Systematics of the Rubidgeinae (Therapsida: Gorgonopsia)

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Introduction

Materials

Institutional abbreviations

Systematic paleontology

Therapsida Broom, 1905

Gorgonopsia Seeley, 1894

Gorgonopidae Lydekker, 1890

Rubidgeinae Broom, 1938

Description

Species accounts

Aelurognathus Haughton, 1924

Aelurognathus tigriceps (Broom & Haughton, 1913) (Reconstruction Figs. 12, Specimen Figs. 515)

Clelandina Broom, 1948

Clelandina rubidgei Broom, 1948 (Reconstruction Figs. 1718, Specimen Figs. 1923)

Dinogorgon Broom, 1936

Dinogorgon rubidgei Broom, 1936 (Reconstruction Figs. 2425, Specimen Figs. 2631)

Leontosaurus Broom & George, 1950

Leontosaurus vanderhorsti Broom & George, 1950 (Reconstruction Figs. 3233, Specimen Figs. 3, 3437)

Rubidgea Broom, 1938

Rubidgea atrox Broom, 1938 (Reconstruction Figs. 3839, Specimen Figs. 4048)

Ruhuhucerberus Maisch, 2002

Ruhuhucerberus haughtoni (Huene, 1950) comb. nov. (Reconstruction Figs. 4950, Specimen Figs. 5153)

Smilesaurus Broom, 1948

Smilesaurus ferox Broom, 1948 (Reconstruction Figs. 5455, Specimen Figs. 5661)

Sycosaurus Haughton, 1924

Sycosaurus laticeps Haughton, 1924 (Reconstruction Figs. 6263, Specimen Figs. 6467)

Sycosaurus nowaki (Broili & Schröder, 1936) comb. nov. (Reconstruction Figs. 6869, Specimen Figs. 4, 7073)

Phylogenetic analysis

Discussion

Rubidgeine higher-level systematics

Ecology of rubidgeines

Biostratigraphy and biogeography of rubidgeines

Conclusions

Supplemental Information

Datafile for phylogenetic analysis.

Data file (TNT format) for phylogenetic analysis.

DOI: 10.7717/peerj.1608/supp-1

Appendix

Character list

  1. Ascending process of premaxilla: (0) terminates posterior to the canine; (1) terminates anterior to the canine. A lengthy ascending (also known as the dorsal or internasal) process of the premaxilla is ancestral for therapsids (Hopson & Barghusen, 1986). In gorgonopsians, however, this character reverts to the pre-therapsid condition: in all known gorgonopsians the premaxilla has a relatively short ascending process.

  2. Posterior margin of palatal premaxillary surface: (0) gently rounded; (1) invaginated. In most rubidgeines, there is a deep invagination in the posterior border of the premaxilla between its main body and vomerine process. This invagination is not solely a side-effect of anterior expansion of the vomer (although it is exaggerated by it), as it is also present in some taxa which lack such expansion (e.g., Arctognathus (see Kammerer, 2015)).

  3. Vomerine process of premaxilla: (0) medially short, almost totally obscured by vomer in ventral view at midline; (1) long at midline, comprising a substantial portion of the expanded interchoanal body in ventral view. In biarmosuchians, a vomerine process of the premaxilla is present (sheathing the vomer laterally) but is not well-exposed ventrally at the midline: the vomer in these taxa nearly abuts the main body of the premaxilla. In gorgonopsians, by contrast, the vomerine process of the premaxilla is a major contributor to the expanded interchoanal body (i.e., the transversely broad median structure made up of the anterior part of the vomer and the vomerine process of the premaxilla), often making up as much of the midline length of this structure as the vomer. In gorgonopsians where the vomerine process of the premaxilla makes up significantly less of the interchoanal body length than the vomer, it is only because the vomer is expanded for more of its length (as in Arctognathus or Sycosaurus)—the premaxilla is still longer in these taxa than in biarmosuchians.

