EVIDENCE AND ABSENCE OF EDIACARAN STYLE FOSSIL ASSEMBLAGES IN THE PROTEROZOIC

The search for Proterozoic megascopic fossils has been marked by extremes. True fossils occur in large numbers at discovery sites and have been found inter-regionally and on more than one continent. In contrast, most Proterozoic dubiofossils are confined to one or a few specimens from restricted sites. Their origins are best explained by deformation of sediment or disruption of microbial mats. All claims for early Proterozoic multicellular fossils require exceptional evidence, and should expect close scrutiny, if only because of the possible implications: 1) repeated evolution of multicellularity; 2) a cryptic multicellular fossil record; 3) an unreliable fossil record; 4) search failure.

The shared characteristics of the Mesoproterozoic “string of beads” form Horodyskia and late Neoproterozoic Ediacara assemblages, provide pointers in the hunt for more Proterozoic mega-fossils. Despite their age difference, these fossil deposits exhibit similar sedimentary settings and taphonomy. They were both preserved on microbial-mat-bound substrates by event beds in prograding deltaic sedimentary settings.

In contrast, 1.6 Ga structures claimed as branching animal burrows (Vindhyan of India), and the discs and paired strings from the 1.7 Ga Stirling Range sandstones (Western Australia), remain isolated in time and space. Study of the sedimentary context may produce alternative explanations for these curious discoveries.

Horodyskia found in both the Bangemall Group (Western Australia) and the Belt Supergroup (Montana), has several taphonomic variants. Although superficially resembling certain Ediacaran forms it has no clear affinities with metaphytes or metazoans.

Ediacaran fossil assemblages are distinguished by many, and diverse, body fossil impressions including one or more Ediacarian index fossils (e.g. Charnia, Palaeopascichnus, Pteridinium, Nemiana). While metaphytes appear to have evolved before the Neoproterozoic, there is still no reliable fossil evidence for metazoans before 575 Ma, or coelomate-grade animals before 560 Ma.