Yuanansuchus maopingchangensis sp. nov., the second capitosauroid temnospondyl from the Middle Triassic Badong Formation of Yuanan, Hubei, China

Skull roof

A small interpremaxillary foramen lies on the midline, close to the anterior border of the premaxillae. The nares are elongated and large (the left measuring 314 mm in length, and the right 275 mm), and are situated near the anterior margin of the skull. The external naris is bordered anteriorly and laterally by the premaxilla, dorsally by the nasal, and posteriorly by the maxilla. The posterolaterally directed suture between the premaxilla and the nasal touches the medial margin of the external naris, and the right suture is displaced anteriorly relative to the left one. In the dorsal view, the maxilla is restricted to the skull margin and does not extend posteriorly to the anterior margin of the orbit. No septomaxilla is evident. The nasal contacts the maxilla posterior to the external naris. The lacrimal contacts neither the orbit nor the naris.

The two large orbits are both ovoid, but differ in size, the right orbit being longer (43 mm in length) and more anteriorly positioned than the left (38 mm in length). Their sizes are slightly larger than those of Y. laticeps, which are 35 mm in length. However, both orbits are situated relatively far anteriorly, lying close to the mid-length of the skull roof. In this characteristic Y. maopingchangensis resembles Y. laticeps, Mastodosaurus giganteus and some juvenile capitisauroids (such as Watsonisuchus aliciae) (Liu & Wang, 2005; Schoch, 1999; Warren & Hutchinson, 1988). The orbits are almost dorsally directed and are raised above the rest of the skull roof, indicating an aquatic or semiaquatic lifestyle in which the animal habitually positioned itself near the water surface. The highest point on the skull roof is located on the postfrontal, just posterior to the orbit. The prefrontal is relatively long (measuring about one quarter of the midline length of the skull), tapers to an anterior point, and is elevated just anterior to the orbit. The frontal participates in the orbital margin, as is the case in many other capitosaurs (Damiani, 2001; Schoch, 2008) but not in Y. laticeps (Liu & Wang, 2005). The frontal is subequal in length to the nasal, although the left frontal is shorter than the right. However, in contrast to the condition in all other capitosaurs, the frontal extends posteriorly beyond the posterior margin of the orbit (Schoch, 2008; Schoch & Milner, 2000). The postfrontal is a nearly quadrangular in shape, and forms the posteromedial margin of the orbit. The postorbital is wide, and its lateral margin extends beyond that of the orbit. However, the postorbital does not extend as far anterolaterally as in many other capitosaurs, but narrows anteriorly and makes only a small posterior contribution to the orbital margin. The jugal forms most of the lateral border of the orbit, and has a long anterior process that extends far anteriorly to the orbit and reaches nearly the same level as the tip of the prefrontal. However, the length of the preorbital part of the jugal is nevertheless less than half that of the snout. Although the suture between the jugal and quadratojugal is unclear, the former is definitely one of the longest bone in the skull roof.

The skull table is very long for the anteriorly positioned orbit. The ratio of postorbital portion to the whole skull is 0.38. This ratio is similar to the ratio in Y. laticeps, bigger than in Mastodonsaurids (∼0.3), much bigger than in other capitosaurids (<1/4) (Liu & Wang, 2005; Fortuny, Galobart & De Santisteban, 2011; Schoch & Milner, 2000; Sidor, Steyer & Hammer, 2014). The parietal foramen is round and small, with a diameter of 6.5 mm, and lies on the midline near the anteroposterior midpoint of the parietals. The parietal is short, and is similar in length to the postparietal, the supratemporal, and the tabular. The postparietal is relatively long, its length exceeding its width. The posterior margin of two postparietals is incompletely preserved but looks transversely aligned as in Y. laticeps and Mastodonsaurus giganteus (Liu & Wang, 2005; Schoch, 1999). The tabular is short and wide, and its lateral process is only slightly posteriorly directed. As a result, the posterior margin of the skull roof is nearly straight in dorsal view as in Y. laticeps, rather than distinctly concave as in most capitosaurs (Schoch, 2008). The tabular contacts the squamosal, enclosing an otic fenestra and preventing the supratemporal from entering the dorsal margin of this opening. The otic fenestra is more or less rounded as in Eocyclotosaurus, not ovoid as in Quasicyclotosaurus and Cyclotosaurus; it embays the squamosal more deeply than the tabular as in Quasicyclotosaurus, Eocyclotosaurus, and Cyclotosaurus mordax (Schoch, 2000; Schoch & Milner, 2000). The squamosal is a large bone in the posterolateral part of the skull roof. The posterior margin of the squamosal is poorly preserved, but this bone does extend far enough posteriorly to overhang the occiput. The posterior margin of the postparietal, tabular and squamosal appears thick (∼7 mm) relative to the total height of the occiput (47 mm).

