The new ichthyosauriform Chaohusaurus brevifemoralis (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China

Cranium

Cranial suture patterns are given in Figs. 4–6, so the contact patterns among bones are minimally reported in the description below to save space. Exceptions are bones surrounding major fenestrae, and ambiguous or polymorphic cases. Unless otherwise stated, descriptions of cranial and mandibular elements below are based on the holotype (Fig. 4) and the paratypes (Figs. 5 and 6), one of which revealing some hidden parts of cranial elements through partial disarticulation.

Premaxilla. The premaxilla is a slender bone that occupies most of the snout in lateral view, where its length is slightly less than half of the skull length. Its extent along the midline is less because the nasals extend anteriorly in-between the two premaxillae. The premaxilla is approximately triangular but has short sub- and supranarial processes emerging from the ventral and dorsal corners of the posterior margin, which forms the anterior margin of the external naris. One of these processes may be missing in some specimens without a clear pattern. The premaxilla bears teeth, as described below.

Maxilla. The maxilla is about half as long as the premaxilla along the jaw margin. It has an ascending postnarial process that excludes the lacrimal from the margin of the external naris. This process is robust, with its base width rivaling the antero-posterior length of the external naris. The suture with the lacrimal appears smooth. The maxilla forms the posterior and postero-ventral margins of the external naris, forming a continuous curve that contributes to about a third of the oval perimeter of the opening. The anterior extent of the maxilla in lateral view is slightly anterior to the external naris, while the posterior end of the bone in lateral view overlaps the anteriormost part of the orbit narrowly. The maxilla bears teeth, as described below.

Nasal. The nasal is a major bone that forms a part of the skull roof from the anterior orbital to prenarial region, behind the major part of the snout. It is widest near the lacrimal, and tapers to a point both posteriorly and anteriorly. The posterior ends of the right and left nasals are widely separated by the frontals while the anterior ends meet along the sagittal line. In the holotype, the right nasal appears to extend far more anteriorly than the left nasal, but this is likely an artifact of preservation (Figs. 4A and 4C).

Frontal. The frontal is a major bone that forms the skull roof in the orbital region. Its shape is approximately an elongated triangle in dorsal view. Its postero-lateral process narrowly participates in the antero-lateral margin of the upper temporal fenestra, forming the most medial segment of the anterior terrace of the fenestra (Fig. 4). The bone is excavated in the terrace region, being separated from the roof part by a conspicuous ridge. The ridge and terrace are confluent with the corresponding structures of the postfrontal.

Parietal. The parietal has the smallest exposure of the bone of the skull roof, comprising the main corpus forming the posteriormost part of the skull roof and a supratemporal process that extends postero-laterally from the corpus. The corpus appears small compared to those of most ichthyopterygians in its relative size to the skull, in width and length. There is a ridge at the posterior end of the skull roof, followed by a slope that is better seen in occipital view than dorsally. They likely represent the parietal ridge and slope that are present in mixosaurs and parvipelvians (Motani, 1999). The inter-parietal suture is not straight: it is bent once in front of the pineal foramen and deeply interdigitated posterior to the foramen (Figs. 4 and 6). It is evident that there is a descending ventral flange along the margin of the upper temporal fenestra, as expected in neodiapsids. The flange is separated from the roof by a conspicuous ridge.

Postfrontal. The postfrontal forms a major part of the bar between the orbit and upper temporal fenestra. It is posterior two-thirds are excavated to form most of the anterior terrace of the upper temporal fenestra (Fig. 4). This proportion of excavated area is unusually high compared to ichthyopterygians. The excavated terrace is separated from the skull roof by a sharp and conspicuous ridge that continues into the frontal.

Prefrontal. The prefrontal is the largest of the bones surrounding the orbit (Figs. 4–6). It has a structure often referred to as an “eyebrow,” a small shelf above the antero-dorsal corner of the orbit. This feature is shared with basal ichthyopterygians, such as U. hataii and Grippia longirostris (Mazin, 1981; Motani, Minoura & Ando, 1998; Motani, 2000; Cuthbertson, Russell & Anderson, 2013b). The bone is swollen laterally around the base of this structure, making it appear unusually massive for a cranial dermal bone.

Lacrimal. The lacrimal appears as a narrow strip of bone between the maxilla and prefrontal in lateral view (Figs. 4 and 5). It is wider away from the orbit. See Orbit below for its participation in the structure.

Jugal. The jugal is a narrow and J-shaped bone in lateral view (Figs. 4–6) but there is indeed much three-dimensionality to the morphology of the bone. The bone is most robust near the postero-ventral corner, where it has a thick and rounded cross-section. From there extends a narrow and horizontal plate of bone anteriorly, forming the maxillary process along the antero-ventral margin of the orbit. This plate widens anteriorly along the orbital rim. This ventral margin of the orbit formed by the jugal is continuous with the anterior wall of the orbit formed by the prefrontal, together giving depths to the orbital rim. Postero-dorsal to the postero-ventral corner extends the postorbital process. Unlike the maxillary process that is flat horizontally, the postorbital process is flat vertically. The postorbital process articulates mainly with the postorbital but it also contacts the quadratojugal and may be partially overlapped by the squamosal.

