Synapomorphies (from Werneburg et al. 2023, after Sawin 1941; Romer 1947; Boy 1990; Milner 1989, 1990; Werneburg and Steyer 1999; Schoch and Hampe 2004; Werneburg 2007; Werneburg and Berman 2012; Schoch and Milner 2014): (1) Enlarged choana medially wide; (2) Ectopterygoid, palatine and vomer only with two or three fangs (without subsequent smaller teeth); (3) Lacrimal reaches anteriorly to the naris or septomaxilla; (4) Enlarged posterior width of skull (pS_w/S_l=0.92–1.10; compare Fig. 2); (5) Posterior part of the cultriform process widened (partly); (6) Interclavicle of adults proportionally small and broadly-ovate in outline; (7) Ilium with vertically directed dorsal process, which is anteroposteriorly widened dorsally.

All three species to be revised, Clamorosaurus nocturnus, C. borealis and Syndyodosuchus tetricus, fulfill the first four criteria of the family diagnosis. The fifth diagnostic feature is not present in all eryopid genera, only in Eryops and Onchiodon. The sixth and seventh diagnostic features concern the interclavicle and ilium, which have not been recorded in all three species. In this respect, all these species can be assigned to the family Eryopidae.

Clamorosaurus nocturnus Gubin, 1983.

Synapomorphy: (1) Very wide interpterygoid vacuities, with the orbitae not obscured in ventral view, in contrast to all other eryopids and shared with the stereospondylomorph Intasuchus (Konzhukova 1956; Werneburg et al. 2020), however, in contrast to Intasuchus, the interpterygoid vacuities of Clamorosaurus are unique in being anteriorly widened.

Characters shared with certain eryopids: (2) Premaxillary snout region laterally constricted at the level of the external naris, shared with Eryops and Osteophorus; (3) Skull very wide, shared with Onchiodon; (4) Fangs on the vomer located on two separate circular tooth pits, one medial to the anterior edge of the choanae and one medial to the choana at its midlength. Shared with Syndyodosuchus.

PIN 1582/1, consisting of the skull roof (skull length 18.2 cm), scapulocoracoid and ventral scales, together with undetermined bony remains, and a partly prepared section of the basal plate of the parasphenoid and the clavicle, both in ventral view.

PIN 1582/4, consisting of the anterior part of a skull with the skull roof in dorsal and the palate in ventral view; PIN 1582/6, consisting of the anterior part of a skull with the skull roof in dorsal view and the fangs of ectopterygoid, palatine and vomer of the palate in ventral view; PIN 1582/2a, consisting of a scapulacoracoid with ribs, PIN 1582/2b representing a clavicle, and PIN 1582/2c consisting of a possible humerus-fragment in two parts).

All this referred material was discovered near the town of Pechora on the lower Pechora River (Komi Republic, Russia) in the Sheshminskian Gorizont (Ufimian), late Kungurian (Cisuralian, Permian).

C. nocturnus has no autapomorphies, but a unique combination of characters: (1) Density of sculpture pattern quantified as the number of pits per in² on frontal plus jugal range between 3.08 and 4.07, shared with C. borealis, Glaukerpeton and close to Syndyodosuchus, but in contrast to all other eryopids; (2) Premaxillary and maxillary teeth are small and circular in cross section, in contrast to C. borealis, O. labyrinthicus, and Eryops; (3) Teeth no. 8 and 9 are the largest in the premaxilla, in contrast to C. borealis and many other eryopids; (4) Tooth no. 6 is the largest in the maxilla, in contrast to C. borealis and many other eryopids; (5) Equal internarial and interorbital width, in contrast to C. borealis, Syndyodosuchus, Glaukerpeton, and E. megacephalus; (6) Narrow interorbital width, shared with C. borealis, Syndyodosuchus, Actinodon, and E. megacephalus; (7) Jugal very wide, shared with C. borealis, O. thuringiensis, and Eryops sp. from the Moran Formation (MCZ1914; Werneburg 2008; Schoch and Milner 2014); (8) Septomaxilla is completely unsculptured and ventrally directed, shared with C. borealis, Eryops, and Glaukerpeton; (9) Short contact between jugal and prefrontal, shared with Syndyodosuchus, Glaukerpeton, Actinodon, and O. labyrinthicus, but in contrast to C. borealis, O. thuringiensis, and E. megacephalus; (10) Supratemporal wide, but longer than wide, in contrast to C. borealis; (11) No interfrontal, in contrast to Eryops and Osteophorus; (12) No lateral line sulci, in contrast to Glaukerpeton and Actinodon; (13) Interchoanal width is equal to internarial width, in contrast to C. borealis; (14) Short and wide palatine, only slightly longer than wide; (15) Ectopterygoid much longer than palatine; (16) Narrow basal plate of parasphenoid, in contrast to C. borealis, Onchiodon, Stenokranio and Glaukerpeton; (17) Cultriform process of parasphenoid longer than median length of vomer, shared with nearly all eryopids, but in contrast to C. borealis.

Three incompletely preserved skulls with median lengths of 16 to 18 cm show the skull roof in dorsal view and parts of the palate in ventral view. They have complementary, congruent features, such as a small, dense dermal sculpture, small, almost oval orbitae, a very narrow interorbital region (IOw/Sl=0.21), a very wide jugal, a wide lacrimal that reaches to the naris in front, and rather small teeth in the maxilla and premaxilla. Therefore, all these skulls belong to the same species.