  4. Median vomerine ridge: (0) absent; (1) present.

  5. Lateral vomerine ridges: (0) absent; (1) present. The vomer in gorgonopsians is always unpaired, with a characteristic ‘triple ridge’ morphology. A pair of lateral ridges originate at the point where the interchoanal body of the vomer starts to expand; these ridges extend anteriorly, often continuing onto the vomerine process of the premaxilla. The median ridge typically originates at a similar position, although it is located more anteriorly in some taxa (e.g., Sycosaurus). Although these three ridges are present in all gorgonopsians in which the vomer is known, this morphology is here split into two characters because of the variable condition in the biarmosuchian outgroups. No vomerine ridges of any kind appear to be present in Biarmosuchus, but Hipposaurus exhibits a tall, distinct median vomerine ridge (most clearly visible in SAM-PK-K252). Most other biarmosuchians exhibit lateral vomerine ridges but not a median one (e.g., Bullacephalus jacksoni (BP/1/5387), Herpetoskylax hopsoni (CGP/1/67), Lemurosaurus pricei (NMQR 1702), Lobalopex mordax (CGP/1/61), Lophorhinus willodenensis (SAM-PK-K6655), Lycaenodon longiceps (NHMUK R5700), or Paraburnetia sneeubergensis (SAM-PK-K10037)). Given the poor preservation of the palate in known specimens of Hipposaurus, it is possible that this genus had the full gorgonopsian complement of three ridges. It is worth noting, however, that the lateral ridges in biarmosuchians are very thin, laminar structures, more like ventral deflection of the vomerine edge than a separate ridge (similar to the ‘scroll-like’ vomerine morphology of anteosaurs (Kammerer, 2011)). By contrast, in gorgonopsians these ridges are relatively robust, discrete structures.

  6. Expanded interchoanal body shape: (0) elongate and relatively narrow throughout its length, making up nearly all of interchoanal portion of vomer; (1) lobate, with anterior and posterior expansions; (2) elongate but tapering, with broad anterior terminus; (3) bulbous, with broad anterior expansion. Vomerine morphology is remarkably diverse within gorgonopsians, a fact that has traditionally been obscured by poor preparation. The ancestral condition, present in biarmosuchians, is to have nearly the entire interchoanal portion of the vomer be expanded, as opposed to the typical condition in gorgonopsians where the interchoanal vomer is made up of an extremely narrow, rod-like posterior portion and a transversely expanded anterior portion. In biarmosuchians this structure may become slightly wider anteriorly, but is generally quite narrow throughout (in comparison to the condition in rubidgeines). The biarmosuchian vomerine morphology seems to be retained in Eriphostoma (based on CT-scans of AMNH FARB 5524 (Kammerer, 2014)) and is also present (probably homoplastically) in Arctognathus (Kammerer, 2015). Gorgonops has an unusual vomerine morphology (described here as ‘lobate’) in which the vomer has double expansions: a posterior one, then a constriction, then an anterior one that becomes confluent with the vomerine process of the premaxilla. The posterior vomerine expansion in Gorgonops is also associated with exaggerated development of the lateral vomerine ridges. This character state represents an autapomorphy of Gorgonops in the current analysis, but it is also present in other gorgonopsians, such as the Russian taxon Sauroctonus progressus (Tatarinov, 1974). In Sycosaurus and to a lesser extent Smilesaurus the interchoanal portion of the vomer is divided into a rod-like posterior and expanded anterior region (unlike biarmosuchians), but expansion begins in a relatively posterior position (compared to other rubidgeines) and proceeds gradually. By contrast, in the remaining rubidgeine taxa the expansion of the vomer is very abrupt and occurs relatively far forward, resulting in a typically ‘bulbous’ anterior interchoanal body.

  7. Vomerine-pterygoid contact: (0) present; (1) absent. The presence of a midline palatine suture excluding the vomer from contacting the pterygoids has long been recognized as a key autapomorphy of Gorgonopsia (Hopson & Barghusen, 1986; Sigogneau-Russell, 1989).

