Cranial morphology of an Early Cretaceous Monjurosuchid (Reptilia: Diapsida) from Liaoning Province of China and evolution of the choristoderan palate

Abstract

A new specimen of Philydrosaurus proseilus from the Early Cretaceous Chiufotang (Jiufotang) Formation preserves the first complete palate of a monjurosuchid choristodere. As in other choristoderes, the palate of Philydrosaurus is akinetic, extended by a broad contact between the vomer and maxilla, and equipped with multiple batteries of palatal teeth. This specimen provides phylogenetically significant information and clarifies the distribution of many apomorphies within Choristodera. Philydrosaurus is primitive relative to the Neochoristodera in that it exhibits only a moderate degree of posterior displacement of the choanae and retains a relatively large interpterygoid vacuity. However, Philydrosaurus also exhibits several derived features previously considered diagnostic of the Neochoristodera, including establishment of a long midline contact of the pterygoids and development of a distinct nasopalatal trough extending from the choana. The choristodere palate exhibits significant modification of the primitive diapsid condition, including elongation of the vomers, establishment of a vomer–maxilla contact, posterior displacement of the choanae, development of the nasopalatal trough, and reduction of the interpterygoid vacuity.

Keywords:

• Diapsid reptiles
• Choristodera
• Early Cretaceous
• Chiufotang Formation
• western Liaoning of China
• evolution of palatal structures

Acknowledgements

This study is primarily based on significant field discovery of fossils; and for this reason, we thank all the crewmembers from Shenyang Normal University for their passionate effort in field collection of fossils from Chaoyang area. The research was supported by the National Natural Science Foundation of China (NSFC grant #40532008), RFDP of the Chinese Ministry of Education, and American Museum of Natural History Carter Fund. Thanks are given to Professor R. C. Fox (University of Alberta) for reading and improving the manuscript, and Alan Turner and Jack Conrad (American Museum of Natural History) for helpful discussions. We also thank Kristin Lamm for her skillful illustration of figure 5.