The stratigraphic and geographic occurrences of Permo-Triassic tetrapods in the Bogda Mountains, NW China — Implications of a new cyclostratigraphic framework and Bayesian age model

The Junggar and Turpan basins of Xinjiang Uygur Autonomous Region, northwest China, host a well-preserved terrestrial Permo-Triassic boundary sequence exposed along the northern and southern flanks of the Bogda Mountains. During the Permo-Triassic transition, this region was located in mid-latitude northeast Pangaea, making it an important comparison to the well-studied higher latitude record of the South African Karoo Basin. Broad similarities are apparent between the tetrapod fossil records of both areas, such as the reported co-occurrence of Dicynodon-grade dicynodontoids and Lystrosaurus in the upper Permian and the high abundance of Lystrosaurus in the Lower Triassic. However, the exact placement of the Permo-Triassic boundary in Xinjiang and South Africa has been debated. In the Chinese sections, the Permo-Triassic boundary falls within the upper Guodikeng Formation (= upper Wutonggou low order cycle), but several horizons have been proposed based on biostratigraphy, chemostratigraphy, and paleomagnetic data. A new Bayesian age model that is calibrated by multiple radiometric dates and tied to detailed litho- and cyclostratigraphic data offers new insight into the location of the Permo-Triassic boundary in Xinjiang and the opportunity to reconsider tetrapod occurrences in a highly resolved chronostratigraphic framework. We investigated the positions of new and historic tetrapod collections relative to the revised Permo-Triassic boundary, including uncertainties about the locations of key historic specimens. The stratigraphic range of Dicynodon-grade dicynodontoids in Xinjiang is poorly constrained: most specimens, including the holotype of Jimusaria sinkianensis, cannot be precisely placed relative to the Permo-Triassic boundary. A new specimen of Turfanodon sp. for which we have reliable data occurs in the upper Permian. Despite their previous treatment as Permian in age, most Bogda chroniosuchians were collected in strata above the Permo-Triassic boundary and the therocephalian Dalongkoua fuae also may be Triassic. Some prior placements of the Permo-Triassic boundary in Xinjiang imply an upper Permian lowest occurrence for the cosmopolitan dicynodont Lystrosaurus, but all Lystrosaurus specimens that we can precisely locate fall above the Permo-Triassic boundary. The high abundance of Lystrosaurus in the well-dated Early Triassic of Xinjiang suggests an Early Triassic age for the interval of greatest Lystrosaurus abundance in the Karoo Basin, where the exact ages of putative Permo-Triassic strata are more controversial. The Xinjiang record is an important datum for determining whether there was a single, globally synchronous time of highest Lystrosaurus abundance.