Dianmeisaurus gracilis: A new Small-sized Eosauropterygian From Anisian Triassic Luoping

A new small-sized eosauropterygian (Diapsida: Sauropterygia) from the Middle Triassic of Luoping, Yunnan, southwestern China

Authors:

Shang et al

Abstract:

A new eosauropterygian, Dianmeisaurus gracilis gen. et sp. nov. is described based on a nearly complete skeleton from the Member II of Guangling Formation (Anisian) of Luoping County, Yunnan Province. It is a small-sized (with a total length of less than 50 cm) sauropterygian with a slender body. This new species is similar superficially to two other smallsized eosauropterygians, Diandongosaurus Shang et al., 2011 and Dianopachysaurus Liu et al., 2011a, which were also collected from the same stratum of Luoping, in the body proportion, the skull with no contracted snout, and an oval orbit extremely larger than supratemporal fenestra. However, the new species is characterized by an extremely narrowed interorbital septum, the mandibular articulation at the level of occipital condyle, the big and stout anterolateral process of the clavicle, the proximal part of the ulna much wider than the distal end, and the presence of 41 presacral vertebrae. Furthermore, the new species differs from Diandongosaurus in having the preorbital region shorter than the postorbital region, the prefrontal with no contact of the postfrontal along the dorsal margin of the orbit, the short mandibular symphysis with the entrance of the splenial, and the premaxillary and anterior dentary teeth fang like but not king-sized. Additionally, the anterolateral process of the clavicle is very sharp and slender in Diandongosaurus. Compared with the Dianopachysaurus, the new species is different in having no pachyostosis of the caudal ribs, no more than three carpal ossifications, and the rounded astragalus. Our phylogenetic analyses suggest that Dianmeisaurus is probably the sister group of Diandongosaurus. The two genera, together with the Majiashanosaurus, Keichousaurus, and Dianopachysaurus are grouped in a monophyletic clade and phylogenetically more closely related to the Nothosauroidea than the European pachypleurosaurs (Dactylosaurus, Anarosaurus, Serpianosaurus, and Neusticosaurus). The coexist of three small-sized eosauropterygians indicates that sauropterygians were a highly diversified group of marine reptiles in the Luoping fauna, Yunnan Province during the early Middle Triassic.