Palate

The palate is moderately vaulted, with the lateral edges slightly below the level of the middle portion. The maxilla bears more than 30 marginal tooth sockets, but no teeth are preserved. The anterior palatal vacuities are paired, oval in outline and completely separated by the premaxilla and the vomer, as in Eocyclotosaurus wellesi, Mastodonsaurus giganteus, and Paracyclotosaurus crookshanki (Damiani, 2001; Schoch, 1999; Schoch, 2000). A small fossa lies on the midline, crossing the suture between the premaxilla and the vomer. This structure may be a fossa subrostralis media, a feature present in Eocyclotosaurus woschmidti (Kamphausen, 1989).

The vomerine plate is longer than wide. The plate is in contact with the premaxilla anteriorly, the maxilla laterally, and the palatine posterolaterally, and it sends out a long posteromedial process that runs lateral to the cultriform process. The vomerine tusk pair is large, and situated at the anterolateral corner of the vomer. A triangular depression lies on the midline, behind the fossa subrostralis media. The transvomerine tooth row is not preserved, but the protruding base shows it is slightly convex posteriorly for the presence of the depression as in Edingerella madagascariensis (Maganuco et al., 2009). However, this depression is not observed in smaller specimen IVPP V 22629. The left side of the plate is poorly preserved, but on the right side a depression is situated posterior to the transvomerine tooth row.

The choana is elliptical in outline, and relatively long, as in most capitosauroids. It is bordered by the maxilla laterally, the vomer medially, and the palatine posteriorly. The interpterygoid vacuities are well-developed, measuring nearly 50% of the midline length of the skull, and are approximately as wide posteriorly as anteriorly. Each vacuity is bordered laterally by the palatine and pterygoid, and medially by the vomer and parasphenoid. The orbits are located at the level of the posterior half of the vacuities, as in Yuanansuchus laticeps. Apart from a small portion of the right orbit, they can be seen through the vacuities in their entirety.

The palatine forms a slender posterior process that meets the ectopterygoid laterally and the pterygoid posteriorly. The palatine tusk pair is not preserved, and even the shapes of their alveoli are unsure. The right ectopterygoid is a slender elongate bone situated medially to the maxilla. Shallow tooth sockets lie on the lateral margin of the bone, medial to the maxillary teeth. However, the part of the palate forming the lateral margin of the left interpterygoid vacuity is so mediolaterally compressed that even the suture between the maxilla and the ectopterygoid is unclear.

Although the middle portion of the vomerine plate is not well-preserved, the length of the midline suture between the two vomers shows that the parasphenoid extends anteriorly to around the level of the anterior margin of the interpterygoid vacuities. The anterior tip of the parasphenoid is far more posteriorly positioned than in most Capitosaurians, including Y. laticeps. The parasphenoid seems smoothly sutured with the vomer, and the fodina vomeralis should be absent here, in contrast to Yuanansuchus laticeps, The cultriform process is a flat bar which narrows laterally in the vicinity of its midpoint, and whose triangular base is smoothly continuous with the vaulted basal plate of the parasphenoid. The width of the basal plate is subequal to its length. The crista muscularis is a transversely oriented ridge located on the posterior rim of the basal plate. The ventrally positioned ‘pockets’ mentioned by Watson (1962) are indistinct and widely separated. The parasphenoid contacts the pterygoid laterally at an anteroposteriorly long suture, the length of the suture is slightly greater than the width of the corpus of the parasphenoid.

The palatine ramus of the pterygoid tapers anteriorly, and contacts the posterior margin of the palatine along a short suture located roughly at the level of the anterior margin of the orbit. The corpus of the pterygoid curves ventrally towards its lateral side; its lateral margin is granular and forms a roughened area. The quadrate ramus has a ventral oblique surface and an tall dorsal oblique ridge (crista obliqua), and extends posterolaterally to form a sutural contact with the main body of the quadrate. The palatine and quadrate rami of the pterygoid participate in medially delimiting the subtemporal fenestra, which is bordered posteriorly by the quadrate and laterally by the quadratojugal and maxilla. Both quadrates are preserved. This bone is nearly triangular in palatal view, and the medial side which contacts the pterygoid is much wider than the lateral side which contacts the quadratojugal. The ventral condyle for the articular is saddle-like. The quadrate bears a boss (hyoid tubercle) above the medial part of the condyle. The occipital condyles are separated from one another and are formed by the exoccipitals, which are in sutural contact with the parasphenoid anteriorly. The occipital condyles are positioned posteriorly to the level of the quadrate condyles. A long, slender, unidentified piece of bone is preserved on the left ventral margin of the skull. It could be part of a rib. The anterior end of this element covers the posterior portion of the upper alveolar row.

The skull of IVPP V 22629 is nearly complete, with a midline length of 21 cm and a width of approximately 15 cm (Fig. 4). The long axis of the orbit measures 27 mm, and the short axis measures 20 mm. Another skull (IVPP V 22630) is slightly smaller (Fig. 5). Both specimens are similar to the holotype in their general features, but differ from the holotype in that the frontal merely comes close to the orbit rather than entering the orbital margin. The frontal extends only to the level of the posterior margin of the orbits, rather than continuing posteriorly beyond this level as in the holotype. In IVPP V 22629, only one vomerine tusk socket lies anterior to the choana, and there is no socket lying posterior to the choana (Fig. 4B). In IVPP V 22629, the area of the contact between the vomer and the cultriform process is poorly preserved, and the anterior extent of the cultriform process is uncertain.