  8. Dentition on palatine boss: (0) extensive; (1) elongate single row; (2) a few teeth in a restricted position. ORDERED. The primitive condition for synapsids is to have extensive palatal dentition (Reisz, 1986). Biarmosuchians typically retain broad patches of teeth across the palatine and pterygoid bosses: in Biarmosuchus, each palatine and pterygoid boss bears over fifty densely packed teeth. The palatal dentition is reduced to varying degrees in all of the ‘advanced’ groups of therapsids: in gorgonopsians, the palatine dentition usually takes the form of an elongate single row of teeth. In the earliest gorgonopsians (represented by Eriphostoma and Gorgonops in the current analysis), this row extends along both the lateral and medial sides of the palatine boss, whereas in later gorgonopsians it extends only along the lateral side. In a few rubidgeines (Clelandina, Rubidgea, and Smilesaurus) the palatine dentition is extremely reduced, with only one to three teeth in a small patch at the anterior edge of the boss. This character is treated as ordered, as state 2 just represents an increase in tooth reduction from the previous states.

  9. Palatine boss shape: (0) delta-shaped; (1) reniform. The palatine bosses of biarmosuchians are large, triangular or delta-shaped structures with extensive dentition. In the early gorgonopsians Eriphostoma and Gorgonops, the palatine dentition is reduced relative to that of biarmosuchians, but the overall shape of the boss is retained. In later gorgonopsians, the boss is reduced in size and typically is ‘bean’-shaped or reniform.

  10. Pterygoid palatal boss: (0) discrete structure distinct from palatine boss; (1) thin ridge extending posteriorly from palatine boss. Ancestrally in therapsids, the palatine boss and pterygoid palatal boss are dentigerous structures of nearly equivalent size, separated by a weak trough. In gorgonopsians, the palatal boss of the pterygoid is always smaller than the palatine boss, but for the most part is still a discrete, dentigerous structure. In the rubidgeines Clelandina, Dinogorgon, Leontosaurus, Rubidgea, Ruhuhucerberus, and Sycosaurus, however, the palatal pterygoid boss is reduced to a thin ridge extending from the back of the palatine boss towards the midpoint between the transverse processes.

  11. Dentition on palatal boss of pterygoid: (0) extensive; (1) reduced; (2) absent. ORDERED. As noted above, biarmosuchians have large palatal bosses of the pterygoids with extensive dentition. As for the palatine boss, the amount of pterygoid dentition is reduced in gorgonopsians, and typically constitutes a small, circular patch of three to six teeth. In most rubidgeines the pterygoid dentition is reduced even further, but this character state does not entirely overlap with Character 10. In most gorgonopsians where the palatal boss of the pterygoid is reduced to a thin ridge, it is also edentulous, but Ruhuhucerberus exhibits a thin, ridge-like palatal pterygoid boss that clearly still bears teeth. This character is ordered, following the same logic as Character 8.

  12. Dentition on transverse process of pterygoid: (0) present; (1) absent. Teeth on the transverse process of the pterygoid are absent in many adult gorgonopsians (although they may be present in juveniles). In the current analysis, they are coded as present only in Eriphostoma, Gorgonops, and the biarmosuchian outgroups.

  13. Parasphenoid morphology: (0) broad parasphenoid with edges separated by a narrow median groove; (1) parasphenoid rostrum forming tall, narrow blade. A blade-like parasphenoid rostrum is characteristic of gorgonopsians. In rubidgeines, however, there is a reversal to the pre-gorgonopsian morphology, with loss of the ‘blade’ and presence of an elongate groove between the edges.

  14. Parasphenoid length: (0) short (<20% basal skull length); (1) long (>20% basal skull length). The parasphenoid and basisphenoid are fused in gorgonopsians to form a parabasisphenoid. For the purposes of this character, parasphenoid length is taken as the length between the parasphenoid-pterygoid suture and the anterior edges of the basal tubera (which are formed by the basisphenoid anteriorly and basioccipital posteriorly). In ‘pelycosaurs’ and biarmosuchians, the braincase is relatively short, with the distance between the transverse processes of the pterygoids and the basal tubera being less than 20% of the basal skull length. In gorgonopsians, the parasphenoid rostrum is typically elongate, making up more than 20% of the basal skull length. Reversal to a short parasphenoid is observed in some rubidgeines, however, including Clelandina, Dinogorgon, Leontosaurus, Rubidgea, and Sycosaurus. Shortening of the parasphenoid in gorgonopsians is correlated with increasing length of the basal tubera (so that is not included as a distinct character). An exceptionally short parasphenoid and elongate basal tuber is present in Smilesaurus.