Postorbital. The postorbital is best seen in AGB7403, where the bone is disarticulated from the rest of the skull (Fig. 6). Its general outline may be compared to the bottom half of the waxing crescent. There are two short processes extending antero-dorsally (postfrontal process) and postero-dorsally (supratemporal process), respectively.

Supratemporal. The supratemporal is a large bone that forms the postero-lateral corner of the upper temporal fenestra. It resembles the squamosal of neodiapsids. it is essentially a triradiate bone, with the postorbital process extending anteriorly, parietal process medially, and descending process ventrally. The descending process is excavated to allow acceptance of the dorsal end of the quadrate, as in Ichthyosaurus (McGowan, 1973).

Squamosal. The identity of squamosal and quadratojugal requires future scrutiny because some specimens show just one of them each. The following description is based on the tentative identification given in Figs. 4–6 with question marks. The squamosal in Chaohusaurus spp. resembles that described for Mixosaurs atavus in that it is slightly swollen laterally and occupies the middle part along the height of the cheek, unlike in most diapsids where it is located dorsally. The bone covers parts of the supratemporal and quadrate superficially, and sometimes also parts of the postorbital and jugal. The bone appearsmissing in some specimens, possibly reflecting its superficial position that makes it easy for the bone to flake off. However, analyses of 3D morphology are necessary to clarify if the bone is truly missing in these specimens. This bone was identified in several different locations and with different shapes depending on the specimen in a previous publication (Zhou et al., 2017). One of them, figured in their Fig. 4B, matches the current description (see “Discussion”).

Quadratojugal. Again, the identity of squamosal and quadratojugal requires future scrutiny because one of the bones may not be clearly present in some specimens (see “Squamosal”). The quadratojugal is a vertically elongated plate of bone that is slightly broader dorsally than ventrally. It is constricted above the ventral end, where the bone is expanded and thickened to form a cup that sits on the dorso-lateral part of the quadrate condyle, as in Ichthyosaurus (McGowan, 1973).

Quadrate. The outline of the quadrate in postero-medial view may be compared to that of a human auricle, with the quadrate condyle occupying the position of the lobule of the auricle. This shape is most evident in the right element in AGB7403 that has been disarticulated and exposed in postero-medial view (Fig. 6). The condyle has two ridges separated by a shallow groove, as in most reptiles (Romer, 1956). When in articulation, the bone is largely concealed by the quadratojugal that covers most of its lateral aspect, and the supratemporal that encloses its dorsal end.

Supraoccipital. The supraoccipital is missing in the holotype but exposes its postero-dorsal view in AGB7403, being disarticulated from but associated with the parietals (Fig. 6). It is a large bone for the occiput, being slightly larger than the roof part of a single parietal. Its antero-dorsal margin is convex, and postero-ventral margin concave. The lateral margins are approximately straight. The shape of the convex margin approximately fits that of the concave posterior margin of the parietal pair, to which it is supposed to articulate. No articular facets are seen on the exposed side, as expected from the condition in other ichthyosauriforms.

Exoccipital. There is a pair of exoccipitals preserved behind the skull of AGB7403, in disarticulation (Fig. 6). The exoccipital resemble that described for Ichthyosaurus in having a shape somewhat reminiscent of a boot in lateral view; it is wider ventrally than the dorsally, the posterior margin is approximately vertical, and the anterior margin is strongly concave (McGowan, 1973). The toe of the boot is conspicuously curled-up in Ichthyosaurus, but the curling is not as pronounced in the present form.

Basioccipital. The basioccipital of Chaohusaurus in general is largely unknown. In AGB7403, there is a bone between the skull and the atlantal pleurocentrum that may represent the basioccipital, being exposed in what appears to be the dorsal view given the presence of a pair of articular facets, probably for the exoccipitals (Fig. 6). The bone is longer than the atlantal pleurocentrum antero-posteriorly, and its length approximately matches the distance between the posterior margins of the parasphenoid and basisphenoid that will be mentioned below.

