PALEOBIOLOGY OF THE MESOPROTEROZOIC BILLYAKH GROUP, ANABAR UPLIFT, NORTHERN SIBERIA

Silicified peritidal carbonates of the Mesoproterozoic Kotuikan and Yu smastakh Formations, Anabar Uplift, northeastern Siberia, contain exce ptionally well-preserved microfossils. The assemblage is dominated by ellipsoidal akinetes of nostocalean cyanobacteria (Archaeoellipsoides) and problematic spheroidal unicells (Myxococcoides); both are allocht honous and presumably planktonic. The assemblage also includes distinc tive mat-forming scytonematacean and entophysalidacean cyanobacteria, diverse short trichomes interpreted as cyanobacterial hormogonia or ge rminated akinetes, rare longer trichomes, and several types of colonia l unicells. Although many taxa in the Kotuikan-Yusmastakh assemblage a re long-ranging prokaryotes, the overall character of the assemblage i s distinctly Mesoproterozoic, with its major features shared by broadl y coeval floras from Canada, China, India, and elsewhere in Siberia. M icrofossils also occur in middle to inner shelf shales of the Ust'-Il' ya and lower Kotuikan Formations. Leiosphaerid acritarchs (up to sever al hundred microns in diameter) characterize this facies. As in other Mesoproterozoic acritarch assemblages, acanthomorphic and other comple x forms that typify Neoproterozoic assemblages are absent. The combina tion in Billyakh assemblages of exceptional preservation and low eukar yotic diversity supports the hypothesis that nucleated organisms diver sified markedly near the Mesoproterozoic-Neoproterozoic boundary. The assemblages also demonstrate the antiquity of cyanobacteria capable of cell differentiation and suggest the importance of both changing peri tidal substrates and evolving eukaryotes in determining stratigraphic patterns of Proterozoic prokaryotes. The permineralized assemblage con tains 33 species belonging to 17 genera. Ten new species or new combin ations are proposed: Archaeoellipsoides costatus n. sp., A. elongatus n. comb., A. dolichos n. comb., A. minor n. nom., A. crassus n. comb., A. major n. comb., A. bactroformis n. sp., Veteronostocale medium n. sp., Filiconstrictosus cephalon n. sp., and Partitiofilum yakschinii n . sp.