  15. Transverse lamina of septomaxilla: (0) absent; (1) present, bifurcating naris into dorsal and ventral compartments visible in lateral view. A foramen between the septomaxilla and maxilla within the naris (usually visible in anterior view) is broadly present in early therapsids, but the main body of the septomaxilla remains tightly appressed to the maxilla overall. In gorgonopsians, the septomaxilla produces a transverse lamina set well above the dorsal edge of the maxilla, which divides the external naris into dorsal and ventral portions in lateral view.

  16. Long axis of facial process of septomaxilla: (0) sloping posterodorsally; (1) subhorizontal. In biarmosuchians and most gorgonopsians, the facial process of the septomaxilla (which extends between the nasal and maxilla) is angled upwards, tapering posterodorsally. In Clelandina, Dinogorgon, and Rubidgea, however, this process extends more posteriorly, with limited dorsal angulation.

  17. Snout width: (0) equal or greater posterior to canine as across canine; (1) narrower posterior to canine than across canine. Taxa with large canines (as in many early therapsids, and gorgonopsians in particular) necessarily have swollen maxillae to house the roots of these enlarged teeth, typically resulting in a broad snout relative to similar taxa in which the canines are small or absent. In most of these taxa, however, the transverse length between the canines is nevertheless not the broadest point in the snout: the snout either continues to expand posteriorly or remains equally broad. The rubidgeines Clelandina, Dinogorgon, Leontosaurus, and Rubidgea are exceptions, however: in these taxa the snout is distinctly constricted posterior to the canines, before expanding again anterolateral to the orbits.

  18. Maxillary emargination: (0) absent; (1) present. In nearly all gorgonopsians, the postcanine-bearing region of the maxilla is somewhat inset from the zygomatic arch, but this separation is typically minor. In some taxa, however, there is a distinct maxillary emargination in which the lateral surface of the maxilla is deeply concave and situated strongly medial to the zygoma. This is developed to the most extreme degree in the taxa which have lost most or all of their postcanines (Clelandina and Rubidgea) but is also present in Dinogorgon and Leontosaurus (indicating that it is not solely a correlate of tooth loss). Outside of rubidgeines, a well-developed maxillary emargination is also present in Eriphostoma (Kammerer et al., 2015).

  19. Lacrimal foramen: (0) confined to orbit; (1) also exits on lateral surface of lacrimal. In biarmosuchians and most gorgonopsians a single lacrimal foramen occurs on the posterior surface of the lacrimal, within the orbital rim. In Clelandina, Dinogorgon, and Rubidgea, however, there is a large, second lacrimal foramen exiting onto the facial surface of the lacrimal.

  20. Frontal contribution to orbital margin: (0) present; (1) absent. In therapsids, the ancestral condition is to have the orbit bordered dorsally by the prefrontal, frontal, and postfrontal. In rubidgeines (with the exception of Smilesaurus), the prefrontal and postfrontal are expanded and contact each other, excluding the frontals from the orbital margin (although it can still make up part of the dorsomedial orbital wall). Prefrontal-postfrontal contact is weakly developed (and may be intraspecifically variable) in Aelurognathus and Ruhuhucerberus, but is extremely well developed in the remaining rubidgeines, with the frontals broadly separated from the orbits dorsally.

  21. Interorbital ridge: (0) absent or weakly-developed; (1) well-developed. A median interorbital ridge is present in many early therapsids (see, e.g., Kammerer, 2011), but the condition in gorgonopsians is a special case. In nearly all gorgonopsians there is a span near the mid-length of the frontals with an increased degree of interdigitation of the mid-frontal suture. In some taxa this is expanded into a low boss. In the rubidgeines Clelandina, Dinogorgon, Rubidgea, and Ruhuhucerberus this span forms a well-developed, elongate ridge extending near the anterior margin of the orbits.

