The Neoproterozoic Dzabkhan Biota: Algae, Microfossils, Trace Fossils and Problematical Remains from Western Mongolia  

The Ediacaran-Cambrian boundary marks one of the most important events in geological history.  However, in spite of the tremendous progress in Ediacaran-Cambrian stratigraphy and palaeontology, the location of this boundary – and indeed the most appropriate way to subdivide Cambrian and Ediacaran stratigraphy – is still a hot topic.

We examined Ediacaran-Cambrian siliciclastic-carbonate sections in southwest Mongolia, which contain both ichnofossils and small shelly fossils.  These fossils allow the age of the strata to be determined.  Treptichnus pedum proved to be an inadequate marker for the Ediacaran-Cambrian boundary in the Dzabkhan Zone.  The association of ichnofossils present alongside the first appearance of T. pedum is typical of younger ichnozones, correlating to the Upper Tommotian–Lower Atdabanian ichnozone III, and the Rusophycus avalonensis ichnozone from Newfoundland.

The Eastern European platform contains Ediacaran sediments which are associated with glacial deposits at their base, and yield an Ediacara-type fauna.  Microfossils have been derived from silicified, carbonate and phosphate layers of the upper part of the Tsagonolom Formation, which lies above the glacial Maikhanuul diamictites.  The Dzabkhan microfauna contains algae, large acanthomorphic acritarchs, multicellular thalli, sponge spicules and problematic remains.  This complex is comparable to Ediacaran assemblages such as Doushantuo of South China, Pertataka of Central Australia and the Ediacaran of the East European Platform.  The Ediacaran-Cambrian transitional layers contain problematic Ediacara-type fossils.  Problematic cyclic organisms from the Tsaganolom Formation are morphologically similar to the common Ediacaran genera Beltanelloides, Beltanelliformis or Nemiana, but poor preservation prohibits a rigorous identification.

The Ediacaran-Cambrian boundary can be established more precisely at the base of the Tommotian Stage by the presence of the SSF associations, trace fossils, microfossils and bioherm-forming algae.