The skeleton measures 159.9 cm along the vertebral column, of which 92.1 cm comprises the tail. There are about 34 presacral, two sacral, and at least 67 caudal vertebrae. Given the incomplete preservation of the vertebral column, the presacral count is based on the number of cervical and dorsal neural spines for the anterior half, and that of ribs for the posterior half, accounting for the presence of right and left ribs. There is a wide crack in the mid-dorsal region, potentially lowering the accuracy of the vertebral count. However, given that some ribs crossover the crack, the count is more accurate than it may appear, although it may be off the true count by plus or minus 1. These counts suggest that the trunk is short compared to ichthyopterygians (Fig. 3e), most of which have 40 to 80 presacral vertebrae, with an exception of Chaohusaurus with 36. There seem to be five cervicals but the exact count is uncertain because the anterior dorsal region was damaged (Fig. 1a) during excavation.

Figure 3: Quantitative comparisons of selected features. (a) Skull proportions relative to the body length in Ichthyosauromorpha. (b) Carpus length relative to body size. The carpus length is measured as the distance between the radius and the second metacarpal, whereas length of the second caudal vertebra is used as the proxy for body size, following a previous work41. (c) Maximum diameter of the upper temporal fenestra (UTF) relative to a body size proxy. (d) Snout proportions relative to the post-snout skull length. The part of the skull anterior to the orbit is considered the snout. (e) Presacral count of Sclerocormus compared to other diapsids, based on published data42. In (a), gray lines represent isoclines for the proportion of the skull length relative to the body length. Symbols with white infill represent estimated positions of newborns, which tend to have a large skull for the body. Two points for Cartorhynchus represent estimated body lengths based on Chaohusaurus and Sclerocormus tail/body proportions, respectively. Sclerocormus has an unusually small skull. Full size image

The holotype skull of Sclerocormus has been dorso-laterally compressed leading to the displacement and plastic deformation of some elements and fracturing of others. It is a particularly small skull, measuring only 6.25% of the total body length (Figs 1a–d and 3a). In basal ichthyosauromorphs, this ratio ranges from about 12% in Chaohusaurus to 15% in Hupehsuchus (Fig. 3a). The snout of Sclerocormus is short (Fig. 3d), edentulous and much narrower than the skull roof, as in Cartorhynchus. This would allow syringe-like pressure concentration common among suction feeders14. The nasal is large, extending anteriorly to the tip of the snout as in Cartorhynchus but unlike the condition in most reptiles. The external naris is preserved as a small slit (due to compaction during preservation) and closely located to the orbit, which is large and contains extensive scleral plates as in most ichthyosauriforms. These plates have been finely fractured from compression unlike the surrounding bones, yet they still retain clear plate boundaries that allow recognition of at least five different plates. Their surfaces lack the long striations seen in other cranial bones. The frontal does not enter the margin of the orbit, unlike Cartorhynchus. The pineal foramen measures 9.1 mm in length, and is located between the parietals and frontals, near the anterior margin of the large upper temporal fenestrae. As with other ichthyosauriforms, the pineal foramen lies between the orbits, whereas it is located behind the orbits in most other reptiles. The antero-posterior length of this fenestra is about 36.1 mm, which is large for the body size compared to that of Chaohusaurus (Fig. 3c). The large size of the upper temporal fenestrae may indicate a strong biting force15,16. However, no teeth appear to be present in Sclerocormus.

The limb elements of Sclerocormus are widely spaced as in Cartorhynchus but slightly better ossified. The mesopodia are unusually long (Fig. 3b) and bear sparse, round elements. The forelimb is strongly bent as in Cartorhynchus, although this may reflect postmortem displacement of the distal part, unlike in the holotype of the latter genus. There are three rows of carpals, probably including two centralia. Five digits, the longest of which comprises up to four phalanges, are recognized in both fore- and hind limbs. The femur is straight, lacking a marked distal expansion, as in Cartorhynchus but unlike in other basal ichthyosauromorphs. Apart from the astragalus and calcaneum, no other tarsal ossifications are present. Despite the poor ossification of limb bones, the type specimen of Sclerocormus is most likely mature given the advanced ossification of the rest of the body, as in Cartorhynchus and Chaohusaurus. Delay in limb ossification is a common feature among basal ichthyosauriforms.

The short trunk is heavily built. The ribs, which are flattened and broad, are single-headed and articulate almost exclusively with the vertebral centra as in all ichthyosauriforms, although rib articulation may minimally extend to the neural arch in the anterior trunk. The ribcage shallows behind the shoulder with a constant slope, forming a nearly linear ventral margin as in Cartorhynchus but unlike in other ichthyosauromorphs where the margin is curved. The trunk of Sclerocormus is covered ventrally by an extensive gastral rib basket (Fig. 1a,h). The gastral elements are in two parallel series per side, of which the lateral series is unique to ichthyosauriforms among Ichthyosauromorpha. The medial series elements are flat triangles that overlap each other, with the caudad element outlying its craniad neighbour. The flatness and overlapping pattern are shared with Hupehsuchia4,12. The absence of symmetrical median gastral elements is unique to Sclerocormus and Cartorhynchus among basal ichthyosauromorphs. Dermal ossicles are present in the cervical region but not on the neural spines, unlike in hupehsuchians. They are small rounded elements of various diameters between 2.0–5.0 mm. Each ossicle resembles the pelvic ossicles of saurosphargids17, although the locations differ.

The cervical and anterior dorsal neural spines are vertical, unlike those of ichthyopterygians, yet they are antero-posteriorly broadened to leave almost no interspinal space as in Hupehsuchia, but unlike in basal Ichthyopterygia where these neural spines are posteriorly inclined with spaces in between them. Despite the expanded width, the neural spines are at least as tall as they are wide, or taller. Strangely, there are craniad and caudad flanges sandwiching the thickened shaft of each neural spine. This peculiar feature is also seen in one specimen of Hupehsuchus12, and among Ichthyopterygia, Merriam18 described the neural spine of Shastasaurus and Californosaurus to have a thickened central shaft with craniad and caudad ridges or flanges. Also, a previous study noticed weak craniad and caudad ridges in the vertical neural spines of Hovasaurus, a freshwater reptile from the Late Permian of Madagascar19. However, the features do not appear homologous according to our phylogenetic analysis. The caudal neural spines are lower than wide and their tops are rounded in side view, yet they also possess craniad and caudad flanges and a central shaft (Fig. 1j). The haemal arches show a unique morphology. The first eight pairs are not fused distally right to left. The ninth and later pairs, however, are fused distally and appear U-shaped in cranial view, not V- or Y-shaped as in typical diapsid haemal arches (Fig. 1a,j). The modification of haemal arches differs from those of placodonts20 and Atopodentatus21 involving antero-posterior lengthening rather than lateral widening, or of drepanosaurs, with distal forking and secondary fusion22.