Sinocyamodus xinpuensis, LI, 2000

Wang, Wei, Li, Chun & Wu, Xiao-Chun, 2019, An adult specimen of Sinocyamodus xinpuensis (Sauropterygia: Placodontia) from Guanling, Guizhou, China, Zoological Journal of the Linnean Society 185 (3), pp. 910-924 : 910-924

SINOCYAMODUS XINPUENSIS LI, 2000

Holotype: IVPP V11872 , a complete, compressed, small subadult skeleton preserved in dorsal view ( Li,

2000), with skull and trunk prepared out from the matrix after the preliminary report.

Referred specimen: IVPP V17051 , a complete large adult skeleton in dorsal view, lacking only the manus and the posterior-most caudal vertebrae.

Locality and horizon: Near Shaxing Village, Xinpu, Guanling County, Guizhou Province, southwestern China; Wayao Member of the Falang Formation (a.k.a. the Xiaowa Formation), Carnian, Upper Triassic.

Emended diagnosis: A medium-sized cyamodontoid placodont ~ 1 m in length, differing from other cyamodontoids in: the short rostrum with bean-shaped premaxillary and anterior dentary teeth; nasal–frontal contact and nasals not separated by premaxillae; elongated postorbital, with the posterior process extended beyond the level of the midpoint of the longitudinal diameter of the supratemporal fossa; proximal portion of forelimbs not covered by the carapace; phalangeal formula of 2-3-4-4- 3 in the manus (based on the holotype) and 2-3-4-4- 2 in the pes; rounded carapace composed of three types of osteoderms with five rows of meridional ridges and vaulted zones on both sides; and sacral region and tail covered by osteoderms, but pelvic shield absent.

DESCRIPTION

Preserved in dorsal view with slight dorsoventral compression, IVPP V17051 is an almost complete skeleton, lacking only the distal parts of both forelimbs and a few terminal caudal vertebrae. Its total length is ~ 1 m, twice that of the holotype ( Fig. 1 ). Considering the deformation of the skull of the subadult type specimen and a preliminary study of its postcranial anatomy, a detailed description is given, focused on the postcranial elements .

Skull

The skull is deeply embedded in the limestone matrix, with dorsal exposure. The dermal roof remains well articulated and visible, except for the most anterior portion of the snout. There is a crack, filled with calcite, through the whole skull, from the anterolateral corner of the left orbit across the skull table to the right squamosal. The skull shows less deformation than the holotype, although the cheek region is slightly compressed dorsoventrally, resulting in the missing premaxillae and the distorted orbits ( Fig. 2 ). The midline length of the skull (from the anterior snout tip to the occipital edge) is 103 mm, approximately twice that of the holotype, but nonetheless smaller than some Cyamodus specimens ( Nosotti & Pinna, 1996; Rieppel, 2001). As recognized in the holotype, the ratio of the longitudinal diameter of the supratemporal fenestra to that of the orbit is relatively lower in S. xinpuensis (1.64 in new specimen and 1.31 in the holotype), whereas Cyamodus skulls are characterized by a much more enlarged upper temporal fenestra (the ratio is greater than two in adult specimens). Moreover, as a result of the compression of the cheek region, leading to crushing of the circumorbital bones, the maximal length of the orbit might be underestimated here.

The premaxilla is unobservable in this specimen and might not have been preserved. The rostral-most bones are edentulous, and the posterior margins tightly suture to other elements (maxilla, prefrontal and frontal) without openings as external nares; the anterolateral concaves of these rostral-most elements might indicate the posterior margins of the external naris. Therefore, these bones are most likely to be nasals instead of premaxillae. An impression of the premaxillary posterior process (or nasal process) can be recognized on the nasal, suggesting that the premaxillae overlap the nasals, but do not separate the nasals from each other.

The maxillae anteriorly meet the posterolateral processes of the nasals, outlining the posterior corner of the naris and resembling Cyamodus kuhnschnyderi ( Nosotti & Pinna, 1996) and Cyamodus rostratus ( Rieppel, 2001) . This condition is unclear in the holotype for S. xinpuensis , because of disarticulation. Together with several fragments most probably belonging to the maxilla, the maxilla extends backwards along almost all of the ventral margin of the orbit. Considering the distortion, the anterior process of the jugal would also join the orbit lower margin. In the dorsal view, the infraorbital foramen marks the posterior portion of the maxilla where it meets the jugal.

The nasals are paired, forming a nasal–nasal contact. The nasal is triangular: the lateral corner contacts the maxilla, the posterior corner sutures to the frontal, and the concave margin between these two corners meets the prefrontal. Compared with the holotype, the nasals in this adult specimen are more laterally expanded. Notably, the nasals vary greatly among different species of Cyamodus ( Nosotti & Pinna, 1996; Rieppel, 2000; Rieppel, 2001), and the nasal of C. rostratus ( Rieppel, 2001) is most similar to that of S. xinpuensis .

