotian sauropod are also subrounded and exhibit smooth edges and well-defined borders (see Lacerda et al. 2025, fig. 3 h), as in MUPA-ATZ0101. Unfortunately, no CT sections are provided for the lesions in MCT.R.495a and, thus, a comparison with the bulk inner structure of MUPA-ATZ0101 cannot be provided. In contrast to MUPA-ATZ0101, the bulks in the vertebral centra of both E. annectens and the lithostrotian sauropod often appear in association with other centrum facet lesions: mound-like and indent lesions (sensu Anné et al. 2022) in E. annectens; and invagination pits, and superficial bone erosion in the lithostrotian sauropod (Lacerda et al. 2025). The bulks’ lesions in these dinosaurs are either interpreted as central osteophytes (Anné et al. 2022) or subchondral cysts resulting from arthritis (Anné et al. 2022; Lacerda et al. 2025). Nonetheless, subchondral cysts are regarded as erosive indented lesions or endplate defects (Witzmann et al. 2016; Barbosa et al. 2018; Rothschild et al. 2023), rather than subchondral protrusions as those observed in MUPA-ATZ0101, E. annectens, or the lithostrotian sauropod. Alternatively, Anné et al. (2022) suggest that the bulks in E. annectens can represent an early stage of mound-like lesions, most probably caused by long-term biomechanical stress, based on the lack of differentiation between pathological and non-pathological bone tissue, and the presence of microfractures. However, a congenital origin cannot be disregarded (Anné et al. 2022). Osteophytes have been occasionally reported in several plesiosaurs (see Suppl. material 2) and are recognized as bony spurs located on or near the centrum facet margins (e.g., Hopley 2001; Sassoon 2019), similar to those found in mammals or other reptiles (Manfrè and Van Goethem 2020; Rothschild et al. 2023). Although it is not clear if the bulks observed in MUPA-ATZ0101 correspond to osteophytes, they clearly represent a lesion of the centrum facet. The etiology of the pathology in this P. multidentatus individual remains unclear, but based on the proposed differential diagnosis, and the similarities discussed above for the dinosaur vertebral centra, a long-term biomechanical stress or a congenital disorder is proposed.

This study identifies several pathological vertebrae in the Triassic sauropterygian Paludidraco multidentatus. This is the first report of a pathology within the eosauropterygian clade Simosauridae and the first documented non-infectious vertebral pathology in Triassic sauropterygians. The documented pathology consists of paired and symmetrically placed bulks in the articular facets of several centra of the posteriormost dorsal region of the vertebral column of the holotypic individual of P. multidentatus. These bulks are indistinguishable in both outer and inner structure from the non-pathological centrum areas and lack a clear separation from the non-pathological periosteum. The differential diagnosis involving extant and extinct reptiles, as well as mammals, ruled out several potential causes for this pathology, including tumoral, infectious, traumatic, degenerative, and metabolic origins. Similar vertebral lesions have never been reported for any marine reptile (considering the morphology, symmetry, and location), having only been rarely documented in some dinosaurs. In conclusion, the most likely etiology for the bulks documented in the P. multidentatus holotype is interpreted to be either long-term biomechanical stress or a congenital malformation.

This study has been funded by the FPU grant (ref. FPU20/01945) for AC. The authors thank Dr. Andrea Guerrero Bach-Esteve and Javier Salas Herrera (Universidad Nacional de Educación a Distancia, Madrid, Spain) for their advice on paleopathology. The authors also thank the valuable and constructive comments provided by Florian Witzmann, Filippo Bertozzo, and an anonymous reviewer which have substantially improved this work. All specimens described in this study belong to a public institution (Museo de Paleontología de Castilla-La Mancha in Cuenca) and were obtained under excavation conditions authorized by the Dirección General de Patrimonio y Museos de la Junta de Comunidades de Castilla-La Mancha.