AMMONOID AGE CONTROL OF THE EARLY TRIASSIC MARINE REPTILES FROM CHAOHU (SOUTH CHINA)

Mesozoic marine reptiles firstly appeared in the Early Triassic (late Olenekian) after the end Permian mass extinction. They diversified quickly and became the top predators of the Mesozoic marine ecosystem soon after their invasion to the sea. Compared with the derived descendants in the Middle Triassic, the most primitive groups from Early Triassic has been poorly known due to the rare and fragmentary material of poorly age control. Recently, Chaohu Fauna of South China from the Lower Triassic (Spathian) Nanlinghu Formation has attracted much attention for yielding abundant and complete marine reptile skeletons along with numerous fishes and invertebrates such as arthropods, bivalves and ammonoids. They provide new insight not only in vertebrate taxonomy but also in biostratigraphy and paleoecology. Previous research on ammonoid stratigraphy suggested that the first appearance of Mesozoic marine reptiles (such as the ‘oldest’ ichthyosaur Utatsusaurus and Chaohusaurus) was in Subcolumbites zone, which indicated the age of late Spathian. Here we described a series of ammonoids from the beds yielding Chaohu Fauna fossils and recognized the occurrence of Procolumbites for the first time in this area. The Procolumbites layer is about one meter above the first appearance of Chaohusaurus, indicating that the oldest Chaohusaurus is within the Procolumbites zone of middle Spathian age. This new age control is significantly older than the previously suggested assignment (late Spathian). To date, Chaohusaurus is the oldest known ichthyopterygian so far, which is also in accordance with the latest phylogenetic analysis of this group.