Chaohusaurus zhangjiawanensis Chen et al., 2013

Chaohusaurus zhangjiawanensis Chen et al., 2013

Diagnosis (modified after Chen et al., 2013; Zhao et al., 2022; Zou et al., 2020).

Small ichthyosauriform about 1 m in adult length; orbit large and with irregular anterodorsal margin formed by a lateral shelf of the prefrontal; postfrontalprefrontal contact present; dentition heterodont, with slender and pointed anterior teeth and robust and rounded posterior teeth (revised character state); anterior tooth crowns ornamented with sparse apicobasal enamel ridges, posterior tooth crowns ornamented with ridges forming an anastomosing pattern (revised character state); plicidentine present at least in posterior teeth (new character state); maxilla anterior process much longer than posterior process; two pairs of sacral ribs, with only the first pair being distally expanded and the second pair being similar to, but smaller, than the adjacent caudal ribs; caudal peak present; forelimb elements tightly packed; proximal carpals completely ossified; manual pisiform present; distal carpal I unossified/absent; intermedium pentagonal; dorsal process of femur well-developed.

Referred specimen

HFUT YAV-10-08, the anterior part of the snout with associated dentition, impressions of the posterior portions of the upper and lower jaws and posterior maxillary and dentary teeth, a single preserved posterior maxillary tooth crown, and abraded fragments of indeterminate skull bones ( Fig. 2 View Fig ).

Type horizon

Uppermost part of the third member of the Jialingjiang Formation, Olenekian (Spathian), Lower Triassic (see Wolniewicz et al., 2023 for a more detailed discussion of the stratigraphy).

Type locality

Zhangjiawan quarry, Yuan’an County, Hubei Province, China .

Description

Skull. Te preserved length of the skull measures 146 mm and is comparable in size to an almost complete skull of Ch. zhangjiawanensis reported by Zhao et al. (2022), which measures 151.22 mm in length. Te anterior part of the snout is exposed in right dorsolateral view ( Fig. 2 View Fig ). Te premaxillae are partially preserved, with their anterior tips as well as their posterior portions missing, but they are articulated and form a clearly visible interpremaxillary suture. Only the anteriormost part of the right maxilla is preserved, whereas the left maxilla is heavily abraded, and the bone is preserved mostly as an impression in the rock matrix. Te anterior portion of the right dentary is preserved in semi-occlusion with the right premaxilla. As revealed by surface striations and suture lines visible in the mould of the specimen, the posterior portions of the jaws are represented by the impressions of the left dentary and left surangular exposed in lateral view, and the right mandibular ramus exposed in medial view ( Fig. 2 View Fig and 5 View Fig ). Several heavily abraded and indeterminate skull bone fragments are preserved in the posterodorsal part of the specimen.

Dentition—general morphology. Te dentition of Ch. zhangjiawanensis is heterodont, as demonstrated by specimens HFUT YAV-10-08 (this work) and YGM- Y4701 ( Zhao et al., 2022) (crushing posterior teeth were also reported in the holotype of Ch. zhangjiawanensis , but were only briefly described and were not figured; Chen et al., 2013). Te anterior premaxillary and dentary teeth are conical and pointed, whereas the posterior maxillary and dentary teeth are robust and rounded

( Figs. 3 View Fig , 4 View Fig , 5 View Fig , 6 View Fig and 7 View Fig ). Te morphological transition from conical to rounded teeth occurs in the anterior part of the maxilla, as seen in YGM-Y4701 ( Zhao et al., 2022). Tis transitional region is also partially preserved in HFUT YAV-10-08, but the transitional teeth themselves are not preserved. Te mode of tooth implantation of the premaxillary teeth is not possible to determine with confidence. Te maxillary teeth are set within alveoli ( Fig. 6 View Fig ), like in Ch. brevifemoralis , but the presence of a shallow dental groove could not be confirmed ( Gu et al., 2024). It is unknown whether an additional tooth row, like the one reported in Ch. brevifemoralis ( Gu et al., 2024; Yin et al., 2021), was present lingually to the only exposed maxillary tooth row. Root impressions indicate the presence of deep, vertical grooves on root surfaces in the posterior dentition, which implies the presence of plicidentine ( Maxwell et al., 2011). Selected measurements of the dentition are given in Table 1 and Supplementary Table 1.

