The first record of a nearly complete choristodere (Reptilia: Diapsida) from the Upper Jurassic of Hebei Province, People’s Republic of China

Abstract

Choristodera are freshwater aquatic reptiles known from the Middle Jurassic to the Miocene. Their record shows a peak in diversity in the Early Cretaceous of eastern Asia, most notably in the Jehol Biota of China but also in Japan and Mongolia. However, until now, the only Jurassic records from Asia have been rare disarticulated elements from Middle Jurassic microvertebrate sites in Siberia and Transbaikalian, with a possible jaw fragment from the Late Jurassic of Xinjiang, China. Here we describe a new, fully articulated, choristodere skeleton from the Tiaojishan Formation of China, considered to be of Late Jurassic (Oxfordian) age. As such, the new specimen represents the first complete Jurassic choristodere recovered worldwide, as well as providing important information on the pre-Cretaceous history of the group in eastern Asia. In its proportions and postcranial characters, the new taxon resembles previously described Asian taxa such as Philydrosaurus, but it is distinct from them in possessing several apparently plesiomorphic characters including a short antorbital region, paired external nares, and an open lower temporal fenestra. Phylogenetic analysis places the new choristodere as the sister taxon of a non-neochoristoderan clade comprising Asian and European taxa, whereas the European Jurassic Cteniogenys becomes the sister taxon to all other Choristodera.

Computed tomography analysis of the cranium of Champsosaurus lindoei and implications for the choristoderan neomorphic ossification

“…Our data support the observations of Matsumoto et al (), suggesting that the neomorph is present in all choristoderes more derived than Coeruleodraco (i.e. all choristoderes other than Cteniogenys ; Matsumoto et al ). Evans () inferred the presence of the neomorph in Cteniogenys based on facets in the neighbouring bones, a conclusion that is now supported by the presence of this element in other choristoderes.…”

“…). In the early choristodere Coeruleodraco , the neomorph appears to only contact the parietal, opisthotic and a small portion of the squamosal (Matsumoto et al ). Although the neomorphic bone is more accurately described here in Champsosaurus , the morphology of the neomorph in the more basal Coeruleodraco (Fig.…”

“…Line drawing of the skull of Coeruleodraco jurassicus (IVPP V 23318) in dorsal view. Modified from Matsumoto et al ().…”

“…The functionality of the neomorphic bone in the early choristodere Coeruleodraco is hard to determine, given that this element is interpreted as quite small and does not appear to be structural. Prior to the discovery of this genus, the neomorph has always been found in conjunction with the pterygoquadrate foramen (pterygoid foramen of Matsumoto et al ), and it is therefore possible that those two structures were related. The discovery of Coeruleodraco refutes this, where the neomorphic bone appears to occur in the absence of the pterygoquadrate foramen, suggesting that this foramen may be a synapomorphy of Neochoristodera.…”

“…Fox () suggested that the neomorph developed to strengthen the connection between the braincase and the pterygoquadrate region, but the small size of the neomorph reported in Coeruleodraco (Matsumoto et al ) makes this unlikely. As such, it is possible that the neomorphic ossification did not develop due to some driving selection pressure, but developed randomly and became fixed in the population because it had no deleterious effects.…”