Basisphenoid. The basisphenoid is known from only one specimen, AGB7403, which exposes more than half the bone, dislocated to be behind the supraoccipital (Fig. 6). There is no sign of a major foramen in the exposed side, so it is most likely the ventral side. Figure 6C represents the estimation of the bone outline through mirroring of the exposed half. The reconstructed shape is reminiscent of what was drawn for Limnoscelis (Romer, 1956). Unlike in Ichthyosaurus, the basisphenoid of Chaohusaurus is not fused to the parasphenoid. There is a pair of processes pointing postero-laterally from the posterior corners of the bone, and this process continues ventrally to the main body of the bone where they become side walls of a triangular area that is excavated from the ventral side. This area seems to serve as the socket for the main body of the parasphenoid. The triangular area matches the shape of the main body of the parasphenoid (Fig. 6C). Conspicuous striations are seen lateral to the triangular area, resembling those that are reported for the parasphenoid below. The basipterygoid process is short, broad, and directly laterally, not antero-laterally as in most reptiles. The anterior surface of the basisphenoid is slightly concave.

Parasphenoid. The parasphenoid is best seen in AGB7403 (Fig. 6). It is a large bone that covers the base of the braincase region, comprising a long cultriform process and a fan-shaped main body. The anterior half of the fan fits into a triangular socket at the ventral part of the basisphenoid, while the posterior half extends more posteriorly than the main body of the basisphenoid, almost reaching the tip of the postero-lateral process of the basisphenoid (Fig. 6C). The main body is covered by strong striations that radiate from the base of the cultriform process. When articulating the parasphenoid to the basisphenoid, these striations would become a part of a series with the striations reported above for the basisphenoid. There is a pair of swelling at the base of the cultriform process, which are probably basal tubera (Romer, 1956).

Pterygoid. The pterygoid is best seen in GMPKU P-3086 (Fig. 5A). It is similar to the pterygoid of basal ichthyopterygians, such as Grippia longirostris. There is a transverse flange marked by a weak ridge that runs from postero-medial to antero-lateral. Pterygoidal teeth are not present. The part posterior to this ridge is elevated and then becomes slightly twisted toward the posterior end, forming the quadrate ramus. The anterior part tapers to a point along the sagittal plane. As with the pterygoid, there is no sign of the margin for the suborbital fenestra.

Palatine. GMPKU P-3086 has a part of the palatine exposed, showing that it is located in its usual position in ichthyopterygians, that is, antero-lateral to the anterior ramus of the pterygoid (Fig. 5A). There is no evidence of the suborbital fenestra in the exposed part of this bone. Otherwise, the exposure is too limited to allow morphological descriptions.

Orbit. The orbit is large and occupies a substantial part of the postnarial skull, with a maximum diameter that is about twice as large as that of the upper temporal fenestra. The orbital rim does not form a circle, ellipse, or ovoid that would fit onto a plane as in ichthyosaurians. Instead, the rim exhibits a conspicuous medial excursion in the dorsal part, off the lateral plane formed by the remainder of the rim (Figs. 4A and 4C). The excursion corresponds to the constriction of the skull roof near the narrow contact between the prefrontal and postfrontal. Similar constriction of the skull roof and an associated excursion of the orbital rim have been recognized in Grippia longirostris, U. hataii and Cartorhynchus lenticarpus (Motani, Minoura & Ando, 1998; Motani, 2000; Motani et al., 2015a) and seems to be a basal ichthyosauriform feature. The excursion makes the eyeball vulnerable to stresses from the dorsal aspect. This vulnerability is partly remedied by the extensive growth of the scleral ring, as mentioned below.

The orbital rim is formed by the prefrontal anteriorly and antero-dorsally, postfrontal postero-dorsally, postorbital posteriorly, jugal ventrally, and slightly by the lacrimal antero-ventrally. The participation by the lacrimal may not be evident in Fig. 4D but it is largely because of the posterior side of the prefrontal being visible in lateral view, making the orbital rim smaller than it is. It is evident in AGB7403 (Fig. 6). The frontal is excluded from the orbital rim by a narrow contact between the pre- and postfrontals.

External Naris. The normal axis for the planed formed by the margin of the external naris points dorso-laterally and slightly anteriorly, being directed more dorsally than laterally in the holotype. The opening is elongated, and surrounded by the maxilla ventrally and posteriorly, nasal dorsally, and premaxilla anteriorly. The lacrimal is completely excluded from the margin, as in basal ichthyopterygians.

Upper Temporal Fenestra. The upper temporal fenestra extends from antero-medial to postero-lateral direction, with its parasagittal extent being less than the lateral extent (Fig. 4). The skull of Chaohusaurus does not appear narrow despite the smallness of the parietals thanks to the wide upper temporal fenestra. It is bordered by the parietal medially and posteromedially, supratemporal posterolaterally, postorbital antero-laterally, postfrontal anteriorly and slightly by frontal anteromedially. There is no evidence for the squamosal to participate in the margin of the fenestra.

Lower Temporal Fenestra. The lower temporal fenestra is a prominent structure in the cheek that is vertically elongated and approximately wedge-shaped. It is completely open ventrally. This morphology is shared with basal ichthyosauromorphs, including Cartorhynchus lenticarpus and Hupehsuchia (Chen et al., 2014c; Motani et al., 2015a; Wu et al., 2016), but not with ichthyopterygians.

