The Triassic U–Pb age for the aquatic long-necked protorosaur of Guizhou, China

Abstract

The ancient marine limestone beds of the upper part of the Guanling Formation, Panxian County, Guizhou Province, SW China, yielded a wide range of high-diversity well- preserved marine reptiles such as the fully aquatic protoro- saur with an extremely long neck Dinocephalosaurus ori- entalis, the oldest mixosaurid ichthyosaurs and lariosaurs. However, there is no precise isotopic age to study the in- triguing origin, evolution and emigration history of the im- portant fauna. We report a sensitive high-resolution ion mi- croprobe (SHRIMP) U–Pb zircon age for a volcanic tuff bed within the upper part of the Guanling Formation. The result indicates that the age of the fossil horizon is 244.0±1.3 Ma, 14 Maearlierthanthepreviouslyestimatedagebasedoncon- odont evidence. We consider that the marine reptiles had a relatively rapid evolution during Middle Triassic time, some 8 Ma after the end-Permian mass extinction.