New digital anatomical data of Keichousaurus hui (Reptilia: Sauropterygia) and its phylogenetic implication

Systematic paleontology

Description and comparison

All skulls are well preserved and are compressed dorsoventrally. The skull of Keichousaurus is generally wedge-shaped in dorsal view and the widest part of the skull is located in the posterior orbital region (Figs. 1, 2A, 2B, 2E and 2F). The skull length of two adult individuals (CUGW VH007 and CUGW VH009) measures 24.7 mm and 24.3 mm, respectively. The external nare is long and narrow with a subtriangular outline. The orbit is pronounced, comprising approximately 25 percent of the skull length. The preorbital region is slightly longer than the postorbital region. The supratemporal fenestra is elongated and about 70% of the orbital length, and the posteromedial margin of the skull roof is concave. Based on previous studies (e.g., Lin & Rieppel, 1998; Holmes, Cheng & Wu, 2008; Liao et al., 2021), this article provides novel and revised anatomical interpretations.

Frontal

The frontal is elongated and flat, forming the medial edge of the orbit. It articulates with the nasal anteriorly and the prefrontal anterolaterally. The posterior end is bifurcated and covered on the parietal (Figs. 1A, 2A, 2B, 2E and 2F). The posterolateral process gradually tapers and inserts into the depression in the anterior region of the parietal, similar to the condition in Qianxisaurus (Cheng et al., 2012), Dianmeisaurus (Shang & Li, 2015) and Dawazisaurus (Cheng et al., 2016). This contrasts with the previous description, which suggested that the posterolateral process is arc-shaped (Lin & Rieppel, 1998; Holmes, Cheng & Wu, 2008) (Figs. 3A and 3B). The frontal is excluded from the supratemporal fenestra by the postfrontal and parietal.

The posterolateral processes of the frontal in Keichousaurus are nearly parallel, which are similar to those of Diandongsaurus (Shang, Wu & Li, 2011; Sato et al., 2014) and Dianopachysaurus (Liu et al., 2011). However, the processes are different from those of Honghesaurus (Xu et al., 2022), Wumengosaurus (Jiang et al., 2008; Wu et al., 2011), and European pachypleurosaurs, such as Anarosaurus (Klein, 2009), Serpianosaurus (Rieppel, 1989) and Neusticosaurus (Sander, 1989), in which the posterior end of the frontals bifurcates into two processes with a significant angle. The frontal in Keichousaurus differs from that of Panzhousaurus, as Panzhousaurus lacks a posterolateral process (Jiang et al., 2018; Lin et al., 2021).

Basioccipital tuber

A pair of short, columnar bony processes extending posterolaterally on the ventral side of the basioccipital are identified as basioccipital tubera (Fig. 4). These tubera extend from the anterior end of the ventral side of the basioccipital and lie on the posterodorsal side of the pterygoid. The well-preserved left tubercle is short, cylindrical, and extends posterolaterally, forming an angle of about 60° with the central axis of the skull. The distal end of the tubercle is smooth and rounded. The right tubercle appears less intact due to the damage in the occipital region.

The basioccipital tuber is similar to those of nothosaurs and plesiosaurs (Rieppel, 1994; Storrs & Taylor, 1996), but it has not previously been reported in pachypleurosaurs. This new finding confirms the presence of basioccipital tuber in Keichousaurus for the first time, suggesting that it may also be present in other early eosauropterygians.

Pterygoid and palatine

The pterygoid of Keichousaurus is long and strip-like, occupying the central position on the ventral side of the skull. In CUGW VH009, the lateral edge of the pterygoid narrows slightly and contacts with the depressed palatine (Fig. 1B). In CUGW VH007, the pterygoid inserts into the groove of the palatine. In CUGW VH017, the pterygoids on both sides do not fully contact the palatine due to preservation, but based on the morphology of the palatine and its articular surface of the pterygoid, as well as the partially preserved lateral edge of the pterygoid bone, it can be inferred that the middle portion at the lateral edge of the pterygoid exhibits a trapezoidal expansion. The anterior portion of the trapezoidal expansion forms an obtuse angle that inserts into the dorsal concave surface of the palatine, whereas the posterior region forms a small triangular buttress for contacting the distal end of the palatine (Figs. 2G, 2H, 3D, 5A). This suggests that the pterygoids originally extended laterally in a trapezoidal and inserted into the palatine at the position corresponding to the posterior edge of the orbit (Figs. 1A, 1B, 2G, 2H and 3D).

