A new therocephalian (Gorynychus masyutinae gen. et sp. nov.) from the Permian Kotelnich locality, Kirov Region, Russia

Nomenclatural acts

The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers (LSIDs) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:CA4D73A1-8FA7-40DD-A464-621AC01421B6. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS.

Cranium

The cranium of KPM 346 is 173 mm in standard basal length (from anteroventral edge of premaxilla to posteroventral edge of occipital condyle) and 208 mm in total dorsal length (from tip of snout to edge of temporal fenestra).

The palatal portion of the premaxilla is not exposed in the holotype because of occlusion of the lower jaw. The only information available concerning the ventral surface of the premaxilla comes from the edges of the incisor alveoli. Based on this, Gorynychus appears to have had an upper incisor count of five. Although no incisors are preserved in place, two isolated teeth (KPM 348 and 349) preserved in association with the skull have root proportions identical to the empty alveoli and are here interpreted as the upper incisors. Only one of these teeth (KPM 348) preserves an intact crown (Fig. 2C). The crown of KPM 348 is elongate, triangular, and weakly recurved with prominent mesiodistal serrations forming distinct denticles. The morphology of these denticles is unique among therocephalians: they are extremely prominent, extend from the tip of the crown right to the root, and each individual denticle is curved in the apical direction. The only comparable morphology among therocephalians occurs in the postcanines of this same specimen. The cutting edges of KPM 348 are heavily worn, but this wear is asymmetrical. As shown in Fig. 2C, one side of the tooth (it is uncertain whether this is the mesial or distal side) has more of its margin worn down than the other. The same style of asymmetrical wear, but with the sides reversed, is present on KPM 448/1 (Fig. 11D). The isolated incisor of KPM 291, by contrast, is relatively unworn (Fig. 11C). It is uncertain whether this is simply due to recent eruption before the death of the animal or from occupying a different position in the tooth row (its crown is shorter and slightly more curved than in KPM 348 or KPM 448/1, which could be because it represents a more posterior tooth position or a lower incisor).

The facial surface of the premaxilla is overlain by the maxilla posteriorly, near the point between the alveoli for I4 and I5 (Figs. 2, 4 and 5). The bone surface of the premaxilla is rugose, with a series of well-developed foramina located 3–4 mm above the alveolar margin. Anteriorly, three foramina are situated in a distinct channel that originates vertically beneath the internarial bar then curves posterolaterally (Fig. 2B). The first and third foramina in this channel are small (<1 mm diameter), but the second is large (1 mm diameter) and situated deep inside the groove. Posterior to this channel, several additional foramina are present on the lateral face of the premaxilla. The internarial bar is angled somewhat anteriorly, such that it overhangs the alveolar portion of the premaxilla in lateral view (Fig. 4). The ascending ramus of the premaxilla frames the anterodorsal border of the external naris and terminates near the posterodorsal narial border.

The septomaxilla consists of a broad ventral plate making up the base of the external naris, a constricted intranarial portion, and a facial process extending between the maxilla and nasal (Figs. 2, 4 and 5). The ventral plate of the septomaxilla is situated immediately dorsal to the premaxillary–maxillary suture. Immediately ventral to this plate is a large, ovoid foramen (1.5 mm diameter) that spans the premaxillary–maxillary suture. A weak groove extends anterior to this foramen for the length of the ventral plate of the septomaxilla. The constricted intranarial portion of the septomaxilla separates the main portion of the external naris from the maxillo-septomaxillary foramen. It has a pointed, anteromedially-directed anterior process as is typical of therocephalians, but not an expanded transverse lamina as in gorgonopsians. Posterodorsally, the septomaxilla makes a broad, irregular contribution to the facial portion of the snout. This facial process notably does not make up the entire posterior margin of the external naris—the nasal occupies the posterodorsal narial corner (Fig. 2B).