Pineal Foramen. The pineal foramen is completely enclosed between the right and left parietals. Its center is located posterior to the midpoint along the inter-parietal suture. Its shape is ovoid, being wider posteriorly than anteriorly. It is located behind the line connecting the posterior margins of the orbits in the holotype, but the location may differ depending on the direction of compaction experienced by relevant specimens.

Scleral ring. In parvipelvian ichthyosaurs, the scleral ring has a central part that is shaped like the wall of a low conical frustum, with the aperture corresponding to the top of the frustum, and a peripheral part that continues from the base of the frustum to form a round wall around the axis of the central frustum (which is expected to be nearly parallel to the optical axis of the eyeball) (Motani, 2005). The central frustum and peripheral wall form an angle of about 110° to 130° (Motani, 2005; Moon & Kirton, 2016). The center of the frustum part may protrude beyond the plane formed by the orbital margin in life. The central part, however, is preserved flattened in most specimens because of preservational compaction, and appear as a flat ring rather than the wall of a conical frustrum. The scleral ring is very thin and deformable even in extant specimens of reptiles, and can easily change its shape plastically through preservation.

This basic arrangement is already present in Chaohusaurus brevifemoralis, although the peripheral wall appears shorter in the present form than in Jurassic forms. The right scleral ring of the holotype suggests that the central frustum formed an angle of about 120° with the peripheral wall, giving depth to the scleral ring. The left ring has a flattened central part, making it appear as if the central part was completely parallel to the plane formed by the orbital margin, but this is most likely an artifact of preservation as in many parvipelvian specimens. The scleral ring is the largest bony structure in the cranial region, apart from the entire skull itself, filling the large orbit (Fig. 4). The aperture of the ring is also large. A part of the dorsal aspect of the ring is exposed through the constriction of the skull where the medial excursion of the orbital rim occurs. It is seen there that the peripheral wall of the scleral ring fills the embayment of the skull roof (Figs. 4A and 4C), solidifying the eyeball that would otherwise lack bony support in the dorsal direction. The ring is inclined, with the normal axis pointing mostly laterally but also tilted anteriorly and dorsally to some extent. The left scleral ring of the holotype seems to comprise about 16 plates, although it is difficult to arrive at the exact number because the preservation is not perfect. There seem to be 18 plates in the right scleral ring of GMPKU P-3086 but the number, again, is not conclusive.

Mandible

The mandible most likely comprises seven elements, including the coronoid that is lacking in derived ichthyosaurs (McGowan, 1973). The mandibular rami are slightly curved and inclined in natural posture, with the anterior part more vertical than the posterior part that is half horizontal. The lateral side of the mandible appears flat in specimens with disarticulated jaws but this is because of flattening by compaction.

Dentary. As with ichthyopterygians, the dentary is an elongated bone that is longer than half the length of the mandible. It is the only dentigerous bone in the lower jaw. The presence of a dental groove is not clearly established because the medial side of the dentary is rarely exposed. However, it is seen in AGB7403 that the teeth are medial to a bony wall, as seen in subthecodont ichthyopterygians. See below for its dentition.

Surangular. The surangular is the most prominent bone of the mandible in lateral view, occupying most of the posterior half of the structure (Figs. 4 and 6). It extends anteriorly beyond the anterior margin of the external naris. Posteriorly, it supports the lateral aspect of the articular, which forms the jaw articulation with the quadrate. There seems to be no direct contact with the quadrate.

Angular. The angular is an elongated bone that covers the ventral aspect of the mandible, from the posterior end to about the midpoint. It is visible both laterally and medially. However, its lateral exposure is limited, unlike in Cartorhynchus lenticarpus and many basal neodiapsids where it occupies the ventral half or more of the posterior part of the lateral surface (Motani et al., 2015a).

Articular. The articular is barely visible in lateral view, although its dorsal margin may be seen as a narrow band surrounding the poster-dorsal corner of the surangular in some specimens (Fig. 6). The articulation with the quadrate condyle seems to occur with the postero-medial surface of the articular, unlike in Ichthyosaurus where the articulation is through the antero-medial surface (McGowan, 1973). Thus, there is no retroarticular process of the articular in Chaohusaurus brevifemoralis.

Splenial. The splenial is a thin and narrow sheet of bone that lies on the medial side of the dentary. Its complete shape is not seen in any of the specimens available, although GMPKU P-3086 shows most of the bone as previously figured (Zhou et al., 2017). The bone tapers to a point posteriorly, and probably anteriorly. The bone is longer than the dentary and seems to extend over about three quarters of the mandibular length.