In the previous description by Lin & Rieppel (1998) and Holmes, Cheng & Wu (2008), the connection between the pterygoids and palatines on both sides was unclear (Fig. 3C). In European taxa such as Neusticosaurus (Carroll & Gaskill, 1985) and Serpianosaurus (Rieppel, 1989), the lateral margin of the pterygoid does not extend in trapezoidal form, while the lateral sides of the pterygoid of Neusticosaurus peyeri do not contact with the palatine (Sander, 1989). Similar lateral extensions of the pterygoid are present in Wumengosaurus (Wu et al., 2011), Panzhousaurus (Lin et al., 2021), Diandongsaurus (Sato et al., 2014; Liu et al., 2021) and Dianmeisaurus (Shang & Li, 2015), but these extensions are not trapezoidal. These observations suggest that the connection mode between pterygoid and palatine in Keichousaurus may represent an autapomorphy (Fig. 5).

Ectopterygoid

The ectopterygoid is “L” shaped in ventral view, extending from the ventral side of the postorbital and curving anteroventrally to the dorsal edge of the dentary. The medial surface contacts the dorsal surface of the lateral region of the pterygoid, although the suture between them is indistinct (Fig. 3D). Holmes, Cheng & Wu (2008) described a similar bone located behind the palatine in several specimens, which was identified as the medial process of the maxilla instead of ectopterygoid. The “L” shaped ectopterygoid is similar to that of Panzhousaurus rotundirostris (Lin et al., 2021), Diandongosaurus acutidentatus (Sato et al., 2014) and Dianmeisaurus gracilis (Shang & Li, 2015). The ectopterygoid is also present in nothosaurs, such as Nothosaurus rostellatus (Shang, 2006) from Guizhou and Simosaurus gaillardoti (Miguel Chaves, Ortega & Pérez-García, 2018) from Europe, but in these taxa, the ectopterygoids are wide and flaky-like, contrasting sharply with the slender, curved “L” shaped ectopterygoid described in Keichousaurus.

Hyobranchium

The hyobranchium is well preserved and rod-like in CUGW VH009 (Fig. 1B). The pair of hyobranchia are slightly curved along the shafts. They are slender and expanded at both the proximal and distal ends, and the proximal end is more pronounced than the distal end. The total length of the hyobranchial is approximately half the length of the orbit (Figs. 1 and 3D). Notably, the expansion of the distal end of the hyobranchium described by Holmes, Cheng & Wu (2008) appears to be the part of the posterior quadrate branch of the pterygoid rather than the hyobranchium (Figs. 1, 2C, 2D, 2G, 2H and 3C). The hyobranchium has been previously reported in Serpianosaurus (Rieppel, 1989), Neusticosaurus (Carroll & Gaskill, 1985; Sander, 1989), Dactylosaurus (Sues & Carroll, 1985), Wumengosaurus (Wu et al., 2011), Dianmeisaurus (Shang & Li, 2015) and Diandongsaurus (Liu et al., 2021), but remains unknown in Dianopachysaurus (Liu et al., 2011). The hyobranchium of Keichousaurus is more slender, compared to these taxa, with a hyobranchial-to-orbital length ratio similar to that of Dianmeisaurus (Shang & Li, 2015) and Diandongsaurus (Liu et al., 2021), but is larger than those of Serpianosaurus (Rieppel, 1989), Neusticosaurus (Sander, 1989) and Dactylosaurus (Sues & Carroll, 1985).

Dentition

The premaxilla, maxilla and dentary teeth are all well preserved (Fig. 6). There are five premaxillary teeth as described by Lin & Rieppel (1998) and Liao et al. (2021). The premaxillary teeth extend anteriorly, with the tips curving downward and posteriorly. They are generally larger than the maxillary teeth, and their size increases posteriorly, reaching the largest size in the fourth tooth. The fifth tooth is slightly smaller than the preceding four teeth (Fig. 6). In CUGW VH007 and CUGW VH009, which represent adult individuals of different sexes, the number of teeth on the right premaxilla increases gradually from one to four. The crown height of the fourth tooth of CUGW VH007 and CUGW VH009 reaches 1.2 mm and 1.4 mm, respectively. The crown height of the fifth tooth decreases to approximately 0.7 mm in CUGW VH007 and 0.9 mm in CUGW VH009. The left premaxillary teeth of CUGW VH007 and CUGW VH009 follow this pattern, but the third and fourth teeth are rarely exposed, respectively (Figs. 2A, 2B and 6), which may be attributed to tooth replacement. However, CUGW VH017 exhibits no significant variation in tooth size, with the fourth tooth being the largest of the premaxillary teeth.