The maxilla is a tall, gently curved bone making up the main lateral surface of the snout (Figs. 4 and 5). The facial surface of the maxilla is intensely sculptured with shallow, crater-like structures and small foramina, similar to that of some other basal therocephalians (see, e.g., Abdala, Rubidge & van den Heever, 2008). A labial emargination is absent, unlike in Lycosuchus. The posterior terminus of the maxilla is a process under the jugal that reaches the midpoint of the orbit. The tip of this process is notably expanded and rugose, but the majority of this process is notable for lacking the dermal sculpturing so prominent on the rest of the maxilla and bearing a weak lateral groove. The maxilla houses four teeth: the enlarged, blade-like canine and three small postcanines. The canine is recurved and has well-developed serrations fore and aft, as is usual in basal therocephalians (van den Heever, 1994; Abdala et al., 2014). Only a single canine is erupted on each side; unlike the usual condition in lycosuchids and a frequent condition in scylacosaurids, there are not simultaneously-occupied anterior and posterior canine alveoli. The tip of a replacement canine is erupting anterior to the right canine, however. The postcanine morphology of Gorynychus is unique among therocephalians—these teeth are labiolingually compressed and “leaf” or “spade”-shaped, with proportionally large denticles (serrations) along both mesial and distal edges (Fig. 6). Although these denticles are not as large or exaggeratedly curved as those on the incisors, they still show distinct curvature (unlike the finer, straight serrations on the canine). As for the premaxilla, the palatal surface of the maxilla is for the most part not visible in this specimen.

The nasal is a long, flat bone (Figs. 2–5). It also bears dermal sculpturing, albeit developed to a lesser degree than in the facial portion of the maxilla. Dermal sculpturing to some degree is present on all the bones of the snout, including (albeit very weakly) on the facial portion of the jugal. The naso-frontal suture is slightly bowed posteriorly and terminates near the anterior margin of the orbit.

The prefrontal is a roughly trapezoidal bone making up part of the snout and the anterodorsal margin of the orbit (Figs. 3–5). Its contribution to the orbital rim is unusually pronounced and rugose. This is also true for the lacrimal, jugal, and frontal, such that that the circumorbital rim is very prominent everywhere but on the postorbital bar. Anteriorly, the prefrontal terminates in a short process extending between the maxilla and nasal. The prefrontal makes up a large portion (roughly half) of the anterior face of the orbital wall, and appears to exclude the frontal from contacting the lacrimal.

The lacrimal is a large facial bone, nearly equivalent in height to the prefrontal albeit not as anteroposteriorly long (Figs. 4 and 5). A similarly-sized lacrimal is also present in Lycosuchus, and although this bone is less dorsoventrally expanded in scylacosaurids it is of similar anteroposterior dimensions in that group as well (van den Heever, 1994). A large (1.5 mm diameter) lacrimal foramen is present on its contribution to the anterior orbital wall; this foramen does not exit onto the lateral surface. A second, smaller lacrimal foramen may be present ventral to the first, but this is uncertain because of damage.

The jugal makes up the majority of the zygomatic arch (Fig. 4). Its facial portion is an attenuate process terminating below the lacrimal, posterior to the anterior lacrimal margin. This facial portion is remarkably small for an early theriodont. In gorgonopsians, the jugal typically terminates in a broad, plate-like contribution to the snout equalling (or exceeding) the lacrimal in anterior extent (see, e.g., Kammerer, 2015; Kammerer et al., 2015). In Lycosuchus and other early therocephalians, the facial portion of the jugal terminates well posterior to the anterior edge of the lacrimal, but still forms a tall plate on the snout and occupies almost the entire ventral margin of the orbit (van den Heever, 1994). The jugal is constricted beneath the posterior margin of the orbit, before expanding to form a major part of the postorbital bar. In the postorbital bar, an ascending process of the jugal extends dorsally behind the postorbital. Posterior to the postorbital bar the jugal makes a tall contribution to the subtemporal bar, narrowing posteriorly and curving downwards. In the posterior half of the subtemporal bar, the jugal is bifurcated by an anterior process of the squamosal. The ventral portion of the jugal extends nearly to the back of the skull, and has a weakly bulbous terminus. In addition to being preserved in the holotype, an isolated jugal is present on the referred block (Fig. 10), identifiable by the characteristic subtemporal curvature and well-developed process contributing to the postorbital bar.

The squamosal is preserved mainly in its zygomatic ramus (Fig. 4), as the occiput and intertemporal region are poorly preserved in this specimen (Fig. 1). It has a deep and fairly restricted squamosal sulcus, giving it a “forked” appearance in lateral view. Medially, it forms the posterior portion of the lateral margin of the temporal fenestra (anteriorly, this margin is formed by the jugal).

