SHIFTING SANDS: ARE LATE EDIACARAN FOSSIL ASSEMBLAGES RELIABLE RECORDS OF ORIGINAL ECOSYSTEMS?

The Ediacaran Nama Group of Namibia (~549–539 Ma) preserves soft-bodied and biomineralized macrofossils of early metazoans. Recent research has revealed information on the feeding habits, reproductive strategies, life histories, and macro-organism response to fluctuating redox conditions of terminal Precambrian oceans. However, assessments necessitate that the fossilised organisms are preserved in the environments in which they originally lived. Using sedimentological evidence from fossil-bearing beds in the Witputz Sub-basin, we demonstrate ubiquitous transport of macro-organisms in sediment gravity flows (debris flows, turbidites, and linked debris flow-turbidite ‘hybrid’ beds), sourced from up-slope shallower-water environments. Transported Rangea and Ernietta are entrained in ungraded-unsorted mud-sand debris flow beds; transported Cloudina and Namacalathus are entrained in debris flow, linked ‘hybrid’ flow, and turbidite beds. Individual macro-organisms act as clasts within beds, with orientations (horizontal, mixed, or chaotic) reflecting sediment transport mode (laminar, transitional, or turbulent flow, respectively) rather than life position or feeding behaviour. Rare storm event beds, conversely, smother and preserve in-situ assemblages of entirely different taxa, including tubes, Beltanelliformis, and simple horizontal burrows in relatively deep-water depositional environments. Distinct in-situ and transported fossil assemblages are preserved in close stratigraphic proximity but may not have been in close environmental proximity or interacted in terminal Precambrian ecosystems. Transport complicates relationships between macro-organism and geochemical redox data, as well as understanding of bed-scale ecology, species interaction, and feeding behaviours of taxa in this region.