Investigations of enigmatic Neoproterozoic eukaryotes

The fossil record of the Neoproterozoic contains evidence of the diversification of eukaryotes, including the origins of animals and macroalgal clades. In this thesis I have sought to understand the timing, mechanism, and causality of eukaryotic evolution in Precambrian seas by examining the eukaryotic fossil record. My research closely analyzes a select set of fossil assemblages from the Neoproterozic and integrates new information about the fossil record with our knowledge of geochemical proxies, modern phylogenetics, and physiology.;Three fossil assemblages are highlighted in this thesis. The first is a tubular macrofossil assemblage from the terminal Ediacaran Nama Formation of Namibia. I analyze the morphology and chemical composition of these abundant fossils using SEM and EDS. I conclude that they were originally flexible organic-walled structures, most likely of algal grade, and discuss the fact that they are the most biomass-rich fossil assemblage in Precambrian rocks. The second fossil group is a collection of large organic walled microfossils, commonly known as acritarchs. I have reclassified many Ediacaran acritarchs, usually interpreted as phytoplankton, as the resting stages of animals. To do this I use evidence from SEM and TEM to compare their ultrastructure and morphology with modern analogs. I also compare the stratigraphic relationships of these putative animal fossils with other Ediacaran macrofossils. My new interpretation is consistent with our understanding of the causal mechanisms of resting stages induction in modern marine invertebrates. It is also consistent with the diversity trends of these unique fossils as oxygen tensions rise in the world's oceans. The third group of fossils are enigmatic scale morphotypes from the mid Neoproterozoic Tindir Group, Yukon. I document additional chert-hosted specimens and also document new limestone-hosted specimens, preserved in sub-micron detail. I show that all the scale microfossils are made of apatite, and thus potentially represent the oldest known evidence for biomineralization in eukaryotes in the fossil record. Using new specimens, I revise existing taxonomic descriptions and describe novel taxa. Taken together, these projects contribute to our growing body of knowledge on the Precambrian history of life and its relationship to the changing Earth system.