Abstract:Hupehsuchus nanchangensis is the most representative species in the Nanzhang-Yuanan Fauna and one of the oldest marine reptiles. The information about the key features of its cranium is very scarce. Therefore, in order to supplement this part of the content, a fossil specimen collected recently has been carefully repaired and studied in comparative anatomy. This study revised the following cranial diagnoses of this species: a narrow bone space along the middle line of the skull develops along the two branches of upper jaws; the premaxilla contacts with each other at the anterior end, while the nasal only contacts at the posterior ends; groove develops on the premaxilla in the same direction as that of the maxilla; top of the skull is flat without sagittal crest; the parietal is larger than the frontal, and the pineal foramen is located in the anteromiddle part of the parietal; upper temporal fenestra is formed by parietal, postfrontal, postorbital and squamosal, excluding supratemporal; its jugal is a triradiate bone with a thin and long anterior process, and a short ventral process; supraoccipital is large and nearly round; exoccipital has clearly visible nerve opening. The comparision shows that Hupehsuchus nanchangensis, Nanchangosaurus and Eretmorhipis all bear bone spaces formed by both sides of upper jaws, which might be the homologous character of Hupehsuchia. The supratemporal of Hupehsuchus nanchangensis does not participate the upper temporal fenestra. In addition, this species has morphological vestiges of the ventral process of jugal and its inferior temporal fenestra is not completely degraded, which is similar to the oldest diapsid reptiles, but different from the advanced ichthyosauromorphs. It is speculated that with the development and evolution, ichthyosauromorph supratemporal gradually moved forward and participated the upper temporal fenestra, while the inferior temporary fenestra gradually closed. The new material also shows that the external naris of Hupehsuchus nanchangensis is upward and its bite force is not strong, so it can be inferred that this species has a unique feeding function different from other species of ichthyosauromorphs.