  22. Cranial pachyostosis: (0) absent; (1) restricted to thickened edges of the orbital and temporal margins; (2) extensive, lateral margins of prefrontal, postfrontal, and postorbital form swollen, rugose bosses. Pachyostosis of the skull occurs independently in several therapsid clades, with particularly extreme cases in burnetiamorphs (Rubidge & Sidor, 2002) and dinocephalians (Boonstra, 1969). Among gorgonopsians, cranial pachyostosis is only present in rubidgeines. Pachyostosis is relatively weakly developed in Aelurognathus, Ruhuhucerberus, and Sycosaurus, taking the form of a thickened dorsal rim of the orbit and anterior edge of the temporal fenestra. The condition in Leontosaurus is similar, although in that taxon a thicker boss is present at the posterodorsal corner of the orbit. In Clelandina, Dinogorgon, and Rubidgea, however, pachyostosis is taken to an extreme degree comparable to that of anteosaurine dinocephalians (Kammerer, 2011): large supraorbital and postorbital bosses are present and the bone surface is highly rugose.

  23. Preparietal: (0) absent; (1) present. A small median preparietal bone, situated anterior to the pineal boss, is present in anomodonts, gorgonopsians, and biarmosuchians (absent in Biarmosuchus itself, but present in Hipposaurus) among therapsids. Loss of the preparietal has occurred in all of those groups; in gorgonopsians the preparietal is absent in Arctognathus (Kammerer, 2015) and rubidgeines. A preparietal is absent in some specimens of Aelurognathus, Ruhuhucerberus, and Smilesaurus, but clearly present in other (probably subadult) specimens, and may fuse late in ontogeny in those taxa. All known specimens of Clelandina, Dinogorgon, Leontosaurus, Rubidgea, and Sycosaurus (including likely juveniles for all of these genera) lack a preparietal.

  24. Postorbital bar: (0) unexpanded; (1) expanded (>10% of basal skull length); (2) greatly expanded (>20% of basal skull length). ORDERED. Anteroposterior expansion of the postorbital bar relative to other gorgonopsians is present in all rubidgeines but Smilesaurus. In most of these taxa, the postorbital bar remains transversely narrow, but is broadened laterally such that it exceeds 10% of the basal skull length. In Clelandina, Dinogorgon, Leontosaurus, and Rubidgea, the postorbital bar is extremely expanded (greater than 20% of skull length) laterally and also thickened transversely, often augmented by the presence of postorbital rugosities or bosses.

  25. Facial portion of jugal: (0) confluent with suborbital zygomatic portion; (1) depressed relative to zygomatic portion. The facial portion of the jugal often has a concave surface in gorgonopsians, but usually this depression smoothly attenuates onto the zygomatic arch. In a few taxa (Clelandina, Dinogorgon, Rubidgea, and Ruhuhucerberus), however, there is a sharp break in slope between the zygoma and a deeply depressed facial portion of the jugal.

  26. Jugal height below postorbital bar: (0) tall (>10% of basal skull length); (1) short (5–7% of basal skull length). The jugal contribution to the zygomatic arch is slightly constricted beneath the postorbital bar in most gorgonopsians, but this is taken to an extreme in the two species of Sycosaurus. In these taxa, there is a deep ventral concavity in the zygoma.

  27. Subtemporal bar angle: (0) straight; (1) deflected (>20° from long axis of skull); (2) strongly deflected (>45° from long axis of skull). ORDERED. Ventral deflection of the subtemporal bar is present in all rubidgeines and the non-rubidgeine gorgonopsian Lycaenops, but the angle of deflection is usually weak (20–30°). In Clelandina, Dinogorgon, Leontosaurus, and Rubidgea, however the bar is deflected to an extreme degree, exceeding 45° relative to the long axis of the skull.

  28. Subtemporal bar width: (0) narrow; (1) transversely expanded. In addition to being dorsoventrally expanded in the majority of rubidgeines, the subtemporal bar is massive, with significant transverse expansion, in Clelandina, Dinogorgon, Leontosaurus, and Rubidgea.

