Syndyodosuchus tetricus Konzhukova, 1956

Werneburg, Ralf & Witzmann, Florian, 2024, The last eryopids: Clamorosaurus and Syndyodosuchus from the late Kungurian (Cisuralian, Permian) of Russia revisited, Fossil Record 27 (3), pp. 353-380 : 353-380

publication ID	https://doi.org/ 10.3897/fr.27.e125460
publication LSID	lsid:zoobank.org:pub:09729723-9CEC-4FBF-B0B1-FB8103534379
DOI	https://doi.org/10.5281/zenodo.14579670
persistent identifier	https://treatment.plazi.org/id/83287D7D-BCCF-59C0-ADAA-226C43DE935F
treatment provided by	by Pensoft
scientific name	Syndyodosuchus tetricus Konzhukova, 1956
status

Treatment

Figs 10 View Figure 10 , 11 View Figure 11 , 12 View Figure 12 , 13 View Figure 13 , 14 E, F View Figure 14

Holotype.

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

Other material from different individuals.

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.

Occurrence.

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).

Diagnosis.

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 in 2 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 .

Comparative description.

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 View Figure 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 View Table 1 ). It consists of a reticulated pattern of small pits and valleys separated by narrow ridges on nearly all skull roofing bones (Fig. 10 A, B View Figure 10 ). The density of the sculpture pattern is quantified as the number of pits per in 2 (6.452 cm 2) 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 View Table 1 ). A dorsal strutting pattern with large ridges on the skull roof is not developed (Fig. 10 View Figure 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 View Figure 13 ). The skull is as wide as long (Table 2 View 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 View 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 View 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 View 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 View 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 11 A, B View Figure 11 , 12 A – C View Figure 12 ).

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. 13 A View Figure 13 ).

The lacrimal is narrower (La w / La l = 0.30; Table 2 View 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 View Table 2 ). The postorbital is triangular in outline and much narrower than in all other eryopids (Po w / Po l = 0.54, Table 2 View Table 2 ). The postfrontal and prefrontal clearly contact each other as in all eryopids. The prefrontal is anteriorly relatively wide and blunt (Fig. 13 A View Figure 13 ). 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 View 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. 13 B View Figure 13 ). 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. 12 A, B View Figure 12 ). 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. 12 A, B View Figure 12 ) 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. 11 A, B View Figure 11 ). Remains of the ascending process are not preserved. A similar large footplate is known from C. borealis (Fig. 8 B View Figure 8 ) 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 11 A, B View Figure 11 , 13 B View Figure 13 ). 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. 11 A View Figure 11 ). 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. 11 A View Figure 11 ). 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. 11 A, B View Figure 11 ).

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. 12 E View Figure 12 ).