New age constraints for the Middle Triassic archosaur Lotosaurus: Implications for basal archosaurian appearance and radiation in South China

Abstract

Archosaurians were the dominant terrestrial tetrapods during the Mesozoic and became one of the most successful tetrapod clades on Earth following the end-Permian mass extinction. The lower Badong Formation in northwestern Hunan Province, South China hosts fossils representing the basal archosaurian Lotosaurus. The Badong Formation has been previously interpreted as Anisian-Ladinian (247–237 Ma) in age based on biostratigraphic correlations. This study presents new detrital zircon UPb age constraints for sandstones hosting Lotosaurus fossils in Sangzhi County of northwestern Hunan. Our data give a weighted mean age of 238.0 ± 1.4 Ma (Ladinian Stage) as the maximum depositional age for the fossil-bearing layer in the lower Badong Formation. This age is consistent with younger estimates from previous biostratigraphic studies. It is also more precise and stratigraphically constrained than previously published radiometric dating results for the units. This age estimate provides critical geochronologic constraint on the radiation of early archosaurians from China into other areas of Pangea beginning in the Early Triassic.