A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction.

Da-Yong Jiang,Jian-Dong Huang,Neil P Kelley,Cheng Ji,Ryosuke Motani,Rong Zhang,Andrea Tintori,Nicholas C Fraser,Olivier Rieppel,Yuan-Chao Hu,Wan-Lu Fu

doi:10.1038/srep26232

Abstract

Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval. However, emerging discoveries of Early Triassic marine reptiles are questioning this traditional view. Here we present an aberrant basal ichthyosauriform with a hitherto unknown body design that suggests a fast radiation of early marine reptiles. The new species is larger than coeval marine reptiles and has an extremely small head and a long tail without a fluke. Its heavily-built body bears flattened and overlapping gastral elements reminiscent of hupehsuchians. A phylogenetic analysis places the new species at the base of ichthyosauriforms, as the sister taxon of Cartorhynchus with which it shares a short snout with rostrally extended nasals. It now appears that ichthyosauriforms evolved rapidly within the first one million years of their evolution, in the Spathian (Early Triassic), and their true diversity has yet to be fully uncovered. Early ichthyosauromorphs quickly became extinct near the Early-Middle Triassic boundary, during the last large environmental perturbation after the end-Permian extinction involving redox fluctuations, sea level changes and volcanism. Marine reptile faunas shifted from ichthyosauromorph-dominated to sauropterygian-dominated composition after the perturbation.

Highlights

Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval
The earliest evolution of the group still remains elusive, recent studies recognize that they form the clade Ichthyosauromorpha with Hupehsuchia, a group of Early Triassic marine reptiles that inhabited a part of the South China block that later became western Hubei Province, China[2,3,4]
We examined the holotype of Cartorhynchus lenticarpus (Anhui Geological Museum (AGB) 6257)[2] and described specimens of Chaohusaurus (Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) V4001, AGB P45-H85-25, AGB P45-H85-20, AGB MT10010)[5,6,7], Hupehsuchus (IVPP V3232, Wuhan Center of China Geological Survey (WGSC) V26000)[8,9], Eohupehsuchus (WGSC 26003)[10], Nanchangosaurus (Geological Museum of China (GMC) V646, WGSC 26006)[3,11], and Parahupehsuchus (WGSC 26005)[12] for comparison

Summary

Introduction

Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval. We present an aberrant basal ichthyosauriform with a hitherto unknown body design that suggests a fast radiation of early marine reptiles. Ichthyopterygia comprise a group of Mesozoic marine reptiles that are commonly referred to as ichthyosaurs. It is best known for the evolution of fish-shaped body profiles among its derived members that are represented by abundant well-preserved fossils[1]. There is a wide anatomical gap between Hupehsuchia and Ichthyopterygia and only one species is so-far known to fill the gap, namely the basal ichthyosauriform Cartorhynchus lenticarpus[2]. The discovery of Cartorhynchus largely advanced our understanding of the early evolution of ichthyosaurs, additional www.nature.com/scientificreports/. We report a second species of basal ichthyosauriform that differs significantly from Cartorhynchus in its anatomical features

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Conclusion