  29. Zygomatic process of squamosal: (0) terminates under temporal fenestra; (1) terminates under postorbital bar. The zygomatic ramus of the squamosal usually terminates near the midpoint of the temporal fenestra in gorgonopsians, even in taxa in which the postorbital bar is anteroposteriorly expanded (e.g., Aelurognathus, Sycosaurus). In Arctognathus and Ruhuhucerberus, this ramus is unusually long, reaching the level of the postorbital bar. In Clelandina, Dinogorgon, Leontosaurus, and Rubidgea, the squamosal curves anterodorsally, such that it also reaches the postorbital bar.

  30. Zygomatic ridge: (0) absent; (1) present. In Clelandina, Dinogorgon, Leontosaurus, and Rubidgea, a thickened ridge runs from the tip of the squamosal up the jugal, ending beneath the posteroventral edge of the orbit.

  31. Zygomatic boss: (0) absent; (1) present. A massive, rounded boss at the ventral edge of the subtemporal bar is present in Clelandina, Dinogorgon, Leontosaurus, and Rubidgea among gorgonopsians.

  32. Squamosal sulcus: (0) extends laterally onto subtemporal bar; (1) restricted to occiput. The squamosal sulcus is homologous with the external auditory meatus of mammals. In most gorgonopsians it extends laterally, extending onto the zygomatic arch. The degree of lateral coverage is variable, but is almost always visible at least in part in lateral view. The exceptions to this are Clelandina, Dinogorgon, Leontosaurus, and Rubidgea, in which the sulcus is restricted to the occiput.

  33. Subtemporal bar lateral margin: (0) smoothly rounded; (1) with concavity between squamosal and jugal. A distinct concavity between the squamosal and jugal is visible in dorsal view in Clelandina, Dinogorgon, Leontosaurus, and Rubidgea. This concavity is not simply a result of the zygomatic ridge, as it is present even in subadult specimens where this ridge is not yet developed (e.g., RC 101).

  34. Temporal fenestra: (0) taller than wide; (1) wider than tall. A narrow temporal fenestra is ancestral for therapsids and is present in all biarmosuchians. In most gorgonopsians, the temporal fenestra is anteroposteriorly expanded by comparison. In the rubidgeines Clelandina, Dinogorgon, Leontosaurus, and Rubidgea, this fenestra reverts to being relatively short. Although accentuated by cranial pachyostosis and expansion of the postorbital bar, once again subadult or juvenile specimens illustrate that even before extreme expansion of the postorbital bar occurs, the fenestra is relatively short (e.g., RC 101).

  35. Squamosal contribution to occiput: (0) narrower than tabular; (1) broader than tabular. The squamosal is a dorsoventrally elongate but transversely narrow contributor to the occiput in biarmosuchians, such that it is thinner in posterior view than the tabular. In gorgonopsians, the squamosal portion of the occiput is greatly expanded, and is always transversely broader than the tabular.

  36. Parietal midline: (0) consists entirely of pineal boss; (1) extends beyond pineal boss. The parietal is extremely anteroposteriorly short in ‘pelycosaurs’ (Reisz, 1986), and similar proportions are retained in biarmosuchians. Indeed, in biarmosuchians the midline of the parietal only encompasses the pineal boss. Gorgonopsians have relatively short parietals compared to other ‘advanced’ therapsid clades, but they are still significantly longer than in biarmosuchians, extending beyond the pineal boss in all known taxa.

  37. Posterior process of parietal: (0) elongate, extending between tabular and squamosal; (1) confined to skull roof. An elongate, tapering posterior process of the parietal makes up part of the occiput in biarmosuchians and most gorgonopsians, extending between the tabular and squamosal. In Clelandina, Dinogorgon, Leontosaurus, Rubidgea, and Ruhuhucerberus, the parietal terminates dorsal to the interparietal, being effectively restricted to the skull roof.

  38. Occipital height: (0) tall (greater or equal to 50% of maximum occipital width, measured from lateral edge of squamosal); (2) short and broad (less than 40% of maximum occipital width). A relatively low, broad occiput, with prominent transverse expansion of the squamosals, is present in the majority of rubidgeines, with only Smilesaurus and Aelurognathus exhibiting a tall, ‘boxy’ occiput. Among non-rubidgeine gorgonopsians, a low, broad occiput is also present in Gorgonops.

