ABUNDANCE AND MORPHOLOGICAL VARIATION OF VASE SHAPED MICROFOSSILS FROM THE LATE TONIAN CALLISON LAKE FORMATION, YUKON

Vase-shaped microfossils (VSMs), interpreted as the remains of testate amoebae, are found in Late Tonian sedimentary rocks around the world. A new assemblage of VSMs has recently been described from the Callison Lake Formation in Yukon, Canada. Found in silicified black shale horizons, sedimentological data suggests these VSMs inhabited a lagoonal or shelf interior environment. Dated with Re-Os geochronology at 752.7 +/- 5.5 – 739.9 +/- 6.1 Ma, these microfossils are indicative of the early diversification of eukaryotic life prior to the Sturtian-age Snowball Earth event, and are roughly coeval with the diverse VSM assemblage from the Chuar Group of Grand Canyon, Arizona. Here we investigate the distribution and morphological variation of the Callison Lake VSMs. Fossil abundance, morphospecies identification, and other characteristics were determined by utilizing point counting techniques on petrographic thin sections. Photographs of each “count,” coupled with high-magnification images of well-preserved and distinctive forms, were analyzed using ImageJ software to examine the morphological variation. In order to distinguish morphotypes we measured features such as test length, width, aperture diameter, and concavity. We found that fossil abundances are as high as ~8% of counts per thin section, but vary dramatically across sections and horizons, declining to <1%, even between samples only 0.2 m apart. Among imaged, distinctive forms, vase-shaped and curved morphotypes were found to be more abundant than hexagonal and ovular forms, which is consistent with assemblages from the Chuar Group. Additionally, analyses of well-preserved specimens reveal taxa comparable to those from the Chuar Group, as well as some previously undescribed morphotypes of VSMs. Overlapping species with Chuar Group VSMs include Melanocyrillium hexodiadema, Cycliocyrillium torquata, Bonniea dacruchares, and Bonniea pytinai. VSMs observed in one horizon indicate that scaled forms may be present. Further taxonomic classification of the Callison Lake VSMs will serve to help reconstruct Late Tonian ecosystems before the Snowball Earth events, expand our understanding of the early diversification of eukaryotic life in the Neoproterozoic, and help assess the viability of VSMs as index fossils for the Late Tonian.