Osteology and phylogeny of Panzhousaurus rotundirostris from the Middle Triassic (Anisian) of Guizhou Province, China, and implications for the origin of Keichousauridae (Pachypleurosauroidea, Eosauropterygia).

Sauropterygia (including Placodontia and Eosauropterygia) is the most species-rich group of marine reptiles in the Mesozoic. Panzhousaurus rotundirostris is a rare eosauropterygian from the early Middle Triassic (Anisian) marine deposits of Panzhou, Guizhou Province, China. Until recently, P. rotundirostris has been known only by two specimens, and its phylogenetic position within Eosauropterygia remains much controversial. A taxonomic revision of P. rotundirostrisis is provided here based on a comparative study of the type material and a new, juvenile specimen from the same locality and horizon as the holotype. The revised description accommodates significant changes to the reconstruction of P. rotundirostris, including the skull roof, circumorbital bones, and jaws. The juvenile status of the new specimen is well confirmed by a series of features (e.g., smaller body size, relatively larger orbit, unfused neural arches, and fewer carpal ossifications and caudal ribs). The wealth of new anatomical data for this species has facilitated a phylogenetic analysis, the results of which recover Panzhousaurus as a keichousaurid and favour the sister group relationships between Keichousauridae and Pachypleurosauridae. The topology provides new insights into the origin and evolution of Keichousauridae, a major clade of eosauropterygian marine reptiles endemic in the Middle Triassic of China.