Trace Fossils, Ichnofabrics and Problematic Sedimentary Structures at the Proterozoic-Cambrian Transition: Implications for the Early Evolution of Animals and their Habitats

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Trace Fossils, Ichnofabrics, and Problematic Sedimentary Structures at theProterozoic-Cambrian Transition: Implications for the Early Evolution ofAnimals and Their HabitatsMary DroserEAR-0074021The Cambrian radiation was the most significant event in the history of complex animals. Critical to this understanding this event is the record of the burrows, tracks and trails made by soft-bodied animals and preserved in the fossil record; these are known as trace fossils. Additionally, animals mix or churn the sediment and produce a mottled fabric known as ichnofabric. The importance of the ichnological record is exemplified by the fact that the Precambrian-Cambrian boundary is defined on the basis of trace fossils (Narbonne et al., 1987). This research project will be the first detailed systematic field analysis of the correlation of changes in ichnofabrics, discrete trace fossils and mat-related sedimentary structures across the Precambrian-Cambrian boundary. For the proposed project PI has selected four sections located in Newfoundland, Namibia, South Australia and western United States. These sections provide a complete range ofshallow-water siliciclastic facies. Examination of terminal Proterozoic trace fossils has applications to questions of the appearance of bilaterian animals. In addition, the rise in bioturbation (sediment mixing) at the terminal Proterozoic-Cambrian boundary interval profoundly and irreversibly changed the benthic habitat. One consequence was the reduction in microbial binding of siliciclastic sediments, which seem to have occurred in fully marine settings. These microbial mats may have been responsible for the unusual style of preservation of the soft-bodied Ediacara-type faunas. It is also likely that unusual sedimentary structures owe their formation and preservation to these mats. To date there has been no detailed examination of the changes in ichnofabric and discrete trace fossils to constrain the notion of the disappearance of a 'taphonomic window'