General skull morphology. The dermal sculpture of the dorsal surface of the skull roof corresponds to the relatively fine sculpture pattern known from some eryopids such as Clamorosaurus borealis, Syndyodosuchus tetricus and Glaukerpeton avinoffi (Werneburg and Berman 2012) (Table 1). It consists of a reticulated pattern of small pits and valleys separated by narrow ridges (Figs 1–3). The nasal, jugal and squamosal show much more radially directed ridges. The density of the sculpture pattern is quantified as the number of pits per in² (6.452 cm²) on the frontal and jugal, which are typically well-preserved bones in eryopid skulls, and as a proportion of those counts to skull length. These intraspecific indices range between elements and specimens of C. nocturnus between 3.08 and 4.07, which are very similar in C. borealis, S. tetricus and G. avinoffi (Table 1). The dermal sculpture of the dorsal surface of the skull roof in other eryopid species has a much coarser pattern with indices from 0.4 up to 1.7. Higher indices between 1.2 and 4.3 occur only in the subadult Eryops (Table 1).

The dorsal strutting pattern with large ridges on the skull roof is well developed (Fig. 6A) and probably increased the mechanical stability of the skull (Sawin 1941; Boy 1990; Werneburg 2007; Schoch and Sobral 2021; Werneburg et al. 2023). A large longitudinal ridge extends from the lateral portion of the tabular and supratemporal to the postorbital. It then runs on the anterior skull table from the suture between prefrontal/frontal and on the lateral part of the nasal to the medial margin of the naris. Additional transverse ridges occur between the longitudinal ridges on frontals and nasals (Figs 3, 4A, B). The areas between these ridges are depressed. Additionally, a short ridge on the jugal is traceable (Fig. 4B). The degree of skull roof ossification appears to be relatively low and the bones may be intermediate between the normally thick bones as in most other eryopids and the 30–50% thinner skull roof bones of Glaukerpeton (Werneburg and Berman 2012).

The combination of the three known skulls of C. nocturnus allowed a tentative reconstruction of the skull roof in dorsal view and of the palate in ventral view (Fig. 6A, B). The skull is slightly wider than long (Table 2; pS_w/S_l=1.04). The lateral margin of the skull is convex in dorsal view. The snout margin is laterally constricted at the level of the naris like in Eryops megacephalus, Osteophorus and C. borealis. The postorbital region of the skull roof is relatively long (H_l/S_l=0.26) and wide (H_w/S_l=0.50) in contrast to that of E. megacephalus and Stenokranio. The preorbital skull is relatively elongate (PO_l/S_l=0.57). The internarial and interorbital width are nearly equal (IN_w/S_l=IO_w/S_l=0.21–22) as in many eryopids, but in contrast to Glaukerpeton in which the internarial width is smaller, and to E. megacephalus, C. borealis and S. tetricus with a smaller interorbital width (Table 2). The occipital margin of the skull roof is only slightly concave as in Onchiodon thuringiensis and C. borealis. The quadrate condyles lie distinctly posterior to the occipital condyles (Qc_l/S_l=0.17; Table 2). The long oval orbits are relatively small compared to other eryopids (O_l/S_l=0.16).

Growth stage. The three skulls of Clamorosaurus nocturnus (PIN 1582/1, 1582/4 and 1582/6) clearly belong to adult animals, as indicated by the following features: (a) The dermal sculpture consists of a reticulated pattern of small pits and valleys separated by narrow ridges; (b) The quadrate condyles lie distinctly posterior to the occipital condyles; (c) The quadrate is ossified dorsally; (d) The orbits are relatively small compared to other eryopids; (e) The pterygoid has a pronounced transverse process; (f) The scapulocoracoid is well ossified (Figs 3, 5F); (g) The skull length of 16–18 cm is relatively large and ranged in the middle-sized group in the family Eryopidae, and Actinodon and Onchiodon labyrinthicus have no larger skulls. However, the degree of ossification of the neurocranium indicates that the three specimens were early adults: (h) Sphenethmoid and basioccipital are not preserved and were probably not ossified in this stage.

Skull roof. The interpremaxillary suture is short and accounts for 7.4% of the midline length of the skull. The alary process of the premaxilla is wide and short. C. borealis and Actinodon have no alary process. The premaxillary tooth arcade has nine tooth loci (only six in C. borealis). The relatively small teeth have a circular cross-section, and only the two posteriormost teeth are slightly larger. This type of dentition contrasts with that of C. borealis, E. megacephalus and O. labyrinthicus, which consists of much larger teeth that are long-oval in cross-section.

The maxilla has a relatively narrow dorsal shelf and is ventrally in contact with the quadratojugal. Its tooth arcade has about 25 tooth loci (only 21 in C. borealis). The teeth have a circular cross-section and they are relatively small. Only the sixth tooth is slightly larger, similar in size to the two larger ones of the premaxilla. This type of dentition contrasts with that of C. borealis, E. megacephalus and O. labyrinthicus, which has much larger teeth that are long-oval in cross-section.

The circular naris is of similar proportional length as in Glaukerpeton or E. megacephalus, comprising 10% of the midline length of the skull. The small septomaxilla is not sculptured (Figs 4A, B, 5A, B) and may be ventrally directed inside the naris (shared with Eryops). The posterior margin of the naris is clearly formed by the nasal, lacrimal and maxilla (Fig. 5A, B).

The lacrimal is roughly diamond-shaped. It is separated from the orbit by a short contact between jugal and prefrontal. The medial part of the lacrimal is wide (La_w/La_l=0.49), and this bone participates in the posterolateral narial margin. The frontal is long and narrow like in most other eryopids and gets narrower in its posterior part where it is restricted by the medially expanding postfrontals.

The jugal is proportionally wider (Ju_w/S_l=0.22) than in all other eryopids apart from Eryops sp. (MCZ1914) from the Moran Formation (Table 2). The postorbital is triangular in outline. The postfrontal and prefrontal clearly contact one another as in all eryopids. The prefrontal is anteriorly relatively wide. The width of the supratemporal is striking; this bone is only 1.1 times longer than wide. Only in C. borealis the supratemporal is much wider than long (see below).

