The cranial characters of a new specimen of Yunguisaurus from Middle Triassic Ladinian Xingyi fauna, Guizhou

The Triassic Pistosauroidea from Europe, North America and China is the key to link the "stem-group" and "crown-group" Sauropterygia, and is important to reveal the origin and the early evolution of Jurassic-Cretaceous "crown-group" Sauropterygia, Plesiosaurus. Yunguisaurus is one of the most primitive genera of Pistosauroidea discovered in South China, with the most well-preserved specimens. However, the cranial osteology of Yunguisaurus is still lack of understanding. A new material of Yunguisaurus, excavated from the Zhuganpo Member(Ladinian) of Falang Formation, Xingyi Fauna in Guizhou Province, provides new detailed cranial information, such as the unfused frontals and the existence of an open space in the occipital region. Detailed comparison shows that the new material is the longest one among the five specimens of Yunguisaurus reported so far, indicating the unfused frontals should be a diagnosis of Yunguisaurus, rather than an ontogenic difference. There is no inner-posterior branch in squamosal of this new material, forming an open occipital part, which is similar to that of Plesiosaurus, indicating a close relationship with this genus.