The tetrapod fauna of the upper Permian Naobaogou Formation of China: 5. Caodeyao liuyufengi gen. et sp. nov., a new peculiar therocephalian

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:EBA6DC8E-45B6-42C7-B570-BFA9559F2795. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS.

Skull

The incomplete skull measures nearly 13 cm along the midline, and the estimated total skull length is ∼14 cm. The snout is short, representing approximately 40% of the skull length. A small portion of the premaxilla is preserved with the last left incisor (Fig. 1), and it is partially covered by the maxilla laterally. A diastema the length of two incisors is present between the last incisor and the upper canine. The ventral margin of the maxilla slopes upwards anterior to the canine but is nearly straight in the postcanine region (Fig. 2). The preserved portion of the maxilla suggests that the facial plate is relatively high, its height roughly equal to half of its anteroposterior length. In dorsal view, the snout is constricted behind the canine on the better-preserved left side (Fig. 1) (the right side is incomplete, with the canine root exposed). The maxilla bears numerous pits, depressions and foramina on its external surface (Fig. 2). A depression extends anteroventrally between the canine buttress and the orbit. The right maxilla houses the crowns of one canine and six postcanines, whereas the left preserves only the canine and last postcanine. The postcanines are conical with a smooth surface. A lengthy diastema is present between the canine and the first maxillary postcanine.

Erosion of the skull roof has exposed the inner surface of the snout (Figs. 1 and 3). The canine boss is transversely narrower than that of Shiguaignathus wangi (Liu & Abdala, 2017a; Liu & Abdala, 2017b). The antrum Highmori is not preserved, and the sinus Highmori is almost triangular in dorsal and medial views. The lacrimo-palatine ridge is nearly vertical and very deep (Figs. 1 and 3). The palatine sinus can be divided into two parts: a lateral part with a flat floor and a medial part forming a pocket. The medial margin of the palatine sinus is a slender parasagittal ridge on the palatine (Figs. 1 and 3).

Only the ventral portion of the left orbit is preserved (Figs. 1–3). The anterior border lies within the anterior half of the skull, an indication of the proportionally short snout. The preserved orbital margin is formed by the prefrontal (preserved only in its ventral portion), lacrimal, and jugal (Fig. 2). The lacrimal is a nearly triangular bone in lateral view, having a short dorsal suture with the prefrontal and forming a thickened ridge on the anterior and ventral margins of the orbit. It bears a pit, the lacrimal foramen, within the orbit wall (Fig. 1). The anterior extension of the jugal is restricted to the middle portion of the orbit; so the deep suborbital bar is formed by the maxilla and lacrimal anteriorly, the lacrimal and jugal towards the center and somewhat posteriorly, and only the jugal on the posterior portion (Figs. 2 and 3). The anterior process of jugal bears a distinct depression on the lateral surface, just posterior to the suture with the maxilla. The jugal has a robust dorsal process, encircling the orbit posteriorly, though the dorsal tip is missing where it contributes to the postorbital bar. The jugal extends posteriorly towards the occiput, and is overlapped laterally by the tall, triangular anterior process of the squamosal. The deep subtemporal zygomatic arch is formed by the jugal and squamosal. Behind the orbit, the zygomatic arch bows laterally, and has a concave ventral margin throughout its length. The temporal fenestra is widest toward its posterior margin (Fig. 1).

The squamosal forms most of the posterolateral portion of the temporal region (Figs. 1 and 2), but the dorsal portion of the bone is missing so its full extent is uncertain. Its posteroventral process bears a notch housing the dorsal portion of the quadrate-quadratojugal complex, which is exposed in lateral view within the ‘squamosal slit’ (Fig. 2). Medioventrally, the squamosal broadly contacts the paroccipital process of the opisthotic (Figs. 4, 5C, 5D). Anteroventrally, it bears a long process, which probably contacted the prootic, epipterygoid, and quadrate process of the pterygoid as in other eutherocephalians. However, only the contact with the pterygoid is preserved. The squamosal has an anteriorly directed, triangular lamina which roofs the post-temporal fenestra (Figs. 5E, 5F), but it is separated from the border of this fenestra by a ventral extension of the tabular in occipital view. A broad, shallow sulcus lies on the posterior surface of the squamosal just lateral to the paroccipital process of the opisthotic (Figs. 5C, 5D).