The parietals anteriorly approach the level of the posterior orbital margin, and the postparietals and tabulars are comparatively short. The tabular horn is modestly elongated (Th_l/S_l=0.09), narrow and its rounded tip points posteriorly and slightly laterally. The width of the cheek is pronounced (W_w/S_l=0.30) and is only exceeded by the relative cheek width in Onchiodon (Table 2). The squamosal is relatively narrow and the quadratojugal is very wide, especially in its anterior part. The quadratojugal reaches far posterior so that its posterior end comes to lie posterior to the squamosal and roofs the quadrate (Fig. 6A, B).

The exposure of the quadrate on the occipital surface of the cheek (Fig. 1) consists of a narrow strip of bone that is directed anteromedially between the squamosal and the quadrate ramus of the pterygoid. A boss-like protuberance at the ventral margin of the dorsal quadrate process like in Stenokranio or Glaukerpeton might have been developed (Fig. 3B). Quadratojugal foramina cannot be determined with certainty. Like in most adult eryopids, lateral line sulci are not present (Witzmann et al. 2010; Werneburg et al. 2023).

Palate and braincase. From the palate, large parts of the vomers, palatines, ectopterygoids, pterygoids and the basal plate of the parasphenoid are preserved. Longitudinal ridges on the palatal bones are not developed. Immediately anterior to the level of the anterior vomerine tusks, the rounded posterior end of the anterior palatal fossae extends on the anterior part of the vomers (Fig. 4D) and probably on the dental shelf of the premaxilla.

The vomer is elongated and narrow. The smallest width of both vomers (=interchoanal width ICw/Sl=0.22) is equal to the smallest width between the narial openings (internarial width) and nearly the same as the interorbital width. The posterolateral corner of the vomer encloses the anterior tip of the pterygoid as in Eryops. The suture between vomer and palatine is much more elongated than in C. borealis, Glaukerpeton and Actinodon. The short palatine is only slightly longer than wide. The ectopterygoid is elongated and c. 1.5 times longer than the palatine. Its posteriormost part is equal in width to the medially neighbouring pterygoid.

The dentition of palatine and ectopterygoid is interesting. The palatine bears a conspicuous fang anteriorly and a much smaller one posteriorly, which has the same size as the two fangs on the anterior part of the ectopterygoid. The fangs on the vomer are of equal or smaller size than those on the ectopterygoid. They are located on two separate circular tooth pits; the slightly larger one is located medial to the anterior edge of the choanae and has space for two small fangs. On the right vomer, only one tooth is preserved in this pit, but there is space for another one. Two teeth have been recorded on the left vomer. A further, slightly smaller tooth pit with one tooth is positioned medial to the choana at its midlength. Apart from C. nocturnus, this second tooth pit on the vomer is only known in C. borealis (here with a pair of fangs) and Syndyodosuchus.

The relatively small choana is of irregular outline, medially expanded and slightly longer than wide. The choanae of Glaukerpeton, Actinodon and Eryops are larger and more elongate, and in the case of Stenokranio about as long as wide (Fig. 15).

The pterygoid has a narrow palatinal ramus; its most anterior part forms a narrow, anteromedially directed tip, which may overlap the posterolateral corner of the vomer. The transverse flange of the pterygoid exhibits a low, angular expansion. In C. borealis, Onchiodon and Actinodon, the entire free lateral margin of the pterygoid is greatly expanded into a right-angled projection. The palatinal ramus and the elongated basipterygoid ramus are strongly curved; thus, the interpterygoid vacuities are extremely wide, especially in their anterior part. The orbitae are not concealed by the pterygoids in ventral view. These characters are shared with C. borealis but are unknown in other eryopids. The stereospondylomorph Intasuchus (Konzhukova 1956; Werneburg et al. 2020) presents similar features, but especially the anteriorly widened interpterygoid vacuities in Clamorosaurus contrast with Intasuchus. Polygonal bony plates covering the interpterygoid vacuities are not preserved. Three larger bony plates are accumulated in the anterior part of the interpterygoid vacuities (Fig. 4C, D), which may represent remains of the epipterygoid. Similar bones are known from C. borealis and Syndyodosuchus (see below).

The basicranial articulation is firmly sutured (Fig. 4C, D). The cultriform process of the parasphenoid is generally narrow in contrast to O. labyrinthicus and Eryops, in which the process is swollen in its posterior half with convex lateral margins. The basal plate has a narrow rectangular shape like in Eryops and Syndyodosuchus. The ventral surface of the parasphenoidal basal plate has curved furrows for the carotid artery below the basipterygoid pockets, but their foramina lie more anterodorsally near the pockets. A large denticle field is developed between these furrows, which may taper anteriorly like in C. borealis to attain a triangular shape. Numerous denticles are present on the vomer, on the palatinal ramus of the pterygoid, partly on the palatine and probably on the ectopterygoid. The basioccipital and exoccipitals were apparently not ossified in this growth stage. The articular condyle of the quadrate is transversely expanded.

The visceral skeleton and mandibles are not preserved.

Postcranium. Few bones of the anterior part of the postcranial skeleton are associated with the skulls: ribs, clavicles, scapulocoracoids and ventral scales. One narrow rib and one rib with expanded proximal and distal ends are preserved. The clavicle has a relatively narrow ventral blade with remains of dermal sculpture (Fig. 5F). The scapulocoracoid (Figs 3, 5F) has an angle of about 90° between the supraglenoid buttress and the anterior margin of the scapular blade. Such an angle is known in most Eryops specimens and Stenokranio (see discussion in Werneburg et al. 2023). This angle is less than 90° in Glaukerpeton and O. labyrinthicus.

PIN 3950/1, consisting of the skull in dorsal and palatal view (skull length 15.3 cm) with associated isolated bones such as sphenethmoid, stapes and left quadrate.

None.

