GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 173-13
Presentation Time: 11:10 AM


BOTTJER, David J.1, YIN, Zongjun2, ZHAO, Fangchen3 and ZHU, Maoyan3, (1)Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, (2)State Key Lab of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS, East Beijing Road,39, Nanjing, 210008, China, (3)State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing, 210008, China

The Ediacaran-Cambrian is a time with globally enhanced production of phosphorites. This includes perhaps the two most famous deposits with variable cell and soft tissue phosphatization in microfossils, the Ediacaran Doushantuo Formation, which contains the Weng′an Biota, and the lower Cambrian Kuanchuanpu Formation and equivalents, found in China. A wide range of variables is responsible for phosphatization during this interval, including depositional and geochemical conditions and microbial activity. Degree of microfossil phosphatization is widely dependant upon original cell and tissue types. Microfossils with non-diagenetic morphologies that are iconic for modern crown group metazoans have had the highest level of acceptance for their interpreted phylogenetic affinity. Microfossils with these iconic morphologies are rare from the Weng′an Biota, but relatively common from the Kuanchuanpu Biota. Thus metazoan affinity for Doushantuo microfossils has commonly relied on relatively simple and less-well-known morphologic features of embryonic organisms, for which there is not a concensus on the nature of their phylogenetic signal. In the Weng′an Biota, the presence of embryonic cellular structures detected in three-dimensional microfossils and studied with synchrotron analysis offers intriguing evidence of bilaterian animal origins. These few scattered clues of a bilaterian presence may be all that can be found from the Weng′an Biota. This may be because animals had not evolved suitable recalcitrant tissues for phosphatization or a through gut that promoted phosphatic preservation of bilaterians by Doushantuo time (~610 Ma). However, the presence of the Eocyathospongia specimen, interpreted as a likely sponge-grade animal, indicates that there is potentially more to be found to inform us from this fascinating interval of the evolution of life on Earth. Differences in the phosphatized microfossil record between the Weng′an and Kuanchuanpu Biotas are thus due to the combination of a primary signal reflecting the initiation of the Cambrian explosion, and a secondary signal driven by a multitude of taphonomic processes operating at different levels of effectiveness during deposition of these two units.