MICROFOSSILS FROM THE GREYSON FORMATION, LOWER BELT SUPERGROUP: SUPPORT FOR EARLY MESOPROTEROZOIC BIOZONATION

The ability to reconstruct or infer relationships between Precambrian paleobiological, geochronological and lithostratigraphic datasets ultimately hinges on establishing a robust paleontological library with sufficient resolution to distinguish macroevolutionary patterns from ecological or taphonomic idiosyncrasies of particular deposits. Progress is being made in dividing up the Neoproterozoic by reference to global-scale macroevolutionary patterns, but the preceding Mesoproterozoic is renowned for its low rate of evolutionary turnover of fossil taxa. A new assemblage of microfossils exhibiting complex cellular morphologies has been recovered from the >1.45 billion-year-old Greyson Formation of the lower Belt Supergroup, Montana, the first to be reported from early Mesoproterozoic deposits of Laurentia. Although dominated by sphaeromorphic acritarchs, the assemblage includes populations of Tappania, Valeria, Satka, Dictyosphaera, Caudosphaera and longitudinally striated tubes, in addition to long-ranging filamentous forms. The Greyson microfossils are conspicuously similar to those from the coeval Roper Group of Australia, though Tappania is more generally distributed throughout the early Mesoproterozoic and has also been reported from deposits in Russia, India and China. Compilation of data from the lower Belt Supergroup and other coeval deposits points to the utility of recognizing an early Mesoproterozoic (~1550-1450 Ma) assemblage zone represented by Tappania plana, Valeria lophostriata, Satka favosa, and Dictyosphaera delicata. The Greyson microfossils, in conjunction with data from coeval assemblages, may be useful for reconstructing the historical transition to the eukaryote-dominated biosphere that followed throughout the remainder of the Proterozoic and Phanerozoic.