Fossil microbial communities in Neoproterozoic interglacial rocks, Maikhanuul Formation, Zavkhan basin, Western Mongolia

Abstract

Glaciogenic deposits are widely distributed in the Neoproterozoic successions around the world, but only few of them are associated with occurrences of authentic macroscopic fossils. Problematical circular remains recovered from intertillite beds of the Neoproterozoic Maikhanuul Formation, Zavkhan basin, Western Mongolia may be a new record of biogenic structures in ancient glacial rocks. The Maikhanuul tillites were deposited between 732 and 777Ma (the underlying volcanic rocks of the Neoproterozoic Dzabkhan Formation) and 632±14Ma (the overlying limestones of the Ediacaran – Nemakit – Daldynian Tsagaanolom Formation). The assemblage of microfossils from the Tsagaanolom Formation is comparable to the assemblage known from the 635–551Ma Doushantuo Formation of South China. The Maikhanuul glacial event may therefore be correlated with the 635Ma Nantuo glaciation. Morphological, taphonomic, biometric and geochemical observations have allowed us to interpret the problematical circular remains from the Maikhanuul tillites as microbial communities preserved in situ on the floor of an ancient glacial basin. Many discoidal fossils from Precambrian strata have been compared with extant microbial colonies that have a similar external morphology. However, ultrastructure of the hypothetical Precambrian microbial communities, to the best of our knowledge, has not previously been reported. Scanning electron microscopy of the circular remains from the Maikhanuul tillites revealed rod-shaped, filamentous and star-shaped bacteriomorph structures. Some microremains and especially the star-shaped particles are considerably enriched in iron (up to 50%) that may be attributed to bacterial activity. Furthermore, the star-shaped microremains can be compared with modern star-shaped bacteria known from freshwater and terrestrial environments and with aggregates of biogenic nanohematite. Although the presence of microbial communities could not be used for global correlation, it does provide information on prokaryotic diversity and ecology as well as on palaeoenvironmental conditions.