The vomer is an elongated element with a long and very narrow interchoanal process, resembling that of Kotelcephalon viatkensis (Tatarinov, 1999). Although the anterior tip of the interchoanal process is missing, the putative contact of the anterior process with the premaxilla appears to be much narrower than in most therocephalians. The interchoanal portion of the vomer generally has a smooth ventral surface, although its posterior tip (near the margin of the internal choana) bears a low ventromedian crest continuing on to its palatal surface. The dorsal surface of the vomer is incompletely preserved, but three close ridges can be observed on the dorsal side, with a narrow space between them (Figs. 1 and 3). Dorsally, the vomer rises as a triangular flange near the posterior margin of the choana (Figs. 1 and 3). The posteriormost surface of the vomer, where it contacts the palatines and forms part of the main palatal surface, is fairly flat. Dorsally, the posterior part of the vomer and the pterygoid form a midline ridge. This ridge diminishes at the level of the transverse process of the pterygoid (Fig. 1).

The palatines are elongate and broad on the palate, and separated medially by the vomer and the pterygoids. On the dorsal side, the palatines extend near the midline of the skull, but are still separated by a narrow section of vomer (Fig. 1). In ventral view, the palatines border the posterolateral margin of the choana, with the maxilla forming the anterolateral margin. The crista choanalis extends medially from the canine boss to the anterior corner of the suborbital vacuity (Fig. 4). The middle section of the crista choanalis is low and indistinct.

The large pterygoid bears the palatal process, transverse process, and posteriorly, the diverging quadrate ramus (Figs. 1, 3 and 4). The palatal process bears a tall median crest, much more developed than the paired intermediate crests lateral to it. There is no interpterygoid vacuity. The ventrally-directed transverse process expands ventrolaterally, terminating in a blunt lateral tip. The anterior face of the transverse process bears a broad, shallow depression. The posteromedial portion of the transverse process is dentigerous. The process posteriorly abuts a well-developed, subtriangular suborbital vacuity with a nearly straight medial margin. It is bordered by the palatine medially, the palatine and ectopterygoid laterally, and the pterygoid posteriorly.

A small ectopterygoid forms part of the transverse process (Figs. 1, 3 and 4). This bone forms the anteromedial border of the subtemporal fossa. The major part of the ectopterygoid is curved, forming a nearly triangular lamina with the transverse flange of the pterygoid. The lateral side of this lamina forms a buttress pressed against the mandible.

The dorsal portion of the occiput and the portion anterior to the foramen magnum are not preserved, and a portion of the inner side of the braincase is exposed (Figs. 1 and 5). There is an enormous fenestra ovalis, exposed in ventral, lateral and medial views, with a diameter of nearly 10 mm (Figs. 5G–5M). It is formed by the prootic dorsally, the opisthotic posteriorly, and the parabasisphenoid and part of the basioccipital ventrally. The prootic is incomplete anterior to the foramen for cranial nerve VII, and its anteroventral process is also incomplete. On its dorsal margin, the prootic incisura is shallow, and the anterodorsal process is short (Figs. 5I, 5J). The ventral side of the prootic is raised as a thickened ridge on the anterior surface of the occiput, bordering the fenestra ovalis and contacting the parabasisphenoid. Posterodorsally, the prootic contacts the supraoccipital. On the medial surface, where the prootic meets the supraoccipital, lies a wide, shallow, conical floccular (subarcuate) fossa for the lateral lobe of the cerebellum (Fourie, 1974) (Figs. 5G, 5J). Posteroventral to this fossa is the crus communis canalium, which represents the dorsal limit of the sinus utricularis. This recess extends dorsally and anteriorly to meet the floccular fossa. Posteriorly, a shallow recess for the posterior vertical semicircular canal runs vertical to the jugular foramen. The canals and fossae of this region are similar to those of Russian therocephalians Chthonosaurus velocidens and Karenites ornamentatus (Ivakhnenko, 2011), but the fenestra ovalis is much larger in Caodeyao. The prootic is sutured with the opisthotic posteriorly and it has an incompletely preserved posteroventral process. The slender posterodorsal process of the prootic is not exposed on the anterior side, only on the occiput (Fig. 5C, 5D). There is no evidence of the presence of a central process of the prootic. The epipterygoid is partially preserved, with the base of its ascending process present above the quadrate process of the pterygoid (Figs. 1 and Figs. 5A, 5B).

