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Environmental Controls on the Taphonomy of Phosphatized Animals and Animal Embryos from the Neoproterozoic Doushantuo Formation, Southwest China

¹ Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740; sdornbos{at}uwm.edu
² Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 2100008, China
³ Division of Biology, California Institute of Technology, Pasadena, CA 91125
⁴ Department of Life Sciences, National Tsing Hua University, Hsinchu, Taiwan, China, and National Museum of Natural Science, Taipei, Taiwan, China

Phosphatized animals and animal embryos of the Neoproterozoic Doushantuo Formation of Southwest China provide what is likely the earliest fossil evidence for animals, including bilaterians. This research utilizes field and petrographic analyses of the animal-fossil-bearing interval of the Doushantuo Formation and general observations of animal and animal-embryo preservation in order to gain insight into the taphonomic processes involved in the preservation of these fossils. Results indicate that there are two genetically related phosphatic lithofacies within the animal- and animal-embryo-bearing Weng'an Phosphorite Member: a lower black facies and an upper gray facies. Within each of these facies, phosphogenesis and phosphatization took place under different environmental conditions. The black facies is a pyrite-rich bituminous phosphorite that lacks dolomite in its lowermost two meters and contains evidence for lower levels of reworking than the gray facies. Deposited on top of a karstic sequence boundary, the black facies is interpreted as a condensed, sediment-starved deposit that shallows upward into the higher-energy gray facies. Abundant matrix-forming dolomite and greater levels of reworking characterize the gray facies. Both facies were deposited in shallow, nearshore-marine environments. The environmental differences between these two members are of great taphonomic importance because of the resulting difference in reworking levels. Lower levels of reworking in the black facies indicate the presence of a larger taphonomic window in this member because fewer phosphatized fossils were destroyed during reworking. The search for future significant Doushantuo fossils should focus on this black facies.

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