The holotypic material was found near the town of Inta (Komi Republic, Russia) in a limestone from the coal mine number nine of the Ufimian Intinskaya Svita, late Kungurian (Cisuralian, Permian) in 1961.

Autapomorphies: (1) Premaxilla with only six teeth, in contrast to nine to 15 premaxillary tooth loci in all other eryopids; (2) Maxilla with only 21 teeth, in contrast to 25 to 43 maxillary tooth loci in all other eryopids; (3) Supratemporal much wider than long; (4) Cultriform process of parasphenoid much shorter than median length of vomer.

Synapomorphies with some other eryopids: (1) Density of sculpture pattern quantified as the number of pits per in² on frontal plus jugal range between 3.40 and 5.23, shared with C. nocturnus, Glaukerpeton and Syndyodosuchus, but in contrast to all other eryopids; (2) Premaxilla without alary process, shared with Actinodon; (3) Some teeth have a long-oval cross-section in labial-lingual direction, shared with O. labyrinthicus and Eryops; (4) Teeth four to six are the largest premaxillary teeth, in contrast to C. nocturnus and many other eryopids; (5) The third tooth is the largest in the maxilla, in contrast to C. nocturnus, and many other eryopids; (6) Lacrimal wide, its width is only exceeded in O. thuringiensis; (7) Internarial and interorbital width differ, shared with Syndyodosuchus, Glaukerpeton, and E. megacephalus, but in contrast to C. nocturnus; (8) Very narrow interorbital width, shared with C. nocturnus, Syndyodosuchus, Actinodon, and E. megacephalus; (9) Small orbits, only Syndyodosuchus has relatively smaller orbits; (10) Jugal wide, shared with C. nocturnus, O. thuringiensis, and Eryops sp. from the Moran Formation (MCZ1914); (11) Septomaxilla is completely unsculptured and ventrally directed, shared with C. nocturnus and Eryops; (12) Elongated contact between jugal and prefrontal, shared with O. thuringiensis, and E. megacephalus; (13) No interfrontal, in contrast to Eryops and Osteophorus; (14) Tabular with elongated tabular horn, shared with Stenokranio and O. thuringiensis; (15) Quadrate condyles lie far posterior to the occipital condyles, only in E. megacephalus is the distance larger; (16) No lateral line sulci, in contrast to Glaukerpeton and Actinodon; (17) Interchoanal width wider than internarial width, in contrast to C. nocturnus; (18) Elongated and narrow palatine, much longer than wide, shared with Syndyodosuchus and Actinodon, but in contrast to C. nocturnus; (19) Ectopterygoid and palatine about equal in length; (20) Greatly expanded transverse flange of pterygoid into a right-angled projection, shared with Onchiodon and Actinodon and much more pronounced than in C. nocturnus or Syndyodosuchus. (21) Wide basal plate of parasphenoid, in contrast to C. nocturnus, Onchiodon, Stenokranio and Glaukerpeton; (22) Triangular denticle field, shared with Onchiodon.

General Skull Morphology. The dermal sculpture of the dorsal surface of the skull roof corresponds to the fine sculpture pattern known from eryopids such as Clamorosaurus nocturnus, Syndyodosuchus tetricus and Glaukerpeton avinoffi (Werneburg and Berman 2012) (Table 1). It consists of a reticulated pattern of small pits and valleys separated by narrow ridges on nearly all skull roof bones (Fig. 7A, B). The density of the sculpture pattern is quantified as the number of pits per in² (6.452 cm²) on the frontal and jugal, which are typically well-preserved bones in eryopid skulls, and as a proportion of those counts to skull length. These intraspecific indices range between both elements of C. borealis between 3.40 and 5.23, and on the prefrontal this ratio is 7.84 (Table 1). The dermal sculpture of the dorsal skull roof in other eryopid specimens consists of a much coarser pattern with indices ranging from 0.4 up to 1.7. Higher indices between 1.2 and 4.3 occur only in subadult Eryops (Table 1).

The dorsal strutting pattern with large ridges on the skull roof is well developed (Figs 7A, B, 9A). A large longitudinal ridge extends from the lateral portion of the tabular and supratemporal to the postorbital. It then runs on the anterior skull table from the suture between prefrontal/frontal and on the lateral part of the nasal to the medial margin of the naris. A transverse ridge connects the longitudinal ridges on the frontals, and anterior and posterior to it the surface of the frontals is depressed.

The degree of skull roof ossification is probably relatively high and the bones may have the thickness commonly present in other eryopids with the exception of Glaukerpeton and C. nocturnus (see above).

The well-preserved skull of C. borealis allows a tentative reconstruction of the skull roof in dorsal view and of the palate in ventral view (Fig. 9). The skull is slightly wider than long (Table 2; Fig. 6B; pS_w/S_l=1.08). The lateral margins of the skull are convex in dorsal view. The snout margin is laterally markedly constricted at the level of the naris like in Eryops megacephalus and Osteophorus. The postorbital region of the skull roof is relatively long (H_l/S_l=0.25) and wide (H_w/S_l=0.52). The preorbital skull is relatively elongate (PO_l/S_l=0.60). The internarial and interorbital width differ from each other (IN_w/S_l=0.27, IO_w/S_l=0.21) with a smaller relative interorbital width. Both species of Clamorosaurus share with Actinodon (IO_w/S_l=0.20–0.21) the narrowest interorbital region in eryopids. The occipital margin of the skull roof is only slightly concave. The quadrate condyles lie distinctly posterior to the occipital condyles (Qc_l/S_l=0.22; Table 2) as in Eryops. The circular orbits are small compared to other eryopids (O_l/S_l=0.14).