The prefrontals are separated by the nasal–frontal contact. The prefrontal is a small, rectangular plate between the frontal and the maxilla, posterior to the nasal, which was interpreted as a laterally elongated large element in the holotype ( Neenan et al., 2015). As in other cyamodontoid placodonts, the prefrontal forms the anterior border of the orbit and is excluded from the external narial margin by a maxilla–nasal connection, which is barely discernible owing to the fragmentation of the type specimen.

The frontal is paired with an obvious medial suture. Anteriorly, it meets the nasal directly with a smooth suture, and posteriorly, it extents close to, but is excluded from, the parietal foramen on the parietal and not beyond the level of the anterior extremity of the supratemporal fossa. The frontal is an elongated dermal bone along the longitudinal axis of the skull and forms part of the anterior orbital roof.

The postfrontal contributes the posterior part of the orbital roof. In this specimen, the margin of the postfrontal is not as angulated as in the holotype, although its posterolateral side is also concave. The postfrontal is excluded from the anterior corner of supratemporal fossa by a relatively broad contact of the postorbital with the parietal.

The parietal is a large, reverse Y-shaped element on the skull roof. It seems to retain a superficial, suturelike midline. However, considering that the skull roof is crushed, it is preferable to regard the parietal as completely fused, as in other cyamodontoids. The parietal develops short anterolateral processes, anteriorly embraced by the frontal and the postfrontal. The posterior ramus of the parietal extends to constitute almost the entire proximal border of the supratemporal fossa, and posteriorly meets the squamosal. Different from the type specimen, the parietal is much more robust and dominant in the adult skull roof of S. xinpuensis , covering more than half of the maximum length of the skull. The parietal foramen is located entirely in the anterior portion of the parietal, and the skull table is weakly constricted in its posterior portion.

The postorbital is long, with three processes. The anterior process projects along the dorsal margin of the jugal, but whether it enters the orbit is not clear, because of the distortion; it does in the holotype. The proximal process, defining the anterior wall of the supratemporal fenestra, extends to contact the postfrontal anteriorly and the parietal laterally. The majority of the lateral margin of the supratemporal fenestra is formed by the posterior projection of the postorbital, and the supratemporal fenestra is elongated, along with the posterior projection. If correctly identified, the postorbital extends further posteriorly than in any other known cyamodontid, or even in any cyamodontoid placodont ( Rieppel, 2001).

Most of the jugal is unexposed in the limestone matrix, anteriorly connecting to the maxilla. The jugal is supposed to constitute the ventral margin of the orbit, according to the interpretation of the type specimen ( Neenan et al., 2015), but in this new specimen, the compression causes dislocation so that the composition of the posteroventral border of the orbit is uncertain. Posteriorly, the jugal tapers ventrally under the quadratojugal around the level of the anterior margin of the supratemporal fossa.

The quadratojugal is similar to that of the holotype, receiving the posterior process of the postorbital and entering the posterior part of the lateral border of the supratemporal fossa. It meets the squamosal posteriorly. Coordinating with the better-developed supratemporal fenestra in this adult specimen, the quadratojugal is also elongated.

The squamosal is a massive rectangular bone enveloping the supratemporal fenestra as the posterior wall and is much more robust than in the subadult individual. In the occipital view, its posterolateral and posterior surfaces are rough and angular, showing three to four facets, possibly for osteoderm attachment. Although there are no osteoderms remaining on the squamosal, two isolated osteoderms are preserved separately: one is embedded in the cervical fragment near the right margin of the left squamosal, and the other is near the left squamosal on the left side (its position is shifted in Fig. 2 ).

The dorsal process of the epipterygoid is visible in the dorsal view through the supratemporal fenestra. It is a broad, disc-shaped element under the lateral, concave margin of the parietal and is more ossified and extended than in the holotype.

Mandible

In the dorsal view, the constituents of the mandible are hardly observable, except for parts of the coronoid and articular. The ascending process of the coronoid extends out through the supratemporal fenestra near the posterior projection of the postorbital, suggesting a well-developed coronoid process. The short retroarticular process formed by the articular lies ventrally under the right squamosal.

Axial skeleton

The new specimen presents a nearly complete vertebral column, despite the loss of the posterior-most caudal vertebrae. Most of the preserved vertebrae are articulated, except the cervical series, which is scattered as fragments between the skull and the carapace. Presumably, the fifth and sixth dorsal vertebrae are entirely covered by the carapace. If so, the vertebral column consists of 42 preserved vertebrae, including a tentative seven cervical, 12 dorsal, three well-recognizable sacral and> 20 caudal vertebrae. The holotype preserves approximately seven cervical, three sacral and> 20 first caudal vertebrae, with the dorsal vertebrae, covered by the carapace, estimated to be more than ten. Cyamodus hildegardis has the same number of dorsal and sacral segments as S. xinpuensis ( Scheyer, 2010; W.W., personal observation). Gastralia or other axial elements could not be observed.

