Boreskov

COMPLEX HETEROTROPHIC EUKARYOTES ATTHE MESOPROTEROZOIC–NEOPROTEROZOIC BOUNDARYNagovitsin K.Trofimuk Institute of Petroleum‐Gas Geology and Geophysics of SB RAS, Novosibirsk, RussiaAlthough the late Proterozoic record of biotic evolution is resolved in very general terms,it has become evident that the rise of biodiversity and morphological complexity ofmicroorganisms was very nonuniform. One of the greatest transformations in the history ofbiosphere occurred in late Mesoproterozoic and early Neoproterozoic: morphologicaldiversity of large complex microfossils in contemporaneous rocks suggests diversification ofeukaryotes and their incorporation into prokaryotic ecosystems. Phylogenetic affinities ofextinct eukaryotes are difficult to establish based on morphological criteria only; however,when taphonomic features of fossil preservation are taken into account, it is possible toelucidate probable role of the extinct organisms in trophic structure and test themorphological criteria.The Lakhanda fossil microbiota in the east of the Siberian Craton provides the bestpaleontological record across the Mesoproterozoic–Neoproterozoic boundary.Morphological and taphonomic features of the microfossils suggest that the Lakhandamicrobiota included saprotrophic eukaryotes:1. Caudosphaera expansa Hermann – organisms that formed mycelia with outgrowingvertical long multiseriate filamental stipes ending with spherical sporangia. Preservation onseveral bedding planes indicates that in life the mycelium was embedded in the sediment,with sporangia protruding above the sediment surface. Most likely, the organisms wereheterotrophs.2. Organisms that formed procumbent (two‐dimensional) netlike thalli with diversecellular structure and growth strategies.2.1. Coenocytic thalli growing by expansion; first, ringlike structures form by fusion ofthe ends of elongated juvenile cell, then processes form and anastomose in the spaceenclosed by the ring, and finally the ring expands and is divided into two new ringlikestructures.2.2. Coenocytic thalli growing by dichotomous branching and anastomosis of claviformprocesses.122