Growth stage. The single skull of Clamorosaurus borealis can be interpreted as adult for the following reasons: (a) The dermal sculpture consists of a dense reticulated pattern of small pits and valleys separated by narrow ridges; (b) The quadrate condyles lie far posterior to the occipital condyles; (c) The quadrate is ossified dorsally; (d) The orbits are small compared to other eryopids. Admittedly, small orbits could also represent a taxon-specific character and not necessarily an ontogenetic one. In general, however, larval and juvenile temnospondyls have proportionally larger orbits than adults, and thus the small orbits in C. nocturnus support our interpretation; (e) the pterygoid bears a pronounced transverse process; (f) The epipterygoid is ossified with a large plate. With a skull length of 15 cm, C. borealis is a middle-sized eryopid similar to Actinodon and Onchiodon labyrinthicus.

Skull roof . The interpremaxillary suture is moderately long and accounts for 11.0% of the midline length of the skull. An alary process of the premaxilla cannot be discerned, and the only other eryopid without this process is Actinodon. Both premaxillaries are strongly curved, leading to a narrow snout with a strongly arched tooth arcade and a lateral constriction anterior to the maxilla. The premaxilla has only six tooth loci, whereas all other eryopids have nine to 15 premaxillary tooth loci (Table 3). Similar to the small orbits, it cannot be ruled out that this is an ontogenetic character since tooth number frequently increases in temnospondyl ontogeny from larvae to adults in the context of proportional snout elongation (see e.g., Witzmann 2005b). However, this possibility is regarded as unlikely by us because (1) Boy (1990) did not document any increase of premaxillary teeth during ontogeny of Onchiodon labyrinthicus, and (2) the other characters listed here argue against a larval or juvenile state of the specimens under study. Only the first two small teeth and the following middle-sized tooth have a circular cross-section. Teeth number four to six are the largest ones and possess a long-oval cross-section in labial-lingual direction, shared with Eryops megacephalus and O. labyrinthicus.

The maxilla has a slightly wider dorsal shelf than C. nocturnus and it is ventrally in contact with the quadratojugal. Its tooth arcade has only about 21 tooth loci in contrast to all other eryopids having 25–43 tooth loci. All maxillary teeth are much smaller than the largest premaxillary teeth but are similar in size to the smallest premaxillary teeth. The 3^rd posterior maxillary tooth is the largest one and causes a small lateral expansion of the skull margin, but this tooth is slightly smaller than the third tooth of the premaxilla. The maxillary teeth are mostly circular in cross-section with few exceptions on the right maxilla which possess a long-oval cross-section in labial-lingual direction.

The circular to oval shaped naris is relatively small as in O. labyrinthicus, its length comprising 8% of the midline length of the skull. The small septomaxilla is not sculptured (Fig. 7A, B) and lies ventrally directed inside the naris (shared with Eryops and C. nocturnus). The posterior margin of the naris is clearly formed by the nasal, lacrimal and maxilla (Fig. 9A).

The lacrimal is triangular with a wide posterior part (La_w/La_l=0.54). It is separated from the orbit by an elongated contact between jugal and prefrontal. The frontal is narrow like in most other eryopids and does not reach anteriorly to the level of the anterior ends of prefrontal and jugal. The jugal is wide (Ju_w/S_l=0.20) and proportionally only slightly narrower than in C. nocturnus. Therefore, the width of the skull at its midlength is similarly large in both species of Clamorosaurus (mS_w/S_l=0.98), comparable to O. labyrinthicus (mS_w/S_l=1.00). The postorbital is triangular in outline. The postfrontal and prefrontal clearly contact each other as in all eryopids. The prefrontal is anteriorly relatively narrow and extends further anterior than the frontal. The posteromedial part of the postfrontal is expanded. The supratemporal is much wider than long – a unique character in eryopids. The tiny parietals extend anterior to the level of the posterior orbital margin. Posteriorly, they do not reach the level of the posterior margin of the supratemporals. C. borealis bears a relatively short postparietal but an elongated tabular with a marked, slender tabular horn (Th_l/S_l=0.09). The cheek is narrower (W_w/S_l=0.26) than in C. nocturnus (Table 2). The squamosal and the quadratojugal are narrow. Similar to C. nocturnus, the quadratojugal reaches far posterior so that the quadrate condyle occupies a position (Qc_l/S_l=0.22) similar to E. megacephalus.

The dorsal exposure of the quadrate consists of a narrow, short process that is directed anteromedially between the squamosal, quadratojugal and the quadrate ramus of the pterygoid. Similar to C. nocturnus, a possible boss-like protuberance is developed at the ventral margin of the dorsal quadrate process (Fig. 8C). Two quadratojugal foramina are detectable close together in posterior view – the paraquadrate foramen and the accessory paraquadrate foramen (Figs 7B, 8C). Both foramina are rarely visible together in other eryopids. However, Cernansky et al. (2016) reported four internal foramina in the quadratojugal of Eryops, so that this feature complex is probably more variable than previously thought. Lateral line sulci are not present.

Palate and braincase. The palate is well preserved so that a reconstruction is possible (Fig. 9B). Longitudinal ridges on the palatal bones and traces of the anterior palatal fossae on the anterior part of the vomers are not preserved.

The vomer is elongated and relatively narrow. The smallest width of both vomers (=interchoanal width ICw/S_l=0.32) is wider than the smallest width between the narial openings (internarial width INw/S_l=0.27). In C. nocturnus both ratios are smaller and equal (0.22). The suture between vomer and palatine is much shorter than in C. nocturnus. The palatine is relatively narrow and elongated, much longer than wide, like in Actinodon and Syndyodosuchus, but in contrast to the short and wide palatine in C. nocturnus. The ectopterygoid is longer than wide and of almost equal length as the palatine. Its posteriormost part is narrower than the adjacent part of the pterygoid.