The cervical elements are fragmented, resulting in ambiguity. The atlas–axis complex is invisible and sheltered under the occipital region of the skull; therefore, the two amphicoelous centra beside the occipital border are tentatively interpreted as parts of the cervical 3 and 4. The neural arches are broad and tall, weakly fused to the centra in the proximal cervical vertebrae. Posterior to the gap near the fourth cervical centrum, the next two vertebrae are interpreted as cervical 6 and 7 owing to their position anterior to the scapular blade and relatively less-extended transverse processes. The sixth and seventh cervical neural arches are lower and wider than those of the anterior cervical segment and present the transitional morphology of the dorsal series ( Fig. 3 ).

The first dorsal vertebra lies beneath the anterior margin of the carapace. Distinct from the cervical elements, the dorsal vertebrae develop much lower vertical neural spines, more flattened neural arches, and the transverse processes are remarkably extended distally ( Fig. 3 ). All of the dorsal vertebrae are firmly articulated by transversely expanded zygapophyses, but without zygosphene–zygantrum articulation. The dorsal neural spines abut, but are not fused to the axial corresponding osteoderms, contributing to the middle-line components of the carapace. All of the dorsal transverse processes are increasingly inclined, from the first to the sixth dorsal vertebra, and then decreasingly from the seventh to the 12th, corresponding to the lateral borders of the carapace. The 12th dorsal vertebra possesses a much shorter left transverse process, articulated with a rod-like dorsal rib, indicating that all the dorsal vertebrae bear ribs ( Fig. 4 ), different from the reconstruction of C. hildegardis ( Scheyer, 2010) .

The three sacral vertebrae are characterized by stout ribs, in contrast to the last dorsal vertebra, although their sacral neural spine and neural arch are similar to the latter ( Fig. 4 ). The neural arches are anteroposteriorly elongated compared with those on the dorsal vertebrae. The first sacral vertebra is the most robust, developing the longest sacral ribs, which are gradually decreased in the other two sacral vertebrae. The sacral ribs have distinct distal expansions that abut the ilia. However, the iliac blades and the third sacral ribs are displaced in this specimen, showing a relatively unconsolidated pelvic girdle adapted to the aquatic lifestyle.

The anterior three caudal vertebrae are exposed in the dorsal view, and the fourth to the 20th in the left lateral view ( Fig. 5 ). Similar to the sacra, the first to the third caudal vertebrae have short neural spines and distinct transverse processes. The neural spines of the fourth to the eighth caudal vertebrae are slightly taller than those of the anterior vertebrae and then gradually decrease in height corresponding to the neural arches, becoming more posteriorly located on the neural arch in the other posterior caudal segments. The transverse processes shorten sharply from the fourth to the eighth and disappear from the ninth to the rest of the caudal vertebrae. The caudal centra taper in height, but do not change distinctly in length. The chevrons are preserved on the central anteroventral rim of the fourth to 20th, but might be associated with more proximal and distal caudal vertebrae.

Pectoral girdle and forelimbs

The dorsal blade of the left scapula is the only observable portion of the pectoral girdle and can be identified based on its orientation and its distinctive bulky nature compared with the transverse processes. The tip of the scapular dorsal blade is exposed, pointing posterolaterally, near the distal end of the first dorsal transverse process and the anterior margin of the carapace. Given the displacement of the anterior dorsal vertebrae, the scapula is likewise slightly dislocated.

The forelimbs are generally short and stout, lying along but outside the anterolateral margin of the carapace. Both propodials and epipodials are preserved, with most of the manual elements lost ( Fig. 6 ). The humeral proximal head is located under the most anterior and marginal tuberculous osteoderms that flank the nuchal excavation. The humerus is faintly curved, expanded proximally, and more extremely expanded and flattened distally, but the humeral shaft is more cylindrical and distinctly constricted. The epicondyles are reduced, with a notched ectepicondylar groove and without an entepicondylar foramen. The preserved ulna (80 mm) is more robust and longer than the radius (68 mm), although both are distally deformed to some extent. The elements of the wrist and manus are too obscure to be identified.

