Emergence and ecological transition of the Mesozoic marine reptiles: Evidence from the Early Triassic Chaohu and the Middle Triassic Xingyi Faunas

Abstract

Two major evolutionary events in the early history of Mesozoic marine reptiles have been recorded in the Triassic of China. The first event evidenced by the Early Triassic Chaohu Fauna was the emergence of new marine communities and their subsequent radiation in the Spathian Subage of the late Olenekian (Early Triassic). The earliest marine reptile fossil horizon was found to be 248.81 Ma in age, therefore about 3.35 myr after the end-Permian mass extinction. The emergence time for the clade Ichthyosauromorpha has been suggested to be between 251.5 and 248.8 Ma. Moreover, the first occurrence of this clade is not thought to have been prior to the end-Permian mass extinction and this is therefore indicative of an exceptionally high evolutionary rate. Globally, twenty eight marine reptile species (six from Chaohu) have been recorded in an interval of just 2 myr in the Early Triassic, and these marine reptiles diversified and became the predators in the Mesozoic marine ecosystems that formed soon after the end-Permian mass extinction. The first event was followed by a transition from coastal to oceanic communities followed by selective disappearance of coastal species and emergence of oceanic species in the Ladinian of the Middle Triassic, as evidenced by the Xingyi Fauna. Here a 5.1 m thick fossiliferous unit yields 17 or more taxa of marine reptiles and can be subdivided into two separate assemblages. The lower assemblage is mainly composed of nearshore sauropterygians, close to that of the Anisian Panxian Fauna and with a strong palaeobiogeographic affinity to western Tethys. In the upper assemblage, the coastal pachypleurosaurian and nothosaurian sauropterygians disappear, to be replaced by the large ichthyosaurs and pistosaurs with a closer palaeobiogeographic affinity to Panthalassa. In this assemblage, there is an example of an ichthyosaur unequivocally feeding on another marine reptile. This is the oldest definitive record of a marine reptile acting as an apex predator. The transition from the lower assemblage to the upper one also records a turnover of Mesozoic marine reptiles from coastal to oceanic forms, and from a strong west Tethyan affinity to an assemblage showing a greater affinity with east Panthalassa by the late Middle Triassic.