Coronoid. The coronoid is one of the most enigmatic bones in Chaohusaurus. The right side of GMPKU P-3086 and the left side of AGB7403 reveal sutures between the coronoid and surangular that resemble each other closely (Figs. 5 and 6). However, this suture seems to be closed on the left side of GMPKU P-3086 and most other specimens. The coronoid occupies the area surrounding the coronoid process, mainly dorsally and medially. The bone is also visible slightly in lateral view. The bone is wide dorsally, forming a shallow basin between the coronoid process and the articular. The width of the bone is evident by comparing the lateral (Fig. 5A) and more dorsal (Fig. 5B) views of the right coronoid of GMPKU P-3086. The coronoid process is very weakly developed.

Prearticular. The prearticular is known from partial exposures only. The right mandibular ramus of AGB7403 is exposed laterally but medial elements have slipped ventrally relative to the lateral elements, exposing their lateral sides (Fig. 6). It is seen there that the anterior ramus of the prearticular is long and slender, tapering anteriorly to a point. Its anterior extent overlaps the most posterior part of the dentary.

Dentition

There are 35 teeth in the left premaxilla of the holotype, leaving about three vacant tooth positions among them. Nine teeth are recognized in the left maxilla. In total, there are 47 tooth positions in the left upper jaw ramus of the holotype. The count is similar in AGB7403, which has ten teeth in the right maxilla, and 29 teeth and about five vacant tooth positions in the left premaxilla. the tooth counts in the left maxilla and right premaxilla are uncertain. Both specimens are male individuals, so the slight difference in the total counts likely reflects individual variation. The mandibular teeth are not exposed in the holotype, but they can be counted in AGB7403, where the dentary has 42 teeth and at least eight vacant tooth positions, making the total tooth count to be about 50. This number is similar to the mandibular tooth count in U. hataii, which was previously estimated to be approximately 50 (Motani, 1996).

The dentition is heterodont, with slender and conical anterior teeth and robust and rounded posterior teeth. In the left maxilla of the holotype, the posteriormost six, of the nine teeth, are robust and rounded, while the anterior three appear similar to the premaxillary teeth in size and shape. The transition between the two tooth types is abrupt. The robust posterior teeth are two to three times wider in diameter than the anterior teeth. The root seems to expand basally and the pulp cavity is open, as in basal ichthyopterygians.

Hyoid

Ceratohyal. There is a pair of ceratohyal, located in-between the mandibular rami near their posterior ends. They most likely represent the first ceratohyals of reptiles (Motani et al., 2013). The ceratohyal I is a small and slender rod, which is unsuitable for suction feeding (Motani et al., 2013).

Vertebral column

Atlas. The neural arch and pleurocentrum of the atlas are exposed in the holotype (AGB7401). The neural arch is divided into the right and left halves, as in some reptiles, including ichthyosaurs (Moon & Kirton, 2016). The pleurocentrum of the atlas has a convex articular surface (Fig. 4), which is most likely the anterior surface given the preserved orientation, and that similar pleurocentra are known for U. hataii and Cymbospondylus piscosus. These bones are comparable to the corresponding axial pleurocentra in size, and are unlikely to be the intercentra. Another specimen (AGB7403) exhibits the posterior view of the pleurocentrum from a slightly lateral angle, where the posterior articular facet is concave. The bone is about only half as long as the subsequent vertebral centra (Fig. 6). There is no bone that can be positively identified as the atlantal rib in any of the specimen, although AGB7043 has a short and slender bone near the atlas, possibly representing the atlantal rib.

Axis. The axial neural spine is broader antero-posteriorly by about twice compared to the subsequent neural spines (Fig. 6). The neural arch has a rib facet laterally, forming the dorsal half of the synapophysis. The ventral half of the synapophysis is on the dorso-lateral part of the atlantal centrum, appearing almost triangular.

Neural arches and spines. The neural spines change their shapes along the vertebral column. In the cervical and anterior dorsal series, the neural spines and arches are about equal in height with each other. Both the neural spine and arch are almost perpendicular to the basal plane of the neural arch, although the axis of the former is shifted posteriorly relative to that of the latter. The neural spines gradually become longer, broader, and inclined posteriorly while the neural arches stay in approximately the same posture and height, with antero-posterior broadening in the mid-dorsal series. The lengthening of the neural spines and steepening of the posterior inclination stop early in the posterior dorsal series, and the neural spine size and angle do not change greatly in the sacral series. Then, the steepening of the neural spines restarts in the caudal series, reaching the extreme inclination, where the neural spines are almost horizontal, by about the 13th caudal vertebra. Then the inclination trend reverses, with the neural spines rapidly becoming less inclined and then start to anticline (i.e., inclined anteriorly as opposed to the normal posterior inclination) at about the 18th vertebra.

