The biostratigraphic conundrum of Siberia: Do true Tonian–Cryogenian microfossils occur in Mesoproterozoic rocks?

Abstract

Abundant and diverse microfossils from shales of the ∼1500-Ma-old Ust’-Il’ya Formation in northern Siberia document early Mesoproterozoic life along the Siberian Platform. Similar to many Meso- and Neoproterozoic microbiota, the Ust’-Il’ya assemblage is dominated by cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, represented primarily by cellular trichomes and empty sheaths) and coccoidal (chroococcaleans, including solitary and colonial specimens). Microbiotas from shallow-water settings that contain morphologically simple and small microfossils were originally taken to be representative of Mesoproterozoic life. However, the Ust’-Il’ya and other compressed organic-walled microfossil assemblages in open shelf facies include diverse microscopic eukaryotes: large (up to ∼1-mm diameter) megasphaeromorph acritarchs, branched filaments that are most likely of eukaryotic algae and other morphologically complex microorganisms.The Ust’-Il’ya assemblage contains numerous taxa previously reported from Tonian and Cryogenian successions (e.g., Elatera, Pterospermopsimorpha, Rugosoopsis and Chuaria), but these are long-ranging genera and no Neoproterozoic index fossils have been recorded. The microbiota is considered to be Mesoproterozoic in age because it contains a few taxa reported from the Calymmian microfossil assemblages (e.g., Lineaforma aff. L. elongata, Satka favosa and Elatera aff. E. minor). The Ust’-Il’ya mixed assemblage of prokaryotic and eukaryotic microorganisms records an evolutionary stage transitional between the prokaryote-dominated Paleoproterozoic and the eukaryote-dominated Neoproterozoic world.As reported in this study, 38 taxa were identified in the Ust’-Il’ya assemblage (of which six forms are described informly) that are assigned to 17 genera of microscopic eukaryotes (presumably 17–20 entities) and prokaryotes. A new species is proposed: Heliconema ustilense.