Conodonts suggest a late Spathian (late Early Triassic) age for Thaisaurus chonglakmanii (Reptilia: Ichthyosauromorpha) from Thailand

Abstract

The timing of the emergence of marine reptiles following the Permian–Triassic mass extinction is crucial for studying the recovery of marine ecosystems in the Triassic. Conodonts, key index fossils in the Triassic, are widely used to date the marine reptiles, especially in the Early Triassic. Triassospathodus anhuinensis is a conodont species originally discovered in the upper Spathian of South China, but lacked a comprehensive description, leading to misidentifications and confusion in previous research. Here, we reevaluated Tr. anhuinensis and summarized its morphological characteristics and spatial and temporal distribution based on a review of the literature. We then critically evaluated the age of the primitive ichthyosaur Thaisaurus chonglakmanii from Thailand based on the updated knowledge of Tr. anhuinensis. We sampled the Khao Thong section where T. chonglakmanii was found for conodont study. Tr. anhuinensis elements were obtained from the bed where T. chonglakmanii was collected and also in the bed five metres above the T. chonglakmanii fossil horizon, suggesting a late Spathian age for T. chonglakmanii. A late Spathian age for T. chonglakmanii questions the hypothesis of Thailand as the place of origin of ichthyosaurs in the late Induan or early Spathain.