The most notable feature of the neural spine is found in the caudal peak neural spine, which is bifurcated into two branches, one dorsal and the other antero-dorsal (cpn in Figs. 3E and 8). As noted elsewhere (Motani et al., 2018), structures that are likely homologous with these two branches also exist in Chaohusaurus chaoxianensis as thickened bars but the two are connected by a thin flange of bones. That flange is absent Chaohusaurus brevifemoralis. The caudal peak vertebra in this context was identified based on the following criteria. First, it is the second vertebra from the last narrow caudal neural spine (lnn in Figs. 3E and 8), which is inclined about 45° posteriorly. The caudal neural spines preceding this last narrow one are steeply inclined posteriorly (Fig. 8), sometimes appearing nearly horizontal. Second, the caudal peak vertebra is in front of the first caudal neural spine to have a completely rounded distal end (fran in Figs. 3E and 8), with clear anterior inclination of about 30–50°. Third, the caudal peak vertebra is usually placed where the curvature of the tail reaches its peak, although preservational bias may obscure this feature. Finally, at least in Chaohusaurus brevifemoralis, the caudal peak neural spine is the first anticline neural spine, although it is not necessarily the case in some specimens of Chaohusaurus chaoxianensis. Anticlination in this case is judged by the angle formed by two lines, one connecting the postero-dorsal corner of the neural spine with the postero-ventral corner of the neural arch, and the other connecting the antero-ventral and postero-ventral corners of the neural arch (Fig. 3E, orange lines). This angle is greater than 90° (i.e., the neural spines are inclined posteriorly, which is the norm) in most neural spines except in the posterior part of the tail, and the first anticline neural spine is where the angle first becomes less than 90° in the caudal series.

Hemal arches and spines. The hemal arches and spines are present starting from the third caudal vertebra to the tip of the tail. GMPKU P-3086 reveals the best representation of the hemal arches and spines in the mid-caudal region, where there is a caudal peak associated with anticlination of neural spine (see above). In front of the anticline neural spine, the hemal spines seem to be absent, while the hemal arch is U-shaped in antero-posterior view (Fig. 8; see fig. 1g of Motani et al. (2018)). The hemal arches appear absent in the caudal peak region of the holotype but this lack probably reflects a preservational bias.

Ribs. The first four ribs of the left side of the holotype have an expanded, fan-shaped head, which most likely represents unified capitulum and tubercle. These ribs are short and do not appear to participate in the formation of the trunk rib cage. The four pairs of ribs most likely represent cervical ribs. If so, there are five cervical vertebrae because the atlas seems to lack associated ribs.

The dorsal ribs are also single-headed, although their proximal ends typically have eight-shaped cross-section, suggesting the head is a combination of the capitulum and tubercle. Anterior and mid dorsal ribs articulate with both the neural arch and vertebral centra via a synapophysis, at least down to the 28th vertebra in the holotype. The articulation with the neural arch is lost somewhere in the posterior dorsal region, as the articular facet on the centrum moves ventrally to be completely located on the lateral side of the centrum alone. The exact location of this transition is obscure. The dorsal ribs become increasingly longer more posteriorly but they start to shorten rapidly in the last five pairs of the dorsal series, so that the last dorsal rib is only as long as the first sacral rib.

There are three sacral ribs in the holotype, identified as such based on a combination of the following observations: short; with thickened shafts; sufficiently wide at the tip to allow articulation with the ilium; oriented so that their tips converge, that is, the first one is postero-laterally inclined, the middle one laterally, and the third one antero-laterally; and located in the sacral region. The same number of sacral ribs were previously identified in AGB6253, a female specimen with embryos (Motani et al., 2014). At least the third sacral rib of the holotype seems to retain the eight-shaped cross-section of the head, seen in the more anterior ribs.

It appears that the first 10 caudal vertebrae have rib facets in the holotype, although only the first three caudal ribs are actually preserved. In AGB6253, at least the first nine caudal vertebrae have rib facets but only the first two caudal ribs are preserved. The parapophyses on the sacral and caudal centra are located at the antero-ventral corners of the centra. The rib heads are not eight-shaped in caudal ribs unlike in the more anterior ribs.

Shoulder girdle

Scapula. The scapula is a lunate plate of bone that rolls slightly to wrap around the ventro-lateral part of the shoulder rib cage, as in basal ichthyopterygians. The anterior margin is round without any indentation or emargination. The glenoid and coracoid facet forms a thickened postero-proximal corner (Fig. 9B).

Coracoid. The coracoid is a plate of bone that appears like a fan with a short and thickened stem (Fig. 9B). The stem ends laterally with the glenoid and scapular facets. The inter-coracoidal suture is straight in the central region only. Both the anterior and posterior margins of the coracoid have concave regions.

