Kotuikan Formation assemblage: A diverse organic-walled microbiota in the Mesoproterozoic Anabar succession, northern Siberia

• Published in: Precambrian research 2015-01, Vol.256, p.201-222
• Main Authors: Vorob’eva, Natalya G., Sergeev, Vladimir N., Petrov, Peter Yu
• Format: Article
• Language: English
• Subjects: Anabar Uplift; Biostratigraphy; Mesoproterozoic; Microfossils; Siberia
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2014.11.011

•The Kotuikan compressed organic-walled microfossil assemblage ∼1500-Ma-old.•The Kotuikan microbiota contains many microfossils of Neoproterozoic appearance.•The microbiota is dominated by large and morphologically complex microorganisms.•The Kotuikan microfossil assemblage does not contain acanthomorphic acritarchs.•The microbiota contains new taxa of microfossils unknown in younger deposits. Abundant and diverse microfossils from shales of the Lower Member of the ∼1500-Ma-old Kotuikan Formation in northern Siberia document early Mesoproterozoic life along the Siberian Platform. Similar to many Meso- and Neoproterozoic microbiota, the Kotuikan assemblage is dominated by prokaryotic cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, which are represented primarily by cellular trichomes and by empty sheaths) and coccoidal (chroococcaleans, including solitary and colonial specimens). However, unlike Mesoproterozoic microbiota of shallow-water settings, the Kotuikan compressed organic-walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes: large (up to ∼1-mm diameter) megasphaeromorph acritarchs, branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans, as well as other morphologically complex microorganisms. The Kotuikan assemblage contains numerous taxa previously recorded from Neoproterozoic successions (e.g., Elatera, Eosolena, Palaeastrum, Pterospermopsimorpha, Rugosoopsis and Navifusa), 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 mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits. The Kotuikan assemblage documents the status of the marine biosphere at an early Mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote-dominated Paleoproterozoic and the eukaryote-dominated Neoproterozoic microbiota. As reported in this study, 35 taxa were identified in the Kotuikan assemblage (of which five forms are described informally) that are assigned to 18 genera of microscopic prokaryotes and eukaryotes. Two new genera and five new species are proposed: Hirudiforma lancetica gen. and sp. nov., H. simmetrica gen. and sp. nov., Lineaforma elongata gen. and sp. nov., E