The frontal is a mostly-flat bone making up the interorbital skull roof (Fig. 3). The medial portion of the frontal is damaged in this specimen, but its contribution to the orbit is well preserved and shows that it was only weakly ornamented dorsally, until the very edge of the orbit (in which it is expanded and rugose, like most of the circumorbital rim). The suture between the frontal and postorbital is poorly preserved and difficult to interpret, but it appears that the frontal makes only a narrow contribution to the dorsal orbital wall (Fig. 4), as in Lycosuchus (van den Heever, 1994).

A distinct postfrontal is absent in Gorynychus; this element may be fused with the postorbital. The postorbital makes up a broad portion of the skull roof at the posterodorsal margin of the orbit and continues as a posterior ramus making up the medial face of the temporal fenestra (Figs. 3 and 4). The latter ramus is badly damaged in this specimen, like most of the posterior skull roof, and its posterior terminus and surface texture is indeterminable. The intertemporal bar is narrow, as is typical of therocephalians. However, this region is too damaged to determine its exact proportions or presence/absence of the pineal foramen or sagittal crest. The occiput is even more badly damaged, being worn off entirely dorsal to the occipital condyle (Fig. 5).

The vomer is not exposed anteriorly, but its posterior interchoanal portion is a tall, narrow blade, like that of scylacosaurids and unlike the broad, vaulted morphology of Lycosuchus (Fig. 7). The vomer is paired—a distinct mid-vomerine suture is visible in the interchoanal portion. Posteriorly, the vomer forms a broad, triangular contribution to the palate, extending between the palatines to contact the anterior margin of the pterygoids posteriorly. An elongate palatal depression or fossa is present medially, originating near the anterior edge of the plate-like palatal portion of the vomer and extending posteriorly until it reaches the transverse processes of the pterygoids. Although the depth of this structure is likely exaggerated by lateral compression in this specimen, its presence is natural.

The palatine is the largest bone of the palate (Fig. 7). It is a topologically complex element composed of a laminar anterior process that forms much of the lateral margin of the choana and a broad main portion with a prominent central ridge bounded by medial and lateral depressions. This central ridge extends anterolaterally to posteromedially and is confluent posteriorly with a ridge on the pterygoid, terminating with the palatal boss. A suborbital fenestra, characteristic of therocephalians, is present at its posterolateral margin, bounded posteriorly by the pterygoid. Presumably it is also bounded laterally by the ectopterygoid, but this element is indistinct if present, probably due to poor preservation of the lateral margins of the palate.

As is typical of early therapsids, the pterygoid consists of three distinct processes: palatal, transverse, and quadrate (Fig. 7). The palatal portion of the pterygoid has an anteromedial-to-posteromedially-angled, strongly interdigitated suture with the palatine. Anteriorly it forms a short portion of the prominent palatal ridges (central ridge of palatine) before expanding into the dentigerous palatal bosses posteriorly. Dentigerous palatal bosses are ancestral for therapsids; although they are present in scylacosaurids and even various eutherocephalians (van den Heever, 1994; Huttenlocker & Smith, 2017), they are absent in lycosuchids (Lycosuchus and Simorhinella) (Abdala et al., 2014). The palatal boss is “teardrop”-shaped, with a narrow posterior tip and broad, rounded anterior. It is curved anterolaterally. The boss bears two tooth rows. On the left palatal boss (the more completely preserved of the two), the posterior tooth row follows the curve of the boss and is made up of five teeth. The anterior tooth row is transversely oriented and is made up of four teeth, for a total of nine (six are present on the right boss, but as mentioned this boss is damaged and this is probably not the complete complement of teeth). The transverse process of the pterygoid extends ventrolaterally: it has a broad base medially, constricts in ventral view toward its midpoint, and then expands into a rounded lateral tip where it braces the mandible. The anterior face of the transverse process bears a broad, shallow depression. Unlike most therocephalians, but similar to lycosuchids, the transverse process is dentigerous in Gorynychus. Unlike Lycosuchus and Simorhinella, however, where the tooth row is relatively long (five teeth) and the teeth erupt directly from the main ramus of the transverse process, in Gorynychus there are only 2–3 teeth situated on discrete, ovoid bosses raised above the medial bases of the transverse processes. On the right transverse process there are only two teeth, a large lateral and small medial one; on the left there are three teeth of equal size, similar to the small one on the right. No interpterygoid vacuity is present; although there is a weak depression between the transverse processes medially, it is entirely bounded by bone dorsally. The quadrate process of the pterygoid is situated dorsal to the transverse process. It extends posterolaterally from a position near the medial base of the transverse process toward the quadrate, weakly curving along its length. The anterolateral margin of the quadrate process forms a tall, narrow ridge; posteromedially it forms a broad, concave plate bounding the lateral edge of the parasphenoid rostrum.