Pelvic girdle and hindlimbs

The exposed elements of the pelvic girdle include a pair of ilia, the distal portion of the left pubis, and the left ischium ( Fig. 4 ). The left ilium is disarticulated, showing the lateral surface of its distal side, whereas the right ilium remains relatively in place and erect, with the posterior end separated from the last sacral rib. The ilium is a roughly rectangular plate, and the maximal length is almost three times the maximal height, equal to the total width of the three sacral ribs. The anterior half of the ilium is rod-like, with a convex ventral margin outlining the acetabular roof. The posterior portion is the iliac blade, with a sharp crest. The barely exposed pubis and ischium both seem to be planar bones, and the curve on the ischiatic lateral edge forms the border of the acetabulum.

The hindlimbs are articulated and complete ( Fig. 7 ). The femur is somewhat longer than the humerus. Its proximal end is more expanded than the distal side, with a stout and straight femoral shaft. The intertrochanteric fossa is recognizable on the proximal ventral surface, whereas the internal trochanter anterior to this fossa is reduced. On the distal end, femoral condyles are small and vestigial, although an obvious intercondylar fossa exists. The fibula and tibia are preserved in parallel and are equal in length, both presenting faintly expanded proximal and distal ends.

There are three tarsals on the right side, with the extra ossification of the tentative distal tarsal 4 beside the astragalus and the calcaneum, whereas only the latter two are preserved on the left, with a possible third ossified element covered by the astragalus. The proximal margin of the astragalus is concave to receive the tibia, whereas the calcaneum is smaller and rounder. Almost all the elements of the pes are preserved. The proximately rectangular metatarsal 1 is the shortest and stoutest of the five, and metatarsal 4 is the longest. The phalangeal formula in the pes is 2-3-4-4-2. Most of the phalanges are stout and roughly square; the terminal phalanges are more triangular.

Dermal armour

The carapace, 340 mm long and 370 mm wide, forms a round shape interrupted by an anterior nuchal excavation ( Fig. 8 ). The posterior margin is obscure, possibly owing to the weak ossification that is not as firm as the middle or the lateral parts of carapace, where the osteoderms are perfectly preserved. This carapace has an extremely regular configuration, which consists of osteoderms in three specific shapes and sizes. The first type is a distinctly enlarged octagonal shape with a sagittal middle keel and a concave posterior margin receiving the anterior convex margin of the next large osteoderm. In the first type, a pair of the most anterior and marginal osteoderms flank the nuchal excavation and are more bulbous and pyramidal than in other types. There are seven rows of these large osteoderms, collectively forming seven longitudinal keels through the carapace, within which the most lateral two rows contribute to the lateral margin of the carapace. The middle row of large osteoderms is arranged along, but not incorporated into, the neural spines of the dorsal vertebrae. The second type of osteoderm is medium sized and hexagonal, and the third type is small and pentagonal. The latter two types align alternately as six longitudinal rows inserting between every two rows of the large osteoderms. Based on the right lateral margin, there appears to be a carapacial lateral wall composed of large osteoderms. All three types of osteoderms meet in very slightly interdigitating sutures and are not fused with each other. Additionally, the osteoderms are slightly ornamented and have an impression of scutes on the surface.

Besides the carapace, additional postcranial osteoderms were developed. The pelvic and proximal caudal portions are preserved in articulation; therefore, the pelvic shield is absent or undeveloped, rather than lost. Instead, an array of tuberous osteoderms possibly form three rows, one dorsal and paired lateral, along the hip and proximal caudal areas. The osteoderms along the caudal vertebrae, with anteroposterior gradient reduction in size, also indicates the presence of the tail armour, at least on the ventral side, even though it is not as heavily developed as on C. hildegardis (PIMUZ T4763). There are only a few sporadic osteoderms preserved abutting to the long bones of the limbs.

Figures

Figure 1. A complete adult skeleton of Sinocyamodus xinpuensis in dorsal view (IVPP V17051).

Figure 2. Skull of IVPP V17051, in dorsal view. Uncertain sutures are drawn with thin dashed lines. Crack is drawn with a wide grey dashed line. The position of the isolated osteoderm near the left squamosal on the left side is shifted, and its original position is shown in Figure 1.

Figure 3. Pectoral girdle of IVPP V17051, in dorsal view.

Figure 4. Pelvic girdle of IVPP V17051, in dorsal view. Endoskeletal elements are shown in white, dermal armour (osteoderms) in light grey, and stippled grey lines indicate mineral-filled cracks.

Figure 5. Tail of IVPP V17051. Caudal 1 to caudal 3 in dorsal view and other caudal vertebrae in left lateral view.

Figure 6. Forelimbs of IVPP V17051. The left forelimb is shown above and the right forelimb below.

Figure 7. Hindlimbs of IVPP V17051. The left hindlimb is shown above and the right hindlimb below.

Figure 8. Carapace of IVPP V17051, in dorsal view. The selected region is magnified and the three types of osteoderms are drawn in different colours.