Contributions to the paleobiology of proterozoic eukaryote evolution

On the southern margin of the North China Platform, supposedly Neoproterozoic (1000 {dollar}sim{dollar} 544 Ma old) successions contain a unique microfloral assemblage of large (ca. 100 {dollar}mu{dollar}m in diameter) acanthomorphic acritarchs (single-celled eukaryotes). Integrative stratigraphic analysis and correlation using petrofabrics, radiometric and carbon isotopic data, however, suggest that these "Neoproterozoic" successions are in fact Mesoproterozoic (1600 {dollar}sim{dollar} 1000 Ma old) in age. This new age assignment implies that morphological diversification of protists inferred from molecular phylogenies may have accelerated in the Neoproterozoic Era but began earlier.; In South China, the Neoproterozoic Doushantuo Formation (570 {dollar}pm{dollar} 20 Ma old) contains two Lagerstatten, a carbonaceous compression assemblage in Miaohe near the Yangtze Gorges and a cellularly preserved biota in Weng'an, Guizhou Province. Miaohephyton bifurcatum from the Miaohe biota preserves parenchymatous cellular structures, apical and intercalary growth patterns, abscission and reproductive structures. In combination these features suggest that Miaohephyton bifurcatum is a phaeophyte, possibly a fucalean. This interpretation indicates that the secondary endosymbiotic event that gave rise to the photosynthetic stramenopiles must have occurred before 570 {dollar}pm{dollar} 20 Ma. In addition, along with other fossils, Miaohephyton bifurcatum implies that the major algal clades not only diverged, but also independently achieved multicellularity during the Neoproterozoic Era.; Doushantuo phosphorites in Weng'an preserve both algal thalli and animal embryos in cellular detail. Globular microfossils previously interpreted as colonial algae are here reinterpreted as equally cleaving, holoblastic animal blastulas; their cleavage geometry, constant size at different cleavage stages, and enclosing envelopes all support an embryo interpretation. It is as yet uncertain whether and how these embryos might have developed into adults. Regardless, these embryos are thought to have had a non-planktotrophic developmental mode because of their relatively large egg size. This and other paleontological evidence implies the antiquity of non-planktotrophy.; Doushantuo embryo fossils also point to a bright future of searching for well preserved micrometazoans in older phosphatic nodules. To understand where and how to look for phosphatized fossils in older rocks, a more complete understanding of phosphatization is necessary. Through SEM examination, it has been demonstrated that phosphatic encrustation and impregnation are the principal processes involved in the phosphatization of Doushantuo soft tissues. In addition, the preservation style of animal embryos is different from that of algal thalli; the phosphatization of animal soft tissues may be facilitated by infesting bacteria.; The Mesoproterozoic large acanthomorphic acritarchs and Doushantuo embryo fossils do not minimize the magnitude and importance of major eukaryotic diversification in the Neoproterozoic Era and metazoan diversification during the Cambrian Explosion. Instead, they suggest that both eukaryotes and metazoans may have a protracted prehistory before their respective explosive radiations, a pattern that has long been recognized in the fossil record of other groups.