The parasphenoid-basisphenoid complex forms a narrow median rostrum originating behind the transverse processes of the pterygoids and extending posteriorly to the basal tubera (Fig. 7). Although tall and blade-like posteriorly, the anterior two-thirds of this complex are divided by a narrow median groove. The posterior terminus of the rostrum is abrupt, with a sharp drop to the base of the basal tubera (typical of early therocephalians) instead of a gradual decrease in height. Dorsally, the parasphenoid forms a narrow median lamina above the pterygoid, forming part of the mid-orbital plate (Fig. 5). Dorsal to this anteriorly is a tall, laminar bone interpreted as the orbitosphenoid, which extends dorsally to contact the frontal-postorbital wall of the orbit. Dorsal to it posteriorly is the epipterygoid, which has an anteroposteriorly broad footplate ventrally that sits atop the pterygoid. The ascending process of the epipterygoid narrows dorsally before expanding again at its contact with the ventral face of the parietal. The basal tubera are broadly separated by a median depression (Fig. 7). They are relatively slender and angled medially at their posterior end. The left stapes is preserved in place, extending from the basal tuber to near the quadrate. The stapes is dorsoventrally narrow but anteroposteriorly broad, although narrowing along its length laterally (similar to Lycosuchus). No foramen or dorsal process is visible, but these could be obscured by matrix. The basioccipital forms a plate posterior to the basal tubera and terminates in the occipital condyle, which is similar to that of other therocephalians.

The dentary is a massive, robust bone with a tall, well-developed, unfused symphysis more similar to that of gorgonopsians than other early therocephalians (Figs. 2, 4, 5 and 11A). The anterior face of the symphysis is densely foraminated. The roots of at least three incisors are exposed due to damage to the right dentary (Fig. 4), but it is probable that more were present, given that these three do not occupy the entirety of the symphysial length. The crowns of the lower incisors are not exposed. The lower canine is not exposed in the holotype but is well-preserved in the disarticulated dentary fragment in KPM 291 (Figs. 10 and 11A). The lower canine is a tall, recurved tooth with well-developed fore and aft serrations. It is proportionally large, taking up much of the alveolar margin of the symphysis. Based on this position, it would have been situated anterior to the upper canine when in occlusion with the cranium. The lower postcanines are mostly obscured in both known specimens of Gorynychus, only a single small lower postcanine is exposed on the right mandibular ramus anterior to the upper PC1 in the holotype. This postcanine is smaller than any of the upper postcanines (2 mm apicobasal length, versus 4 mm in the uppers) but is otherwise similar in morphology, being “spade”-shaped with well-developed denticulation. The dentary is constricted behind the symphysis, then expands posteriorly, with a well-developed angular process (Fig. 4). The dorsal and ventral margins have raised edges, and a distinct lateral fossa is present between them, extending anteriorly almost to the level of the symphysis. Although superficially similar to the masseteric fossa of cynodonts, this fossa is likely non-homologous; there is no evidence that the superficial masseter was present in therocephalians. The raised ventral margin of the dentary terminates posteriorly in a broad, flattened rugose region serving as the attachment site for adductor musculature. The coronoid process extends freely above the postdentary bones and has a broad posterodorsal terminus, as in the other basal therocephalians (van den Heever, 1994). Unlike Lycosuchus, however, in which the posterior edge of the coronoid process is broadly rounded, in Gorynychus this edge is nearly straight (slightly concave).