The anterior maxillary teeth (1–5) of Keichousaurus are well preserved, while the posterior teeth (6–15) are either missing or concealed by the maxilla. It can be inferred that there are 15 teeth on each side based on CT reconstruction, consistent with the description by Holmes, Cheng & Wu (2008), and similar to that of Liao et al. (2021). In all three specimens, the anterior maxillary teeth are slightly inclined, with the tips directed downward. The first three teeth are relatively small, conical in shape. In the adult CUGW VH007, the fourth and fifth teeth are widened. The worn crowns of the fourth and fifth teeth measure only 0.8 mm and 1.0 mm in height, but their anteroposterior length exceeds 0.4 mm, and their transverse width measures 0.3 mm and 0.4 mm, respectively, making them significantly thicker than the anterior three teeth. In another adult, CUGW VH009, the fourth tooth does not present a canine morphology. The fifth tooth, with a crown height of approximately 1.4 mm, and an anteroposterior length and a transverse width greater than 0.4 mm, is identified as a canine (Fig. 6). In CUGW VH017, the fourth teeth are poorly preserved, and the crowns of the fifth teeth are worn. The crowns of the left and right fifth teeth measure 0.6 mm and 0.7 mm in height, but the anteroposterior length and transverse width are both 0.3 mm, which are significantly larger than the anterior teeth. These are considered as canines and might continue to grow. Variations in tooth size across different individuals may result from tooth replacement and preservation. However, the fourth and fifth teeth of the maxilla of Keichousaurus are described as canines by Holmes, Cheng & Wu (2008) and Liao et al. (2021). From the sixth maxillary tooth, the teeth become smaller and more conical in shape, with a cone-shaped crown that forms a 60° angle with the horizontal. Canines in the maxilla have been reported in Dianopachysaurus (Liu et al., 2011), Dawazisaurus (Cheng et al., 2016), Diandongosaurus (Sato et al., 2014; Liu et al., 2021), Dianmeisaurus (Shang & Li, 2015). In Dianopachysaurus, the fourth of the six teeth in the right maxilla was described as significantly enlarged (Liu et al., 2011). In Dawazisaurus, the eighth of the twenty teeth was described as the largest of maxillary teeth and fang-like (Cheng et al., 2016). Five teeth were found in the anterior part of the maxilla of Diandongosaurus, and the third and fourth teeth were identified as canines. However, according to Sato et al. (2014) and Liu et al. (2021), the space between the anterior edge of canines and the premaxilla can accommodate one or two additional teeth. In Diandongosaurus, the canines on the maxilla may be located in the position of the fourth and fifth teeth, which is similar to the canines in Keichousaurus (Sato et al., 2014). In Dianmeisaurus, at least four canines are present on the premaxilla, with the third tooth on the maxilla identified as a canine, which is slightly different from Keichousaurus. The shapes of canines and conical teeth are similar to Keichousaurus (Shang & Li, 2015).

In CUGW VH009, the anterior eight dentary teeth are well preserved, with a generally supine orientation and recurved tips. The dentary teeth are staggered with the premaxillary teeth and the maxillary teeth. The first, second, fifth and seventh teeth are larger than other dentary teeth, with crown heights ranging from 0.8 mm to 1.1 mm. The posterior dentary teeth (starting from the ninth tooth) are obscured by the maxilla. However, some very small teeth are visible through the orbit. A total of 21 dentary teeth are observed based on CT scanning images.