The palatal dentition corresponds to that of Syndyodosuchus and differs in a few characters from that of C. nocturnus. The palatine bears a larger fang anteriorly and a slightly smaller fang posteriorly. The large palatine fang is smaller than the largest teeth of the premaxilla. The ectopterygoid fangs are of nearly the same size as those from the palatine. The fangs on the vomer are of equal or smaller size than those on the ectopterygoid. One fang is located anteromedial to the anterior edge of the choanae on both vomers, at the same level next to the posteriormost premaxillary teeth. A further, somewhat smaller tooth locus with two fangs on both vomers is positioned medial to the choana at its mid-length. This fang pair is located on a prominent ridge which forms the posteromedial margin of the choana and almost reaches the anterior palatine fang. This second tooth locus on the vomer medial to the choana is only known from C. nocturnus and Syndyodosuchus tetricus (however, as described in the present study, only with one fang in these species).

The large choana is longer than wide and medially expanded; it is larger than the choana of C. nocturnus and S. tetricus.

The anteriormost part of the palatinal ramus of the pterygoid is relatively broad and blunt. The transverse flange of the pterygoid is greatly expanded into a right-angled projection, which is more pronounced than in C. nocturnus or Syndyodosuchus, but similar to Onchiodon and Actinodon. The palatinal ramus and the elongated basipterygoid ramus are strongly curved, leading to the great width of the interpterygoid vacuities, also in their anterior part. The orbitae are not obscured by the pterygoids in ventral view. Among eryopids, these characters are shared only with C. nocturnus (see above); additionally, the stereospondylomorph Intasuchus (Konzhukova 1956; Werneburg et al. 2020) presents similar features except for the anteriorly widened interpterygoid vacuities. Polygonal bony plates that covered the interpterygoid vacuities are not preserved.

The large bony plate lying between the basipterygoid ramus of the pterygoid plus parasphenoid on one side and the skull roof on the other side is interpreted as footplate of the epipterygoid (Fig. 8A, B). Remains of the ascending process are not preserved. A similar large footplate is known from Eryops (Sawin 1941: pl. 9; Schoch and Sobral 2021: fig. 6). Syndyodosuchus presents a similar large footplate (see description below and Fig. 11A, B).

The elongated transverse, rod-like basipterygoid process of the pterygoid overlapped the wide basipterygoid pocket of the parasphenoidal basal plate and might have formed a movable articulation (Figs 8A, B, 9B). The cultriform process of the parasphenoid is relatively narrow and much shorter than the vomer in contrast to all other eryopids. The sphenethmoid (Fig. 7C, D) is clearly wider (20 mm) than the cultriform process (5–6 mm). Its posterior part was probably attached to the underside of the skull roof in the mid-part of the interorbital region. It bears a longitudinal ridge in ventral view. The basal plate of the parasphenoid has a relatively wide rectangular shape like in Onchiodon but unlike C. nocturnus, Eryops and Syndyodosuchus. Its ventral surface has curved furrows for the carotid artery below the basipterygoid pockets, but their foramina lie more anteromedially near the pockets. A large denticle field is developed between these furrows, which has a triangular shape like in O. labyrinthicus. Numerous denticles are present on the vomer and on the palatinal branch of the pterygoid. The articular condyle of the quadrate is bilobed and transversely expanded. The posteromedial part of the quadrate bears a foramen near a narrow boss-like process. The basioccipital and the exoccipitals are not preserved.

Visceral skeleton. The stapes has a slender, elongated shaft without a quadrate process. It is proximally pierced by a stapedial foramen and has a wide footplate in which dorsal and ventral proximal heads can be well distinguished. In general morphology it is similar to Glaukerpeton (Werneburg and Berman 2012) and E. megacephalus (Sawin 1941), but much slenderer than in O. thuringiensis (Werneburg 2007).

The mandible is only partly preserved without details. No bones of the postcranial skeleton are available.

Syndyodosuchus tetricus Konzhukova, 1956.

As for species by monotypy.

Syndyodosuchus was interpreted as a basal stereospondylomorph by Konzhukova (1956); Gubin (1984) and all later workers (e.g. Shishkin et al. 2000 and Schoch and Milner 2000) followed this assignment. However, Syndyodosuchus bears the first four characters listed in the eryopid diagnosis (see above).

PIN 570/40, consisting of the skull in dorsal and palatal view (skull length 16.2 cm).

PIN 570/6, consisting of a right anterolateral skull edge; PIN 570/41, consisting of a right anterior dentary; PIN 570/2, consisting of a poorly preserved skull of about 17 cm length; PIN 570/3, consisting of an indeterminable bone.

The material of Syndyodosuchus was found together with that of Intasuchus near the ‘Greater Inta River’ in about 100 m depth in coal beds of a mine from the Ufimian Intinskaya Svita, late Kungurian (Cisuralian, Permian).

Autapomorphies: (1) Tabular tiny in length and width; (2) Internarial width is very large (INw/Sl=0.32); (3) Postorbital very narrow (Po_w/Po_l=0.54).

