Carbonaceous Compressions are a widespread preservational style for fossils, yet their taphonomy remains poorly understood. Previous studies focusing on the taphonomy of carbonaceous compressions have primarily looked at exceptionally preserved faunas in plane view. The precious nature of these fossils leaves destructive techniques of analysis out of the question, but these techniques are necessary if the taphonomy of carbonaceous compressions is to be deciphered. This study analyzes Neoproterozoic carbonaceous compressions from the Yangtze Gorges area in order to address this issue. Chuaria fossils from the Jiulongwan, Sixi, and Sifangtan sections of the Doushantuo Formation and Vendotaenia fossils from the Wuhe and Miaohe sections of the Denying Formation are microchemically analyzed in both plane view and cross section in order to gain a greater understanding of the makeup of carbonaceous compressions. Results confirm and elaborate on previous studies. Likely clay coats are detected on some Chuaria specimens, while they are absent on less thermally mature specimens. Evidence for sulfate reduction in association with carbonaceous compressions is found. Sulfur enrichment, rather than clay coats, is found in association with Vendotaenia fossils. These observations lead to the hypothesis that while organic remains require a very precise set of taphonomic conditions in order to be preserved as carbonaceous compressions, there may be more than one set of conditions that allow for preservation. More studies of a greater taxonomic and taphonomic range of carbonaceous compressions are needed, however, if the mechanisms which control this preservational pathway are to be fully understood. / Master of Science

Macroscopic “carbonaceous” fossils such as Grypania, Katnia, Chuaria, and Tawuia play a critical role in our understanding of biological evolution in the Precambrian and their environmental implications. Unfortunately, understanding of these fossils remains limited by their relative simplicity of form, mode of preservation, and broad taphonomic variability. As a result, debate continues as to even the fundamental taxonomic affinity of the organisms. Megascopic coiled forms (i.e. Grypania and Katnia), for instance, have been interpreted as trace fossils, multicellular algae, prokaryotic filaments, macroscopic bacteria, cyanobacteria, or a transitional form from macroscopic to megascopic bacterial life. Similarly, Chuaria and Tawuia have been interpreted as compressed prokaryotic colonies, algae or algal reproductive stages, and multicellular plant material. Accessibility of new material and increasingly sophisticated means of analysis warrant a new look at these ancient fossils. Understanding the biological affinity of Grypania, in particular, is critical because current opinion is split as to whether these megascopic structures are more likely represent either multicellular bacteria or multicellular algae. Confirmation of either a bacterial or algal affinity would strongly influence fundamental understanding of biospheric evolution, particularly in terms of ocean oxygenation and the availability of bioessential trace metals. Although estimates for the degree of oxygenation required for a Grypania-like multicellular algae are only about 10 % present atmospheric levels (PAL), this estimate is still substantially higher than estimates based on geochemical data suggesting that oxygen levels may not have reached 10% PAL until the latter Neoproterozoic. It has been hypothesized that protracted oxygen of the Proterozoic biosphere may have played a critical role in the availability of redox-sensitive nutrients necessary for bacterial nitrogen fixation and the limiting of eukaryotic evolution. Within this context, our understanding of the taxonomic affinity of Grypania may profoundly affect our understanding of Earth’s biospheric evolution. This thesis provides morphological and geochemical analyses of Grypania spiralis from more than 100 newly collected specimens from the Belt Supergroup for comparison to previously collected specimens from all other known Grypania-bearing localities. Data is used to explore questions regarding the morphology, structural complexity, mode of preservation, and chemistry of fossil material, and to hypothesize on the taxonomic affinity of Grypania spiralis and its implications for biospheric evolution.