Compared with the traditional European pachypleurosaurs, the number of premaxillary teeth in Keichousaurus is similar to that of Neusticosaurus pusillus (5) and Neusticosaurus peyeri (5–6), but slightly fewer than that of Neusticosaurus edwardsi (6) and Serpianosaurus mirigiolensis (6–8). Compared with the Chinese pachypleurosaurs-like forms, the number of premaxillary teeth of Keichousaurus is the same as Dawazisaurus brevis (5), and similar to Panzhousaurus rotundirostris (at least 5 premaxillary teeth), Dianopachysaurus dingi (at least 5 premaxillary teeth), Diandongosaurus acutidentatus (at least 5 premaxillary teeth) and Dianmeisaurus gracilis (at least 4 premaxillary teeth), but slightly fewer than Qianxisaurus chajiangensis (8), and significantly fewer than Honghesaurus longicaudalis (10 premaxillary teeth perhaps) and Wumengosaurus delicatomandibularis (38).

Compared to traditional European pachypleurosaurs, the number of maxillary teeth in Keichousaurus is similar to that of Serpianosaurus mirigiolensis (15–16), but slightly more than Neusticosaurus peyeri (10–12) and Neusticosaurus pusillus (12), and possibly fewer than Neusticosaurus edwardsi (19 or more). The number of the maxillary teeth of Chinese pachypleurosaurs-like forms such as Wumengosaurus delicatomandibularis, Panzhousaurus rotundirostris, Dianopachysaurus dingi and Diandongosaurus acutidentatus is unknown. However, Keichousaurus has significantly fewer maxillary teeth than Wumengosaurus delicatomandibularis, slightly fewer than Honghesaurus longicaudali (17–18) and Dawazisaurus brevis (20), the same as Qianxisaurus chajiangensis (15), and possibly slightly more than Dianmeisaurus gracilis (at least 13 teeth).

The number of dentary teeth in Keichousaurus is fewer than that of Wumengosaurus delicatomandibularis (at least 65 dentary teeth) and traditional European pachypleurosaurs, such as Neusticosaurus peyeri (24), Neusticosaurus pusillus (25) and Serpianosaurus mirigiolensis (31∼32). A detailed comparison of tooth morphology between Keichousaurus, other pachypleurosaur-like forms and pachypleurosaurs is provided in Table 2.

Phylogenetic analysis

This study uses the data matrix of Wang et al. (2022) which is mainly based on the data of Neenan, Klein & Scheyer (2013) with the combination of information from other researchers including Liu et al. (2011). Six characters of Keichousaurus have been modified (Table S1, data from Wang et al., 2022).

The new matrix containing 181 characters and 63 genera, was analyzed by phylogenetic software TNT (Goloboff, Farris & Nixon, 2008). All characters are equally weighted and unordered. Phylogenetic analysis was conducted using a traditional search method with 100 replications of Wagner trees, one random seed and 10 trees saved per run. A total of 351 most parsimonious trees (TL = 811, CI = 0.293, RI = 0.685) were obtained in this analysis (Fig. S3).

In the strict consensus tree, the clades within Sauropterygia are poorly resolved. Five taxa were excluded from the reduced strict consensus tree (Fig. 7), including Majiashanosaurus, Eremtmorhipis, Palatodonta, Hanosaurus, and Chaohusaurus. The European pachypleurosaurs Neusticosaurus, Serpianosaurus, Dactylosaurus, Odoiporosaurus, and Anarosurua, form a monophyletic group, but the Pachypleurosauria from China do not form a monophyletic group. Qianxisaurus and Wumengosaurus are assigned to be Eosauropterygia. They form a polytomy with the European pachypleurosaurs and a branch that consists of the remaining genera of Eosauropterygia. On the contrary, Dawazisaurus, Dianopachysaurus, Dianmeisaurus, Diandongosaurus, and Keichousaurus form a monophyletic group with the remaining genera of European Eosauropterygia excluding Panzhousaurus. Dawazisaurus, Dianopachysaurus, Dianmeisaurus, and Diandongosaurus form a polytomy with a monophyletic group composed of Keichousaurus, Plaudidraco, Simosaurus, Wangosaurus, Germansaurus and two branches which are Nothosauridae and Pistosauroidea. This branch, which includes the Chinese pachypleurosaurs-like taxa except for Qianxisaurus and Wumengosaurus shared three derived characters: fully fused parietals in adults; a dorsal vertebrae count of 16–20; blunt and expanded ungual phalanges in pes that are wider than the proximal phalanges. The branch containing Keichousaurus and other genera of eosauropterygians is characterized by unambiguous synapomorphies: reduced nasals; three tarsal ossifications; short, blunt ungual phalanges in the pes that are not expanded.