The opisthotic is a stout bone with a wide, long posterodorsal process, a slender internal process, and a stout paroccipital process. The posterodorsal process, medial to the post-temporal fenestra, contacts the tabular laterally and the supraoccipital dorsally (Figs. 5C, 5D). The internal process forms a thin lamina between the fenestra ovalis and the jugular foramen (Figs. 4 and 5M). In ventral view on the left side, this process is exposed as a ridge (Fig. 4). The paroccipital process can be divided into an anteroventral flange and a posterodorsal flange, which join to form a V-shaped cavity, the paroccipital fossa of the opisthotic (Fig. 1). The anteroventral flange contacts the posteroventral process of the prootic anteriorly, and their dorsal surface forms the floor of the post-temporal fenestra (Figs. 5C–5F). The lateral side of the paroccipital process is subdivided into a mastoid process on the occipital side and a quadrate process on the palatal side, the latter more massive than the former (Fig. 4). The sacculo-cochlear recess is located anteromedial to the internal process of the opisthotic and medial to the fenestra ovalis (Fig. 5M).

The large, elliptical post-temporal fenestra is positioned at the same level as the foramen magnum. It is bordered by the paroccipital process ventrally, the posterodorsal process of the prootic medially, and the tabular and squamosal dorsally and laterally. On the occiput, the tabular, rather than the squamosal, forms the margin of post-temporal fenestra (Figs. 5E, 5F), as in the Russian Viatkosuchus and Annatherapsidus (Ivakhnenko, 2011), but differing from the South African Moschorhinus and Theriognathus (Durand, 1991; Huttenlocker & Abdala, 2016).

In occipital view, the round foramen magnum is formed by the supraoccipital dorsally, the exoccipital laterally, and the basioccipital ventrally (Figs. 5C, 5D). The supraoccipital contacts the tabular laterally, the opisthotic ventrolaterally, and the exoccipital ventrally. Its dorsal margin is not preserved. The paired exoccipitals do not meet in the midline. In occipital view, the exoccipital can be divided into a flat triangular dorsal part and a ventral projection that should form part of the occipital condyle. The posterior part of the basioccipital, which should form the median portion of the tripartite occipital condyle, is missing.

The parabasisphenoid complex is broken and the middle part is missing (Figs. 4 and 5). Anteriorly, it is buttressed against the pterygoid posterior to the medial portion of the quadrate process. An interdigitated suture is present between these two elements, and near the suture are the paired internal carotid foramina. A parasagittal fossa extends posterior to the opening of the internal carotid foramen on the ventral surface of the parabasisphenoid. No cultriform process is observed. The anteriormost portion of the parabasisphenoid features a short and low median keel, with the anterior portion of the keel bifurcated into two ridges. These ridges continue anteriorly onto the pterygoid, with a concave area between them. Posteriorly, the basisphenoid flares to form the deep spheno-occipital tubercles with the basioccipital.

The left quadrate and quadratojugal are displaced dorsolaterally, suggesting a loose articulation with the squamosal (Figs. 1–5). The quadratojugal is a slender rod tightly sutured to the quadrate (Fig. 1). These bones are separated by a quadrate-quadratojugal foramen. The quadrate is composed of the dorsal process and the ventral transversely-elongated trochlea (Figs. 3, 5A). In anterior view, the dorsal process has a concave depression with lateral and medial crests. The medial crest is partially exposed as a flat surface that probably contacted the paroccipital process (Fig. 3). Only the lateral condyle of the quadrate trochlea is exposed (Figs. 2, 5A). Dorsolaterally, the trochlea supports the base of the quadratojugal.

Mandible

The left mandible and anterior portion of the right ramus are occluded to the skull. The left mandible measures 126 mm in length. The mandibular rami diverge posterolaterally with an angle of 45° in ventral view.

