Evolutionary paleoecology and taphonomy of the earliest animals: Evidence from the Neoproterozoic and Cambrian of southwest China

The Neoproterozoic and Cambrian soft-bodied fossil deposits, or lagerstätten, of southwest China provide invaluable windows into life during the initial radiation of animals. This dissertation examines various aspects of two of these lagerstätten: the Neoproterozoic Doushantuo Formation and the Early Cambrian Chengjiang fauna. Of primary interest is the taphonomy of the earliest known animal fossils, the phosphatized animal embryos of the Neoproterozoic Doushantuo Formation, and the evolutionary paleoecology of animals during the Cambrian “explosion”, as preserved in the Early Cambrian Chengjiang fauna and adjacent strata. A sedimentological and petrographic study of the embryo-bearing interval of the Doushantuo Formation demonstrates that there are two distinct phosphogenic environments in which the Doushantuo fossils were phosphatized. These results may explain the distribution of probable fossils described from the Doushantuo Formation. A detailed specimen-based taphonomic study of the Doushantuo embryos indicates that there is a taphonomic bias toward early cleavage stages and away from later cleavage stages and adults. One possible explanation for this pattern is that earlier cleavage stages were more physically robust and thereby better able to withstand the abundant reworking inherent in phosphogenic settings. Analysis of the paleoecology of benthic metazoans in the Chengjiang fauna and the sediments in which they are preserved indicates that a majority of these benthic metazoans were adapted to survive on substrates characterized by low levels of bioturbation more typical of the Neoproterozoic. Furthermore, a comparison between the paleoecology of benthic metazoans in the Chengjiang fauna and the younger Middle Cambrian Burgess Shale fauna of British Columbia, Canada reveals that the Burgess Shale fauna contains a larger percentage of benthic metazoans adapted to survive on more intensely bioturbated substrates more characteristic of the Phanerozoic. Increasing bioturbation levels thereby likely had a profound effect upon the early evolution of animals.