  39. Dorsal edge of tabular: (0) narrow, tapering tip; (1) broad. The tabular remains fairly broad throughout its height in biarmosuchians, whereas in gorgonopsians the dorsal portion of the tabular is elongate, strongly tapering upwards. A reversal to the primitive condition occurs in various rubidgeines, however, including Clelandina, Dinogorgon, Leontosaurus, Rubidgea, and Ruhuhucerberus.

  40. Supraoccipital shape: (0) height roughly half of width; (1) very low (less than third of width) and ribbon-like. The supraoccipital is generally roughly rectangular in biarmosuchians and gorgonopsians, but in most rubidgeines is extraordinarily low and broad, such that it forms a ribbon-like structure in posterior view.

  41. Mandibular ramus morphology: (0) straight; (1) sinusoidal. The mandibular ramus in biarmosuchians is essentially straight, with only minor curvature between the symphysis and point of articulation. Gorgonopsians, by contrast, have a distinctly sinusoidal angulation to the jaw in ventral view, curving around the canine and the transition between the dentary and postdentary bones.

  42. Splenial process: (0) absent; (1) present. The mandibular symphysis of gorgonopsians is very robust compared with that of biarmosuchians and therocephalians. In most gorgonopsians, the posteroventral tip of the symphysis (i.e., the midline suture of the splenials) forms a thickened, posteriorly-directed process. This process is particularly well-developed in Clelandina and Rubidgea.

  43. Dentary postcanines: (0) present; (1) absent. Lower postcanine dentition is completely absent in Clelandina, Leontosaurus, and Rubidgea.

  44. Coronoid process of dentary: (0) tightly appressed to dorsal margin of mandible; (1) free-standing process. Separation of the dentary coronoid process from the main body of the mandible is a synapomorphy of Theriodontia: a free-standing process is present in gorgonopsians and therocephalians, but not biarmosuchians.

  45. Lateral surface of reflected lamina: (0) lobate sculpturing; (1) well-developed dorsoventrally-oriented bar, with weakly-developed crossbar. The major therapsid clades each display a unique morphology of the reflected lamina. In gorgonopsians this takes the form of a vaguely cruciate pattern, with a well-developed, dorsoventrally-oriented bar above a weaker anteroposteriorly-oriented one.

  46. Dorsal edge of reflected lamina: (0) free; (1) attached to angular body. The reflected lamina is typically attached to the main body of the angular at its anterior edge in therapsids. In gorgonopsians, it is more extensively attached, such that both the anterior and dorsal margins are attached to the rest of the mandible.

  47. Reflected lamina position: (0) immediately adjacent to the point of jaw articulation; (1) broadly separated from point of jaw articulation by the body of the angular. In biarmosuchians, the reflected lamina is so close to the articular that it almost overlaps the jaw articulation laterally. Gorgonopsians differ from this in having a broad lateral exposure of the main body of the angular, separating the reflected lamina from the articular.

Character matrix

Additional Information and Declarations

Competing Interests

The authors declare that they have no competing interests.

Author Contributions

Christian F. Kammerer analyzed the data, wrote the paper, prepared figures and/or tables.

Data Deposition

The following information was supplied regarding data availability:

The research in this article did not generate any raw data beyond the phylogenetic character matrix, which is included in the current contribution both within the text and as a supplemental file.

This publication was registered for an LSID: urn:lsid:zoobank.org:pub:B3A78FFF-D388-413C-B9E9-1AFD7BEC195E.

Funding

Support for my research was provided by the Deutsche Forschungsgemeinschaft (KA 4133/1-1), the Museum für Naturkunde (Berlin), a Gerstner-Kalbfleisch fellowship at the American Museum of Natural History and Richard Gilder Graduate School, and a Sofja Kovalevskaja award from the Alexander von Humboldt Foundation (donated by the German Federal Ministry for Education and Research) to Jörg Fröbisch. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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