FAST RADIATION OF EARLY TRIASSIC MARINE REPTILES IN THE WAKE OF THE END-PERMIAN EXTINCTION

It is widely held that the Triassic biotic recovery from the end-Permian extinction was prolonged for 5 to 8 myr due to excessively hot climate, global ocean anoxia, and ocean acidification caused by strong volcanism, and that early Mesozoic marine reptiles evolved slowly in the Early Triassic. However, recent discoveries of Early Triassic marine reptiles from southern China challenge this traditional view. Here we present two aberrant basal ichthyosauriforms and two ichthyopterygians from the mid Spathian Upper Member of Nanlinghu Formation at Chaohu, Sclerocormus parviceps is larger than coeval marine reptiles, about 1.6 m long, and has an extremely small head and a long tail without a fluke. Its heavy-set body bears flattened and overlapping gastral elements. Its sister taxon, Cartorhynchus lenticarpus, is a small species but has a larger forelimb and shorter trunk, and shares a short snout with rostrally extended nasals with Sclerocormus. It now appears that at least 26 marine reptile species occurred in the Middle-Late Spathian (in just 2 myr), amongst which 15 come from the South China Block. These include five hupehsuchians, two basal ichthyosauriforms, four basal ichthyopterygians, and four sauropterygians. This demonstrates that marine reptiles underwent a fast radiation before the end of Early Triassic, earlier than previously believed. These diverse marine reptiles and fishes such as Saurichthys, occupied the apex level as predators and are associated with abundant invertebrates such as bivalves, cephalopods and arthropods in the Early Triassic marine ecosystem. This shows the first glimpse of a recovering biotic structure reaching a high trophic level.

The high-resolution record of carbonate carbon isotope documents eccentricity modulation of carbon cycling through the Spathian and shows a strong obliquity signal. Based on this data, the age of the lowest stratigraphic level of marine reptile fossil found at Chaohu is 248.81 Ma, about 3.35 myr after the end-Permian mass extinction, and 1.35 myr after the large Smithian-Spathian event. It appears that the first occurrence of marine reptiles corresponds to a change in the response of the oceanic carbon reservoir to astronomical forcing, indicating the recovery of the ocean circulation that may have triggered the onset of a complex marine ecosystem.