Synapomorphies with some of the eryopids: (1) Density of sculpture pattern quantified as the number of pits per in² on frontal plus jugal ranges between 4.69 and 5.00, shared with Glaukerpeton and Clamorosaurus, but differs from all other eryopids; (2) Lateral margin of skull roof is slightly concave to straight; (3) Both premaxillae form a relatively straight snout, like in E. megacephalus; (4) Snout margin is laterally constricted slightly below the level of naris; (5) Premaxilla with elongated and relatively wide alary process, shared with O. labyrinthicus; (6) Elongated interpremaxillary suture, shared with E. megacephalus; (7) Premaxillary, maxillary and dentary teeth are heterodont with a circular cross-section, in contrast to C. borealis, O. labyrinthicus, and Eryops, but shared with C. nocturnus, and others; (8) In the premaxilla, teeth number nine and ten are the largest teeth, in contrast to C. borealis; (9) In the maxilla, teeth number six to ten are the largest teeth, in contrast to C. borealis, and other eryopids; (10) Lacrimal is very narrow and long, three times longer than wide, only similar in Actinodon; (11) Different internarial and interorbital width, in contrast to C. nocturnus; (12) Small orbitae, similar in C. borealis; (13) Very short contact between jugal and prefrontal, shared with O. labyrinthicus; (14) No interfrontal, in contrast to Eryops and Osteophorus; (15) Supratemporal much longer than wide, shared with Onchiodon, Glaukerpeton and Actinodon; (16) No lateral line sulci, in contrast to Glaukerpeton and Actinodon; (17) Occipital margin of skull roof is well concave, shared with Glaukerpeton, O. labyrinthicus and E. megacephalus; (18) Elongated and relatively wide palatine, much longer than wide, shared with C. borealis and Actinodon, but in contrast to C. nocturnus; (19) Ectopterygoid and palatine of about equal length; (20) Palatinal ramus of pterygoid relatively narrow with poorly developed transverse flange, like in Glaukerpeton; (21) Short basipterygoid process of pterygoid; (22) Narrow interpterygoid vacuities, in contrast to Clamorosaurus; (23) Orbitae are partly covered by pterygoids in ventral view, in contrast to Clamorosaurus; (24) Cultriform process of the parasphenoid is longer than the vomer, in contrast to C. borealis; (25) Narrow basal plate of parasphenoid, shared with C. nocturnus and E. megacephalus; (26) Basal plate with foramina for carotid artery in ventral view; (27) Vomer with additional fang medial to the choana, shared with Clamorosaurus.

One skull with a median length of 16 cm preserves the skull roof in dorsal and the palate in ventral view (PIN 570/40). Two right dentaries from additional individuals of the same species with heterodont dentition complete the description (Fig. 12; PIN 570/6 and PIN 570/41).

General skull morphology. The dermal sculpture of the skull roof corresponds to the fine sculpture pattern known from certain eryopids such as Clamorosaurus, Syndyodosuchus and Glaukerpeton (Werneburg and Berman 2012) (Table 1). It consists of a reticulated pattern of small pits and valleys separated by narrow ridges on nearly all skull roofing bones (Fig. 10A, B). The density of the sculpture pattern is quantified as the number of pits per in² (6.452 cm²) on the frontal and jugal, and as a proportion of those counts to skull length. These intraspecific indices range between both elements of S. tetricus between 4.69 and 5.00. The dermal sculpture of the dorsal surface of the skull roof in other eryopid specimens is of much coarser sculpture pattern with indices from 0.4 up to 1.7, higher indices between 1.2 and 4.3 occur only in the subadult Eryops (Table 1). A dorsal strutting pattern with large ridges on the skull roof is not developed (Fig. 10).

S. tetricus and C. nocturnus have slightly thinner bones than the bones in other eryopids, whereas Glaukerpeton has 30^_50% thinner skull roofing bones (Werneburg and Berman 2012).

The well-preserved skull of S. tetricus allows a tentative reconstruction of the skull roof in dorsal view and of the palate in ventral view (Fig. 13). The skull is as wide as long (Table 2; pS_w/S_l=1.01). The lateral margin of the skull is slightly concave to straight in dorsal view. The snout margin is laterally constricted slightly below the level of naris but not so impressive as in E. megacephalus, Osteophorus and Clamorosaurus. The postorbital region of the skull roof is relatively long (H_l/S_l=0.24) and wide (H_w/S_l=0.45) (Table 2). The preorbital skull is relatively elongate (PO_l/S_l=0.60). The internarial width is very large (IN_w/S_l=0.32) and presents the largest ratio in eryopids (Table 2). The interorbital width (IO_w/S_l=0.23) is small like in Clamorosaurus, Actinodon and E. megacephalus. The occipital margin of the skull roof is well concave. The quadrate condyles lie distinctly posterior to the occipital condyles (Qc_l/S_l=0.14; Table 2) but not so wide posterior than in E. megacephalus or C. borealis. The long-oval orbits are the proportionally smallest ones compared to other eryopids (O_l/S_l=0.12).

Growth stage. The holotypic skull of Syndyodosuchus tetricus was an early adult animal, as indicated by the following features: (a) The dermal sculpture consists of a dense reticulated pattern of small pits and valleys separated by narrow ridges; (b) The quadrate condyles lie far posterior to the occipital condyles; (c) The quadrate is ossified dorsally; (d) The orbits are very small compared to other eryopids; (e) The pterygoid has a transverse process; (f) The epipterygoid is ossified with a large footplate. The skull length of 16 cm ranges in the middle-sized group of the family Eryopidae, and corresponds to Actinodon, C. borealis and O. labyrinthicus. However, the incomplete ossification of the occiput indicates that it was an early adult and did not reach the late adult stage.

Skull roof. The interpremaxillary suture is elongated and accounts for 13.1% of the midline length of the skull. The elongated and moderately wide alary process of the premaxilla is clearly detectable. Both premaxillae form a relatively straight snout like in E. megacephalus, but its lateral constriction is formed by the lateral margin of the maxilla posterior to the naris. The premaxilla has 10 or 11 tooth loci in its tooth arcade, like most other eryopids with the exception of Clamorosaurus which has less tooth loci (Table 3). All teeth of premaxilla, maxilla, dentary and palatal elements have a circular cross-section. The ninth and tenth teeth of the premaxilla are the largest ones. Premaxilla and maxilla have a heterodont dentition, with the size differences giving a wave-like profile of the tooth row in lateral view (Figs 11A, B, 12A–C).

The maxilla has a relatively narrow dorsal shelf and it is in contact ventrally with the quadratojugal. Its tooth arcade has 25 or 26 tooth loci. No maxillary tooth reaches the size of the largest premaxillary teeth, and the maxillary teeth are similar in size to the smallest premaxillary teeth. The sixth to tenth maxillary teeth are the largest ones but cause no lateral expansion of the skull margin.