Interclavicle. The interclavicle is one of the most poorly known bones in Chaohusaurus in general. The complete outline of the interclavicle is seen only in two specimens that may or may not belong to Chaohusaurus brevifemoralis, namely AGB6607 and 7400 (see “Systematic Paleontology”). The bones in these specimens are approximately V-shaped plates (Fig. 9F), with a convex anterior margin and a concave posterior margin. The anterior process is shorter than in Cartorhynchus, and coarsely and strongly striated in approximately radial orientations. The overall shape of the bone would be similar to that of U. hataii if the posterior process of the latter interclavicle was removed. Some other specimens show in-between the two clavicles one or two pieces of bone that may appear to be a part of the interclavicle (Figs. 9B and 9C). However, after examining many specimens, these parts are most likely parts of the clavicle that wraps around the anterior side of the interclavicle and that have been cracked. It appears that the interclavicle is minimally exposed when the dermal girdle is in articulation, leading to the difficulty of seeing the bone in most specimens.

Clavicle. The clavicle is the most prominent bone of the shoulder girdle. It is a J-shaped bone that forms a complete U-shape when the right and left elements are articulated (Fig. 9B). AGB7400 shows a different posture of the clavicles (Fig. 9C) but this is the only specimen where the clavicle has been rotated so that its distal tip points laterally, while virtually all other specimens, including the holotype and GMPKU P-3086, have the clavicular distal tips pointing posteriorly. The condition in AGB7400 is unlikely to reflect the natural posture because the other posture is seen in over a dozen specimens.

The clavicle is thickest in two places, near the proximal end and in the midshaft region where an angle is formed antero-ventrally (Fig. 9). This midshaft angle is located in-between the coracoid and scapular articulations. The part proximal to this angle is the main body of the clavicle, which wraps around the anterior margin of the interclavicle medially and faces the gap between the coracoid and clavicle (see below) laterally. The thick proximal ends of the right and left clavicles seem to overlap slightly in front of the interclavicle, thus concealing the anterior aspect of the latter bone completely.

Forelimb

Humerus. The humerus typically has an anterior flange extending preaxially from the main shaft of the bone, as in basal ichthyosauromorphs. This flange in Chaohusaurus brevifemoralis is divided into a proximal and distal sub-flange as in Chaohusaurus chaoxianensis, but with a notable gap between the two unlike in the latter species. The relative extent of this gap changes ontogenetically, together with that of the anterior flange. In the smallest individual, the sub-flanges appear virtually absent (Fig. 9A), with the anterior margin of the humerus being concave. The concave margin has radial, rather than longitudinal, striations, suggesting the presence of a very narrow anterior flange along the margin. The sub-flanges appear proximally and distally and extend toward the center of the anterior margin with growth. The gap between them therefore narrows with growth, although it never disappears completely. In Chaohusaurus chaoxianensis, a notch-like incision exists between the two sub-flanges, instead of an obvious gap as in Chaohusaurus brevifemoralis.

The head of the humerus is located postero-proximally. Its ossification is usually incomplete, with only the largest specimen revealing a humeral head that is partly ossified (Fig. 10D). The degree of ossification of the humeral head is used to judge the relative degree of osteological maturity in thunnosaurian ichthyosaurs (Johnson, 1977). Similar usage is difficult in Chaohusaurus brevifemoralis because of the poor ossification in almost all specimens (Fig. 10).

Radius. The radius is a flattened long bone that is wider proximally than distally. The peripheral shaft of the bone seems to be absent because the entire anterior margin of the bone has radial orientation of striation that are perpendicular to the margin, rather than those that are parallel to the main axis of the bone as in the proper shaft region. As a result, there is a band of “anterior flange” extending along the entire peripheral margin of the bone, making the bone appear wide. The anterior flange widens proximally, where there is the antero-proximal flange of the radius in Chaohusaurus chaoxianensis. In Chaohusaurus chaoxianensis, the contrast in widths between this antero-proximal flange and the more distal part of the flange is conspicuous, with an abrupt change. However, in the present species, the difference is much smaller because the flange gradually widens proximally without any abrupt change. As a result, the antero-proximal flange of the radius appears small at the first sight.

Ulna. The ulna is an elongated and flattened bone with a proximal shaft and a distal fan, as in basal ichthyopterygians. The distal fan of the ulna typically appears asymmetric relative to the longitudinal axis of the bone, developing more postaxially than preaxially. This morphology differs from that in a typical ulna of Chaohusaurus chaoxianensis where the fan is more symmetrical except in one individual where one of the ulnae has a weakly asymmetrical distal fan (AGB6259; compare Motani et al. (2015a: fig. 1C) and Motani et al. (2015b: figs. 2E, F)).