The dentary is a robust bone with a stout, vertical symphysis forming a chin (Fig. 2). A robust, vertical anterior symphyseal region is known in several therocephalian groups including basal therocephalians such as Gorynychus, Lycosuchus, and some scylacosaurids (Abdala et al., 2014b; Abdala, Rubidge & Van den Heever, 2008; Kammerer & Masyutin, 2018; Pusch et al., 2020). This condition is also present in some eutherocephalians, including akidnognathids such as Moschorhinus, Olivierosuchus, and Promoschorhynchus (“mental protuberance”) (Botha-Brink & Modesto, 2011; Durand, 1991; Huttenlocker, Sidor & Smith, 2011) and bauriamorphs such as Nothogomphodon (Ivakhnenko, 2011; Liu & Abdala, 2015), and Microgomphodon (Abdala et al., 2014a). Development of a tall mandibular symphysis is extreme in the Russian Purlovia, in which there is a marked ventral chin (Ivakhnenko, 2011: Fig. 8).

The dentary height decreases posteriorly and reaches its shallowest point near the dentary mid-length, where it is slightly constricted such that the dentary is somewhat concave ventrally. The lateral dentary surface bears a well-developed sulcus immediately behind the symphysis, ending at the posterior margin of the bone, above the angle of the dentary (Fig. 2). The angle of the dentary, formed by the confluence of the posteroventral margin of the horizontal ramus and the anteroventral margin of the ascending ramus, is about 140°. The coronoid process is well developed, with a wide lateral surface, and its dorsal margin ends slightly above the level of the base of the orbit. Posterodorsally, the coronoid process slightly expands in width and its posterodorsal margin is curved dorsally and medially (Fig. 3). The posterior margin of the dentary borders a dorsoventrally narrow, elongate mandibular fenestra. Four incisors appear to be present on the right side of the mandible, but one of them may prove to be an upper tooth. The left dentary bears one canine and six postcanines.

The splenial is a long bone which covers the lower half of the dentary medial surface and does not reach the lower margin of the dentary (Figs. 2 and 4). The splenials from both sides meet on the posteroventral surface of the symphysis.

Only the posterodorsal corner of the coronoid bone is exposed, the rest being covered by the transverse flange of the pterygoid (Fig. 3). It extends ventrally to partially cover the anterior process of the prearticular.

The thin prearticular extends anteriorly into the medial trough of the dentary, runs along the medial side of the angular, and ends posteriorly to contact the articular, but sutures with the latter are not visible. (Figs. 3 and 4)

The angular is located medial and posterior to the dentary (Fig. 2). It forms most the lateral surface of the postdentary area. Its ventral margin is located dorsal to that of the dentary. It borders the narrow mandibular fenestra ventrally. The reflected lamina is elongated posteriorly with a spade-like shape. It is ornamented with fine, radiating ridges and grooves and bears a deep dorsal notch that is positioned at the level of the posterior margin of the coronoid process of the dentary. The ventral margin of the reflected lamina is at the same level as that of the dentary. The angular is bordered dorsally by the surangular.

The surangular is exposed posteriorly in the convex mandibular margin dorsal to the angular in lateral view, with the major portion of the bone covered by the dentary laterally (Figs. 2 and 3). It forms the dorsal rim of the mandibular fenestra on the medial side of the dentary. Anteriorly, it contacts the coronoid and the prearticular. An irregular fossa is formed medial to the dentary, between the surangular and the coronoid, behind the transverse process. A shallow fossa lies on the medial surface of the surangular, dorsal to the posterior end of the prearticular, possibly for the insertion of a portion of the pterygoideus musculature (Fig. 3).

The articular lies on the posterior end of the lower jaw and forms the jaw joint with the quadrate-quadratojugal complex. This complex is displaced from its original position and thus the articular condylar surface is exposed. It bears two concave facets for the quadrate condyles and a wide medial triangular flange.

Humerus

The left humerus is the only preserved postcranial element (Fig. 6). It is a robust, elongated bone with slightly twisted proximal and distal ends. The proximal end has a proximodorsally bulbous head. The proximal half is curved dorsally relative to the distal half. The keel-like deltopectoral crest is well developed, extending down more than half the length of the bone. The distal end is incomplete and deformed with an unnatural strong curvature. A narrow fossa is developed on the dorsal surface of the mid-shaft. A prominent entepicondylar foramen is present.