The circular to oval naris is relatively small as in C. borealis and O. labyrinthicus, its length comprising 7% of the midline length of the skull. The septomaxilla is not recorded. The posterior margin of the naris is clearly formed by the nasal, lacrimal and maxilla (Fig. 13A).

The lacrimal is narrower (La_w/La_l=0.30; Table 2) and longer than the nasal, a pattern that is further present only in Actinodon among eryopids. It is separated from the orbit by a short contact between jugal and prefrontal. The frontal is narrow like in most other eryopids and reaches anteriorly to the level of the anterior ends of prefrontal and jugal. The jugal is relatively wide (Ju_w/S_l=0.14) and of equal width as the orbit at midlength. The left jugal constricts the orbit by a small medial expansion, whereas the orbital rim of the right jugal is not preserved in this part. The width of the skull at midlength is small (mS_w/S_l=0.86; Table 2). The postorbital is triangular in outline and much narrower than in all other eryopids (Po_w/Po_l=0.54, Table 2). The postfrontal and prefrontal clearly contact each other as in all eryopids. The prefrontal is anteriorly relatively wide and blunt (Fig. 13A). It reaches anteriorly only up to the anterior level of the frontals. The posteromedial part of the postfrontal is markedly expanded like in C. borealis. The supratemporal is 1.2–1.4 times longer than wide. It reaches posteriorly clearly below the level of the occipital condyle. The parietals extend anterior up to the level of the posterior orbital margin. They do not reach as wide posterior as the supratemporals. Syndyodosuchus bears a relatively short postparietal. The tabular is tiny – it is the proportionally smallest one in eryopids. A tabular horn is not developed. The cheek is relatively narrow (W_w/S_l=0.27; Table 2) and does not reach the relative width of the cheek in C. nocturnus or Onchiodon. Correspondingly, the squamosal and the quadratojugal are relatively narrow.

The dorsally exposed part of the quadrate between the squamosal and quadratojugal is very small, but its dorsoventral contact to the quadrate ramus of the pterygoid is extensive. Quadratojugal foramina are not visible. Lateral line sulci are not present.

Palate and braincase. The palate is well preserved and can be reconstructed (Fig. 13B). Longitudinal ridges on the palatal bones and traces of the anterior palatal fossae on the anterior part of the vomers are not preserved.

The vomers are elongated, posteriorly narrow and anteriorly widened. The smallest width of both vomers (= posterior interchoanal width ICw/S_l=0.30) is smaller than the smallest width between the narial openings (internarial width INw/S_l=0.32), but the anterior interchoanal width is equal to the internarial width. The suture between vomer and palatine is relatively short but longer than in C. borealis. The palatine is much longer than wide. The ectopterygoid is longer than wide and of almost equal length as the palatine. Its posteriormost part is wider than the neighbouring part of the pterygoid.

The palatal dentition corresponds to that of C. borealis. The palatine bears two relatively large fangs, one on the anterior and one on the posterior part. The palatine fangs are of similar size as the largest premaxillary teeth. The ectopterygoid bears a fang pair on its posterior part which is only slightly smaller than the fangs of the palatine. The fangs on the vomer are located on two separate tooth places and have the same size as the ectopterygoid fangs. One fang is located anterior to the choana (Fig. 12A, B). A further, somewhat larger tooth locus with one fang is positioned medial to the choana in its anterior half. This second tooth locus on the vomer, medial to the choana is otherwise only known from C. nocturnus and C. borealis.

The choana is longer than wide, but posteromedially expanded (Fig. 12A, B) like in Actinodon, C. nocturnus and E. megacephalus.

The palatinal ramus of the pterygoid is overall narrow. The transverse flange of the pterygoid is poorly developed like in Glaukerpeton. The palatinal ramus and the short basipterygoid ramus are only slightly curved and therefore, the interpterygoid vacuities are narrower than in Clamorosaurus and more similar to the vacuities of the other eryopids. The orbits are partially obscured by the pterygoids in ventral view, again in contrast to Clamorosaurus. Polygonal bony plates covering the interpterygoid vacuities are not preserved.

The large bony plate lying between the basipterygoid ramus of the right pterygoid and the skull roof is interpreted as footplate of the epipterygoid (Fig. 11A, B). Remains of the ascending process are not preserved. A similar large footplate is known from C. borealis (Fig. 8B) and Eryops (see above).

The short basipterygoid process of the pterygoid overlapped the wide basipterygoid pocket of the parasphenoidal basal plate and might have formed a movable articulation (Figs 11A, B, 13B). The cultriform process of the parasphenoid is narrow and longer than the vomer, like in all other eryopids with the exception of C. borealis. The basal plate of the parasphenoid has a narrow rectangular shape; its ventral surface has no clear furrows but foramina for the carotid artery between the basipterygoid pockets and the denticle field in ventral view (Fig. 11A). The large denticle field starts widened on the anterior part of the basal plate and extends anteriorly between the carotid foramina up to the base of the cultriform process. This anterior field is not clearly triangular or rounded. Further denticles are only preserved on the palatinal branch of the pterygoid. The articular condyle of the quadrate is well bilobed and transversely expanded. The posteroventral part of the quadratojugal bears a narrow bridge whose posterior boss-like end sutures with the quadrate. This bony bridge is pierced from posterolateral to anteromedial by the paraquadrate foramen (Fig. 11A). The sphenethmoid, basioccipital and exoccipitals are not recorded. The stapes of the visceral skeleton is only preserved with a small part of the shaft (Fig. 11A, B).

Mandible. The anterior part of an isolated right dentary has a heterodont dentition with a wave-like profile of the tooth series. One large symphyseal tooth and a possible tooth place are preserved (Fig. 12E).