Shale-hosted biota from the Dismal Lakes Group in Arctic Canada supports an early Mesoproterozoic diversification of eukaryotes

The Mesoproterozoic is an important era for the development of eukaryotic organisms in oceans. The earliest unambiguous eukaryotic microfossils are reported in late Paleoproterozoic shales from China and Australia. During the Mesoproterozoic, eukaryotes diversified in taxonomy, metabolism, and ecology, with the advent of eukaryotic photosynthesis, osmotrophy, multicellularity, and predation. Despite these biological innovations, their fossil record is scarce before the late Mesoproterozoic. Here, we document an assemblage of organic-walled microfossils from the 1590–1270 Ma Dismal Lakes Group in Canada. The assemblage comprises 25 taxa, including 11 morphospecies identified as eukaryotes, a relatively high diversity for this period. We also report one new species, Dictyosphaera smaugi new species, and one unnamed taxon. The diversity of eukaryotic forms in this succession is comparable to slightly older assemblages from China and is higher than worldwide contemporaneous assemblages and supports the hypothesis of an earlier diversification of eukaryotes in the Mesoproterozoic.

Taxonomy, paleoecology and biostratigraphy of the late Neoproterozoic Chichkan microbiota of South Kazakhstan: the marine biosphere on the eve of metazoan radiation

Carbonaceous bedded cherts of the late Neoproterozoic (Cryogenian) ∼800 to 750 Ma old Chichkan Formation of South Kazakhstan contain an abundant, diverse assemblage of exquisitely preserved microorganisms. Like many Proterozoic microbiotas, the Chichkan assemblage is dominated by prokaryotic cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, represented primarily by cellular trichomes and empty sheaths) and coccoidal (chroococcaleans and pleurocapsaleans, including solitary, colonial, and stalk-forming specimens). However, unlike Proterozoic microbiotas reported from peritidal settings, the Chichkan fossils, permineralized in cherts deposited in the open shelf facies of the formation, include diverse microscopic eukaryotes: vase-shaped testate amoebae, spiny (acanthomorphic) phytoplanktonic unicells, large (up to ∼1 mm diameter) megasphaeromorphic acritarchs, and sausage-shaped vaucheriacean green alga-like filaments.Given the composition of this biota and the presence in it and similarly aged assemblages of numerous taxa typical of late Neoproterozoic deposits (e.g., Cerebrosphaera, Jacutianema, Melanocyrillium, Stictosphaeridium, Trachyhystrichosphaera, and Vandalosphaeridium), the Chichkan Lagerstätte appears representative of the Cryogenian biota as now known, thereby documenting the status of the marine biosphere at a time closely preceding the radiation of the Metazoa. As such, we interpret this and other coeval mixed assemblages of prokaryotic and eukaryotic microfossils as representing an evolutionary stage transitional between the predominantly prokaryote-dominated Precambrian and the eukaryote-dominated Phanerozoic biospheres.As reported here, the Chichkan assemblage is composed of 39 taxa (of which two forms are described informally) that are assigned to 23 genera of microscopic prokaryotes and eukaryotes and that include two new species: Polybessurus crassus n. sp. and Vandalosphaeridium koksuicum n. sp.