Osteology of Yunguisaurus Cheng et al., 2006 (Reptilia; Sauropterygia), a Triassic pistosauroid from China

Triassic pistosauroids are closely related to the Plesiosauria which flourished later in the Mesozoic, but their fossil record has been poor due to the fragmentary nature of the known specimens. Yunguisaurus liae Cheng et al (2006) (Reptilia, Sauropterygia) from China was the first Triassic pistosauroid represented by an almost complete skeleton, and we provide a full description of the holotype specimen based on the result of complete preparation of the skull and posteranium. A revised diagnosis characterizes Y liae by a mixture of primitive and derived features for sauropterygians, such as the high number of cervical vertebrae (similar to the Plesiosauria) with large zygapophyses (shared with basal sauropterygians). The holotype skeleton likely represents a juvenile individual. In a revised phylogeny, relationships among the Triassic pistosauroids are fully resolved but weakly supported; the revised phylogeny differs from the existing hypothesis on a sauropterygian relationship, likely due to the previous reliance on fragmentary taxa. The long neck and hyperphalangy give an impression that Yunguisaurus is very "plesiosaurian," but detailed comparison revealed morphological differences in various parts of the postcranial. These differences, in combination with the revised phylogenetic hypothesis, suggest that Yunguisaurus was not necessarily comparable to the Plesiosauria in terms of body plan and mode of swimming.