TAPHONOMY OF VASE SHAPED MICROFOSSILS FROM THE LATE TONIAN CALLISON LAKE DOLOSTONE, YUKON

Vase-shaped microfossils (VSMs) are interpreted as the remains of testate amoebae, and are found in Late Tonian sedimentary rocks around the world. A new assemblage of VSMs has recently been described from the Callison Lake dolostone in Yukon, Canada. Dated with a Re-Os isochron age of 739.9 +/- 6.1 Ma, these microfossils are indicative of the early diversification of eukaryotic life before the Sturtian-age Snowball Earth event, and are roughly coeval with diverse VSM assemblages from the Chuar Group of Grand Canyon, Arizona. Here we investigate the taphonomy of the Callison Lake VSMs. We analyzed fossils in petrographic thin section, on etched rock surfaces, and from HF macerations. EDS maps were used to analyze the spatial distribution of elements in all three preparation techniques to interpret the taphonomic pathways. We found that silica is pervasive in all samples from thin sections and etched surfaces. In most samples, there are accumulations of carbon at the edges of fossils, and in some, a high concentration of carbon throughout. In other cases, however, we do not see heightened concentrations of these elements. Rather, aluminum and other clay minerals were found to be associated with fossil rims. Fossils from macerated samples are comprised entirely of organic carbon, as all silica would have been removed during maceration. We hypothesize that these macerate fossils represent the internal molds of VSMs. This suggests there are multiple modes of preservation for VSMs in this assemblage. We suggest that at least three taphonomic processes are involved: authigenic mineralization, original preservation, and internal molds. By selecting fossils in thin section and using EDS techniques on these same specimens we are able to more easily and quickly associate certain taphonomic pathways with VSM specimens. This will allow us to determine which mode is dominant in the preservation of VSMs and better understand their relationship to other VSM assemblages, which are preserved via different taphonomic pathways. Understanding the preservation of VSMs will help us to better determine why these fossils are absent from younger rocks, and give us insight into their potential use as index fossils for the Late Tonian.