A new species of Largocephalosaurus (Diapsida: Saurosphargidae), with implications for the morphological diversity and phylogeny of the group

Abstract

Largocephalosaurus polycarpon Cheng et al. 2012a was erected after the study of the skull and some parts of a skeleton and considered to be an eosauropterygian. Here we describe a new species of the genus, Largocephalosaurus qianensis, based on three specimens. The new species provides many anatomical details which were described only briefly or not at all in the type species, and clearly indicates that Largocephalosaurus is a saurosphargid. It differs from the type species mainly in having three premaxillary teeth, a very short retroarticular process, a large pineal foramen, two sacral vertebrae, and elongated small granular osteoderms mixed with some large ones along the lateral most side of the body. With additional information from the new species, we revise the diagnosis and the phylogenetic relationships of Largocephalosaurus and clarify a set of diagnostic features for the Saurosphargidae Li et al. 2011. Largocephalosaurus is characterized primarily by an oval supratemporal fenestra, an elongate dorsal ‘rib-basket’, a narrow and elongate transverse process of the dorsal vertebrae, and the lack of a complete dorsal carapace of osteoderms. The Saurosphargidae is distinct mainly in having a retracted external naris, a jugal–squamosal contact, a large supratemporal extensively contacting the quadrate shaft, a leaf-like tooth crown with convex labial surface and concave lingual surface, a closed dorsal ‘rib-basket’, many dorsal osteoderms, a large boomerang-like or atypical T-shaped interclavicle. Current evidence suggests that the Saurosphargidae is the sister-group of the Sauropterygia and that Largocephalosaurus is the sister-group of the Saurosphargis–Sinosaurosphargis clade within the family.

A Middle Triassic stem-turtle and the evolution of the turtle body plan

“…It shares the T-shaped cross-sectional outline of the broad trunk ribs with both taxa. Saurosphargid reptiles also have broad trunk ribs 20,21 , but these differ from those of stem-turtles in being flatter (with the shaft not offset), straighter, single-headed, and uniform throughout the trunk region. By contrast, the thoracic ribs of Pappochelys and Odontochelys vary in shape with respect to their position, and some ribs have asymmetrical anterior and posterior flanges.…”

First evidence of centralia in Ichthyopterygia reiterating bias from paedomorphic characters on marine reptile phylogenetic reconstruction

“…This leaves Sauropterygia as the only clade for which a basal member with manual centralia is unknown, although such may not be the case if saurosphargids are basal to Sauropterygia as proposed by Li et al (2014). The inferred presence of centralia in the hind limb of Chaohusaurus suggests that pedal centralia loss occurred independently at least in Ichthyopterygia and thalattosaurs (M€ uller et al, 2005).…”

A new specimen ofHupehsuchus nanchangensisYoung, 1972 (Diapsida, Hupehsuchia) from the Triassic of Hubei, China

“…This is supported by the scapular facet that is partly seen on the lateral surface of the posterodorsal edge of the clavicle and by the fact that no clavicle facet is seen on the lateral margin of the scapula. In this point, Hupehsuchus is comparable to Parahupehsuchus (see Chen et al 2014a, Figure 1A), sauropterygians and saurosphargids (Wu et al 2011;Li et al 2014), although this contact cannot be certain in WGSC 26004 owing to the poor preservation of the bone. Chen et al (2014b) considered the clavicle-scapular articulation to be anterolateral in Hupehsuchus and Parahupehsuchus (see their coding of character 54 for the two taxa), but they did not provide any evidence.…”

“…The ventral process is much longer than in Nanchangosaurus and extends anteroventrally to the base of the postorbital bar. The supratemporal bone, which is seen in saurosphargids (Li et al 2014), is also present, although its relationship with the squamosal is uncertain at the posterolateral most portion.…”