The occipital region is better preserved in two specimens. The occiput is relatively low because the occipital condyle is nearly situated at the same dorsoventral level as the quadrate condyle, as in Yuanansuchus laticeps (Liu & Wang, 2005). The posttemporal fenestra is triangular. The foramen magnum is wide. The right stapes is preserved in situ in both specimens, and the one in IVPP V 22630 is complete. The stapedial footplate is large, and the stapedial shaft is slab-like as in Tatrasuchus wildi (Schoch, 1997). The foramen for nerves IX–X (the glossopharyngeal and the vagal nerves) is visible on the lateral side of the exoccipital.

Lower jaw

IVPP V 22631 is a robust and long left lower jaw, missing the anterior tip and the postglenoid area (PGA) (Fig. 5). The intact specimen is estimated to have measured 27 cm in length and 65 mm in height. The preserved anterior tip of the ramus (approximately the midway between the mandibular symphysis and the forward edge of the adductor fossa) has a height to width ratio of approximate 1.5. The height to width ratio immediately behind the postglenoid ridge approximately is 1.7. These ratios are similar to those of Stanocephalosaurus birdi (Jupp & Warren, 1986). The lower jaw increases in height posteriorly until the posterior end of the coronoid, then begins to decrease in height on the lateral side of the adductor fossa. As a result, the dorsal margin of the lateral wall of the adductor fossa is strongly convex. The lateral surface of the angular shows well-developed radial ornamentations, which become less evident on the postsplenial. A distinct mandibular sulcus extends posterodorsally across the lateral surface of the angular, beginning at the ventral margin.

The dentary bears more than 20 tooth sockets, which are anteroposteriorly compressed. Only two broken teeth are preserved on the dentary. The middle and posterior coronoids both bear a single row of teeth, but only a few conical teeth are actually preserved. The medial side of the postsplenial bears a small anterior Meckelian foramen, posterior to which is an additional tiny foramen. The ratio of the length of anterior Meckelian foramen to the adductor fossa length is less than 0.1. The posterior Meckelian foramen is bordered by the postsplenial anteriorly and ventrally, the prearticular dorsally, and the angular posteroventrally. It is elongated and much larger than the anterior Meckelian foramen, but still far less than half as long as the adductor fossa. The center of posterior Meckelian foramen lies anterior to the vertical line passing through the anterior-edge of the adductor fossa. The medial wall of the adductor fossa is incomplete, but the original height of medial wall could be deduced from preserved portion. The height of the medial wall should be slightly greater than 2/3 height of the lateral wall at the middle of the adductor fossa length. The prearticular extends anteriorly beyond the anterior margin of the posterior Meckelian foramen, and contacts the postsplenial below the posterior coracoid. The hamate process of the prearticular is not complete, but still rises above the level of the glenoid fossa. Most of the glenoid facet lies above the level of the dorsal surface of dentary.

Although the postglenoid area is nearly completely missing, the following features on this area still can be deduced from the preserved specimen: the prearticular does not extend into the PGA, and the angular does not participate in the PGA. So the PGA should be type I of Jupp & Warren (1986) for this specimen.

Postcranial skeleton

Many isolated bones have been discovered at this locality, but only a limited number have been successfully excavated because they are quite fragile and the matrix is very hard. Among the elements that have been recovered, three interclavicles and nine clavicles have been identified as temnospondyl in origin and tentatively referred to Y. maopingchangensis (see ‘Discussion’).

The interclavicle is well preserved in IVPP V22632 (Fig. 6). It is rhomboidal in shape, lacking a distinct anterior stylus, and is much wider than long. In contrast, the interclavicle is longer than wide in Mastodonsaurus giganteus (Schoch, 1999). On the dorsal side, this bone bears three converging low ridges (Figs. 6B and 6C). The anterior two ridges lie in the anteroposteriorly middle portion of the bone, and the posterior one is longitudinally aligned. The anterior and posterior margins of the interclavicle are broad and nearly straight. The ventral surface displays the characteristic temnospondyl dermal sculpture pattern, with furrows and ridges radiating outwards from the ossification center.

Among the nine clavicles, only two right clavicles are nearly complete (Fig. 6). The ventral blade of the clavicle is slightly longer than wide, and shows a radial ornamentation on the ventral surface. The medial margin is convex, making the blade fanlike in shape rather than triangular. The dorsal process is posterodorsally directed and similar to that of Stanocephalosaurus pronus (Schoch & Milner, 2000; Howie, 1970). The lateral surface of the dorsal process combines with the ventral blade to form a prescapular cavity, and the dorsal process tapers to form a sharp dorsal tip.