Abstract: Ammonoids (order Ammonoidea), one dominate taxon of cephalopods during Mesozoic, which appeared in the Devonian and went extinct in Cretaceous−Paleogene event. They had an important position in ancient ecosystems, both as predators and important prey for small fish, sharks even some marine reptiles. Based on their fossil records, Ammonoids inhabited surface to deep waters of epicontinental sea, and were present cosmopolitan, both in the southern and northern hemisphere extend subtropical to polar areas. Their diversity shifts are closely correlated to environmental changes, including extinction events and climate variations.
As one of the most important climate shifts during upper Triassic, Carnian Pluvial Episode (CPE) marks a time of profound changes to life, in the ocean and on land. CPE started at the base of Julian 2 and end in Tuvanlian 2 and the sedimentological phenomenon is an increase in rainfall that caused input of terrigenous sediments and black shales deposition into sedimentary basins and the carbonate production meet crises. Previous studies believed some clades of ammonoidea met crisis during the Julian-Tuvalian boundary interval. However, there were few ammonoid diversity research during the Julian 1- Julian 2 boundary interval, which marked the beginning of CPE.
In this study, we described an ammonoid assemble from Julian 1- Julian 2 boundary interval of Luxi county, Yunnan province, south-west China. A total of more than 200 ammonoids were collected, and 13 species of 9 genera were recognized, among which Yakutosirenites and Anasirenites were the first discovered in south-west China. Basing on our research and data from Paleobiology Database (PBDB) we find that at the genus level, the Trachyceratidae experienced an explosive radiation in Julian 2, such as Anasirenites, Sirinites, Neosirenites, Seimkanites, Pamphagosirenites, Striatosirenites, Austrotrachycerasand Neoprotrachyceraswere flourished during this time. The reason of Trachyceratidae explosive radiation may caused by CPE. To be more specific, during CPE, the heavy rainfall carried a lot of terrigenous clastics into the sedimentary basins. It caused the seafloor became flattened and broaden the epicontinental sea field. It provided a suitable environment to Trachyceratidae living and reproduction. Secondly, an amount of terrigenous clastics input the marine would also bring different nutrients into sedimentary basin. It may increase the key elements which Trachyceratidae needed. Thirdly, major climate change reformed the whole marine ecosystem. It may create some new ecological niches that can be occupied by Trachyceratidae.
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