Microfossil paleontology and biostratigraphy of the early Mesoproterozoic Belt Supergroup, Montana

Abstract

The Belt Supergroup is one of few early Mesoproterozoic sites worldwide that record paleobiological evidence of enigmatic ancestral eukaryotes. Using low manipulation acid maceration techniques, two new assemblages of microfossils have been recovered from the Greyson and Chamberlain Formations of the early Mesoproterozoic Belt Supergroup of Montana. These microfossils provide a window into the paleobiology of early Mesoproterozoic Laurentia for the first time. By comparison with fossils from other deposits of comparable age, these assemblages may be used to infer key biochronological, paleoenvironmental and ontogenetic aspects about the original organisms. The Greyson Formation assemblage from outcrops along Newlan Creek includes populations of Tappania, Valeria, Satka, Dictyosphaera, Coneosphaera, Caudosphaera and longitudinally striated tubes. The assemblage also includes microbial mat networks of Siphonophycus and isolated occurrences of Oscillatoriopsis, Rugosoopsis and Obruchevella. The Newlan Creek microfossils are conspicuously similar to those from the broadly coeval Roper Group of Australia, pointing 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 Chamberlain Formation assemblage from drillcore near Black Butte includes Valeria, Leiosphaeridia, Synsphaeridium, Coniunctiophycus, Satka, Symplassosphaeridium and longitudinally-striated tubes. The Black Butte microfossils expand the assemblage diversity previously reported from Chamberlain Formation outcrops near Neihart, Montana. The Black Butte assemblage partially overlaps with, but is notably distinct from, the assemblage from Newlan Creek. The differences between the respective assemblages are most likely attributable to paleoecological zonation of the original organisms. The Black Butte microfossils are broadly consistent with assemblages reported from supratidal to intertidal shallow water deposits of the late Mesoproterozoic. By contrast, the Newlan Creek microfossils are comparable to the distal shelf environment assemblage of the Roper Group of Australia. Correlations between morphometric attributes of the fossils indicate the presence of modestly diverse, environmentally-partitioned ecosystems that included protistan-grade organisms capable of a variety of cell replication processes by the early Mesoproterozoic. Microfossils and macrofossils of the Belt Supergroup provide an unparalleled opportunity for resolving ecological and macroevolutionary relationships among some of the Earth's oldest known eukaryotic organisms.