Kwangsisaurus orientalis Dated From the Anisian Triassic of China

The age of Triassic marine reptile Kwangsisaurus orientalis (Sauropterygia) from Wuming, Guangxi, China

Authors:

Shang et al

Abstract:

The holotype of Kwangsisaurus orientalis Young, 1959 is the only Triassic marine reptile specimen known from Guangxi Zhuang Autonomous Region in southern China. This fossil was collected from Wuming County in 1950s. The exact provenance of the specimen is unclear, but it has long been regarded as an Early Triassic sauropterygian following Young’s original description (Li, 2006b; Li et al., 2008; Zhao et al., 2008). The uncertainty regarding its age has introduced confusion into the studies of biogeography and evolutionary history of Chinese Triassic sauropterygians. Based on recent field work and the help of several local geologists who have knowledge of the specimen’s history, we have determined that the holotype of K. orientalis was from a quarry situated 2 km southeast of Fupeng, Suliang Village, Xianhu Township, Wuming County (Fig. 1). The type specimen was obtained from a fossiliferous bed within thin-bedded marlite, which is considered as the lower part of the Banna Formation based on lithological features. This fossiliferous bed contains abundant ammonites such as Balatonites, Protrachyceras, and Leiophyllites, in addition to some fragmentary vertebrate bones and plant debris (Figs. 1, 2). It can be identified as Middle Triassic based on the ammonites. Above it lies a tuff layer with a thickness of 10-20 cm, which is a marker bed of the Banna Formation. The tuff layer was dated to (244.2±0.7) Ma by High-precision SIMS zircon U-Pb analyses, making the age of K. orientalis Pelsonian (Anisian, Middle Triassic). The Anisian age of K. orientalis is concordant with its reidentification as a pistosaurid by Rieppel (1999a). This age assignment also refutes the longstanding interpretation that sauropterygians appeared in the Youjiang Basin during the Early Triassic, and shows instead that K. orientalis is almost synchronous with the Panxian marine reptile fauna. The Chinese record of sauropterygians shows that this group had dispersed over a large area and adapted to a diverse range of environments by the Anisian.