The postdentary bones are damaged on both sides of the skull: the surangular and articular are more complete on the right side but the reflected lamina of the angular is broken off ventrally; the surangular and posterior portion of the angular are broken off on the left side but the reflected lamina is more complete (Figs. 4 and 5). In general, the postdentary elements are very similar to those of lycosuchids and scylacosaurids. The reflected lamina is large and occupies the entire lateral surface of the angular (typical of early therapsids, but distinct from gorgonopsians in which it is usually widely separated from the articular). The surface structure of the reflected lamina is typical of early therocephalians: an anterodorsal depression becoming a single broad ridge posteroventrally that then ramifies into ventral and posterior ridges. The surangular is exposed laterally as a narrow, curved element atop the angular, contacting the articular posteroventrally. A short angle is present at the posterior base of the coronoid process of the dentary where it overlies the surangular, and the dentary is weakly raised laterally anterior to this point, accommodating the internal anterior process of the surangular. The splenial is a tall, “ribbon”-like bone occupying the medial face of the anterior portion of the jaw ramus (Fig. 7). It lacks any lateral exposure, being restricted to the internal surface of the jaw. It covers almost all of the medial surface of the dentary anteriorly, but decreases in height posteriorly before terminating at the level of the transverse process of the pterygoid. Dorsal to the splenial is a narrow, laminar bone: the prearticular. This element broadens posteriorly and becomes more raised and rod-like posterior to the dentary, terminating in an indistinct contact with the articular. Dorsal to the prearticular at roughly the mid-length of the jaw ramus is a short, narrow, laminar element interpreted as being the coronoid. The articular is poorly preserved on both sides in this specimen, but a bulbous, cup-like terminus articulating with the (equally poorly preserved) quadrate can be discerned in medial view on the left side (Figs. 4 and 7). Although mostly worn off, the base of a large retroarticular process is present.

Postcranium

Little of the postcranium is preserved in the holotype of G. masyutinae. The majority of the cervical series is preserved in articulation with the skull and some ribs and pectoral elements are preserved posterior to this (Fig. 1). The atlas-axis complex is somewhat damaged and obscured by the skull and surrounding matrix and C5–7 are broken and badly worn, but C3 and 4 are well-preserved and exposed on both sides (Fig. 8). The cervical vertebrae are amphicoelous (except, presumably, the atlas) and are separated ventrally by small, wedge-shaped intercentra. The transverse processes are short and blunt. At least one cervical rib is preserved on the right side of the specimen in association with (but disarticulated from) vertebra C3. This rib has a broadly falcate head and mediolaterally narrow main body. Well-developed, stout prezygapophyses (12 mm anteroposterior length) extending anterior to the centra are present in C3 and 4, contacting the associated postzygaphophyses of the anterior vertebrae ventrally (and slightly laterally, as the prezygapophysis bulges out somewhat at its anterodorsal edge). No anapophyses are present. The neural spines of C3 (17 mm tall) and 4 are constricted immediately above their point of origin but expand dorsally into broad, rounded tips (anteroposteriorly 11 mm long in C3). The axial neural spine is definitely anteroposteriorly broader than that of the subsequent vertebrae (Fig. 8), but its exact dimensions (including height relative to subsequent neural spines) are uncertain due to crushing. An isolated vertebra interpreted as the axis in the referred specimen KPM 291 preserves a tall, broad neural spine (Fig. 10), suggesting that the axial spine was substantially taller and longer than subsequent neural spines, as is typical for theriodonts (Jenkins, 1971). The other vertebrae preserved in KPM 291 have relatively lower, longer centra and shorter neural spines (Fig. 11B) than those of C3 and 4 in the holotype. They likely represent dorsals or even anterior caudals, although most are too poorly preserved to identify with any confidence.

The non-vertebral postcranial elements in the holotype are mostly damaged (Fig. 9) The remains of at least eight ribs are present, but little of their morphology is exposed other than simple, curved shafts. A robust, curved elongate bone exposed in worn cross-section probably represents the clavicle, but nothing more about its morphology can be said. The left scapulocoracoid is preserved almost entirely as impression (there is a small chunk of actual bone from the anterior margin of the procoracoid). This impression shows that the coracoid-procoracoid base of the structure was very anteroposteriorly long. The scapula is broad ventrally but narrows markedly dorsally, where it curves anteriorly at tip. A prominent ridge on the scapula originates at the posterior margin of this element before curving anteriorly along the length of the scapular spine. An elongate bone preserved as part of KPM 291 (Fig. 10) is here identified as a fibula. This element lacks the curvature seen in the cervical-thoracic ribs on this block and in KPM 347, and is too long to be a lumbar rib. Additionally, it is expanded at both ends, unlike a rib. The morphology of this element is similar to the fibula of other therocephalians, in which it is usually a narrow, simple bone (Fourie & Rubidge, 2009).