Anterior (conical) dentition. Te anterior teeth of HFUT YAV-10-08 include ten premaxillary teeth (p1–10) and six dentary teeth (d1–6) ( Figs. 3 View Fig and 4 View Fig ). Te teeth are conical and terminate in a pointed tip (e.g., tooth p2; Fig. 4A View Fig ). Te crown surfaces are ornamented with sparse apicobasal (longitudinal) ridges (e.g., teeth p2, p4, and p8; Fig. 4 View Fig ). Te roots seem to lack grooves on their surfaces (e.g., teeth p3, p5, and d3; Fig. 4B, D View Fig ). A slight constriction seems to be present at the crown-root boundary in some of the conical teeth (e.g., teeth p2 and p4; Fig. 4A, B View Fig ).

Ten teeth form a single row in the right premaxilla and around five vacant tooth positions are visible among them—between teeth p2 and p3 (two vacant tooth positions), between p6 and p7 (two vacant positions) and between p9 and p10 (one vacant position) ( Fig. 3 View Fig ). Tis indicates a total tooth count of 15 teeth for the preserved part of the right premaxilla. Tooth spacing ranges from 1.59 to 3.15 mm (full data available in Supplementary Table 1), but the maximum tooth spacing value of 3.15 mm (between teeth p4 and p5) is likely overestimated, since p4 represents a replacement tooth, which has not reached its maximum size ( Fig. 4B, F View Fig ). In general, tooth size increases posteriorly along the jaw length. Teeth p4 and p8 likely represent replacement teeth, because they are much smaller than the remaining premaxillary teeth. Even though some premaxillary teeth are abraded, details of tooth microanatomy (such as the morphology of the pulp cavity and the presence of infolded dentine) are not preserved due to extensive damage (e.g., teeth p6 and p7, Fig. 3 View Fig ; teeth p3 and p9, Fig. 4 View Fig ). Te right dentary preserves only six teeth; tooth d3 ( Fig. 4D, H View Fig ) is the best-preserved one. Te anterior dentary teeth are only very partially exposed, but they share the same general morphology with the anterior premaxillary teeth, being slightly smaller in size ( Fig. 3 View Fig ; Table 1).

Posterior dentition. Te posterior teeth of HFUT YAV-10-08 are represented by impressions left by four left maxillary teeth ( Figs. 5 View Fig and 6 View Fig ) and three left dentary teeth

( Figs. 5 View Fig and 7 View Fig ). Tey are rounded with a blunt or slightly pointed (e.g. i/lm 2; Figs. 5 View Fig and 6 View Fig ) crown tip. Teir crown surfaces are ornamented with ridges forming a vermiform (anastomosing) pattern, superficially similar to that present in the posterior teeth of the early-diverging durophagous ichthyosaurians Tolodus and Xinminosaurus ( Mulder & Jagt, 2019) . Te widely exposed roots of the maxillary teeth bear clearly visible vertical grooves

( Figs. 6 View Fig and 7 View Fig ).

Te impressions demonstrate that the maxillary teeth were set in shallow sockets, similar to the condition in Ch. brevifemoralis ( Gu et al., 2024) . However, it is not possible to determine if a shallow dental groove was present along the maxillary tooth row.

Te maxillary teeth become more robust posteriorly. Tooth lm1 is the smallest preserved maxillary tooth and likely represents a replacement tooth ( Fig. 6 View Fig ). Tooth lm4 is the only maxillary tooth that has its crown partially preserved, although it is heavily abraded ( Fig. 6 View Fig ). Te crown shape index of tooth m4 has a value of 0.51, indicating that it has the most robust crown out of all preserved maxillary teeth and tooth impressions. Te impression of tooth rd8 indicates that the posterior dentary teeth were also rounded. Its root is well exposed and ornamented with sparse, broad, vertical grooves ( Fig. 7 View Fig ). Teeth rd7 and rd9 only left partial impressions of their crowns. Te vermiform (anastomosing) pattern of crown ornamentation is clearly visible in all the preserved impressions of dentary teeth ( Fig. 7 View Fig ). Tere is a large empty space present between the impressions of teeth d8 and d9, indicating vacant tooth positions for likely two additional dentary teeth.