Ediacara fossil assemblage in the upper Vindhyans of Central India and its significance

Abstract

A fossil assemblage of soft-bodied megascopic metazoans possessing faunistic, ecological and taphonomic affinities to known classical Ediacaran assemblages has been discovered from the Proterozoic Vindhyan Basin of India. The assemblage is represented by nine coelenterate genera ( Tribachidium, Eoporita, Kaisalia, Cyclomedusa, Ediacaria, Nimbia, Paliella, Medusinites and Hiemalora), one arthropod genus Spriggina and a few unnamed possible new forms belonging to sponge and coelenterate. These fossils show facies-controlled temporal distribution forming two fossil zones: F 1 and F 2 located, respectively, in the Lakheri and Sirbu Formations (Bhander Group). A majority of them is common to the Ediacara assemblages of several continents, thereby suggesting their global biogeographic distribution. F 1 and F 2 in the local stratigraphy are separated by a thick stromatolitic carbonate facies devoid of metazoan remains. The Vindhyan Ediacara and host rock sequences reveal energetic (wave-tide-storm induced), shallow marine and near-shore environments of deposition of siliciclastic terrigenous facies. The carbonate facies parting suggests interruption of typical Ediacara environments by a broad spell of lime-rich quieter water settings favoring selective growth of metaphytes. The Vindhyan fossils show both Nemiana (higher relief forms without finer features in sandstones, e.g. Cyclomedusa and Ediacaria) and Beltanelliformis (lower relief forms with finer features in shales, e.g. Kaisalia and Hiemalora) types of preservation. Wide biogeography and ecological uniformity qualify the Vindhyan fossils as biostratigraphically significant group that fixes the age of the Lakheri–Sirbu segment at latest Neoproterozoic, helps in intercontinental correlation of the Bhander Group and focuses supra-Sirbu sequence as potential bearer of early Cambrian organic signatures. Predominance of forms representing Cycloza, an archaic group having monaxonic heteropolar symmetry of infinitely high order (e.g. Cyclomedusa) in F 1 and Inordoza having high order radial symmetry (e.g. Hiemalora) and Trilobozoa, an advanced group with fixed order radial symmetry (e.g. Tribachidium) in F 2 is consistent with the early evolutionary trend of coelenterates. Association of bilateral arthropod Spriggina, commonly thought to be ancestral to Arthropoda and Annelida now separated by huge phylogenic gulf, with possible discoid sponge of high-level organization and coelenterates suggests phyletic diversity of the Vindhyan Ediacara.