Carpals. Much of the carpus region is without hard tissue. There are three disk-shaped carpal ossifications, corresponding to the intermedium, ulnare, and fourth distal carpal, in nearly all individuals regardless of body size. Exceptions are a large specimen which seems to have only two carpal bones (AGB6260; Fig. 10C), the terminal embryo described elsewhere that lacks any carpal ossification (AGB6253), and the largest specimen with four carpals, adding the radiale to the list above of three bones (AGB7408). The largest individual of Chaohusaurus chaoxianensis also has four carpals (Motani et al., 2015c), but this individual is smaller than large specimens of Chaohusaurus brevifemoralis with only three carpals. Carpal ossifications in Chaohusaurus brevifemoralis appear smaller than those of Chaohusaurus chaoxianensis relative to the carpus length.

Metacarpals. There are five metacarpals in all specimens, except in one specimen (AGB6258) where six metacarpals are present in both right and left forelimbs (Fig. 10B). The extra metacarpal in this exceptional individual is located preaxially, as in some hupehsuchian ichthyosauromorphs. Unlike in hupehsuchians, however, there is no ossification for an extra preaxial distal carpal that would form the base of the extra metacarpal. Also, there are no phalanges that extends distally from this extra metacarpal. The first and fifth distal carpals lack a complete shaft along their peripheral margins, which are notched. These bones are not completely lunate as in most basal ichthyopterygians because of the notch.

Manual phalanges. The forelimb is hypophalangeal, with a phalangeal formula is 1-3-3-3-1. The third phalanx of the second digit is not always present, making the formula 1-2-3-3-1 in some specimens. The phalangeal formula does not seem to depend on size. As in basal ichthyopterygians, most phalanges are flattened cylinders that are approximately symmetrical across the long axis but elements along the peripheral margin of the limb tend to be asymmetrical. The phalanges are well spaced from each other, more so than in Chaohusaurus chaoxianensis.

Pelvic girdle

Ilium. The ilium changes its shape with growth. In juveniles, it is essentially a short and flattened rod of even thickness that curves slightly (Fig. 11A). In adults, the curvature becomes steeper and a conspicuous iliac blade is formed in the dorsal side of distal part, which is approximately horizontal in natural posture (Fig. 11C). The acetabular facet also expands, resulting in a constriction of the bone between the acetabulum and the iliac shaft.

Pubis. The pubis is the largest of the pelvic bones, although it is not remarkably larger than the ischium (Figs. 11A and 11B). The bone is thickened along its antero-lateral margin, toward the acetabular facet that is located at the posterior end of this margin. The acetabular facet is robust and faces postero-laterally. There is an obturator foramen that opens posterolaterally, just medial to the acetabular fact (Figs. 11A and 11B). Because of this location, the medial margin of the foramen is thick while the lateral margin is much thinner. The inter-pubic margin is approximately straight, although the right and left elements are usually preserved at a distance from each other.

Ischium. The ischium is a lunate bone (Fig. 11). Its antero-lateral corner is thickened to form the acetabular facet, which is thinner than the corresponding facets in the pubis and ilium. The facet is also narrower than the corresponding facet of the pubis. As a result, the acetabulum is not oriented completely laterally but slightly posteriorly (Fig. 11B).

Hind limb

Femur. The femur is a robust bone that is wider distally than proximally, with a weakly constricted shaft in-between. The shaft in many specimens may appear wider than in life depending on the degree of compression during preservation. There is a trochanter on the ventral side of the bone, located slightly distal to the head. Its homology with the known ventral femoral trochanters of reptiles, such as the internal trochanter and fourth trochanter (Romer, 1956), is unknown. There is no fossa on the femoral sides. The distal condyle of the femur is flattened, making the knee joint inflexible.

Tibia. The tibia is a cylindrical bone in the holotype, unlike other limb elements that are flattened at least to some extent. In most other specimens, however, the bone has been compacted to be a flat element. The tibia is wider proximally than distally, and slightly curved posteriorly toward the distal end.

Fibula. The fibula is a flattened long bone with a fan-shaped distal end. The fan develops anteriorly to the shaft, curving the anterior margin of the shaft more strongly than the posterior margin, which in turn is almost straight. This relationship between the shaft and the fan is not obvious in most specimens because of compaction, but is seen at least in better-preserved specimens, including the holotype and AGB6253.

Tarsals. Much of the tarsal region is without hard tissue. There are three tarsal ossifications of various sizes, representing the calcaneum, astragalus, and the fourth distal tarsal. This number seems to be constant regardless of body size. They are all disk-shaped and without three-dimensional sculpture. The astragalus is usually the largest, and the fourth distal tarsal the smallest, although the relative size between the astragalus and calcaneum is highly variable depending on the individual.

Metatarsals. The metatarsals are similar to the metacarpals in general shape, although the former have shafts that are more slender than the latter, at least in the holotype. This difference is not the result of a preservational bias because the left metacarpals and metatarsals are preserved next to each other in the specimen (Fig. 2).

Pedal phalanges. The pedal phalangeal formula is 1-2-3-3-1 where countable. As with the metatarsals, the pedal phalanges are more slender than corresponding manual phalanges.