Evidence and significance of eukaryotic diversification in the ∼1800 million-year-old Changzhougou Formation, North China

The evolution of cellular complexity is both an exciting and contentious topic in paleobiological research. Traditionally, the Mesoproterozoic has been considered the time of major eukaryote diversification; however, evidence suggests that origin of eukaryotic clades occurred earlier. Recognizing eukaryotic microfossils and understanding eukaryote diversification patterns in pre-Phanerozoic rocks is based in part upon morphological characters (e.g., acritarch processes) that are either not abundant or not present in the early parts of the Proterozoic Era. Regardless if the paucity of these features is due to preservational issues or if it predates their evolution, new and refined approaches are required to understand the earliest branching patterns of the Eukarya on the tree of life.;The research herein focuses on fossils and fossil-like structures from the ∼1800 Ma Changzhougou Formation (Yanshan Range), North China. Chapter II describes how macroscopic compression-like structures can be misinterpreted as compressions of a biogenic origin. Carbonaceous compressions farther upsection in the Changcheng Group (∼1600-1800 Ma) are more promising, but are not the focus of this thesis.;Pangjiapu locality acritarchs (Changzhougou Formation) described and discussed in Chapters III and IV show that microscopic fossils in the lowermost Changcheng Group lend better insight into eukaryotic diversification. Although previous research at Pangjiapu (e.g., Yan, 1991) provided inflated estimates of eukaryotic diversity, examination of original specimens, along with acritarchs extracted by refined palynological techniques, indicates that at least 13 distinct eukaryotic morphotypes are present in the Changzhougou Formation. This assessment is based upon the following factors: primary shape, number of wall-layers, vesicle flexibility, and size. The revised eukaryotic diversity is about five times less than the original report by Yan (1991), yet it still upholds that at least a modest number of taxa from the Domain Eukarya were already present by ∼1800 Ma.;Further technological improvements of alternate analyses (e.g., biomarker, TEM, Focused Ion Beam Electron Microscopy) will continue to improve our understanding of early eukaryotes; nevertheless, this study clearly demonstrates that transmitted light study of acritarchs will continue to be an invaluable aspect of understanding and updating our knowledge of the diversification patterns of early eukaryotes.