UNUSUAL PATCHINESS OF EDIACARAN BENTHIC ASSEMBLAGES RELATIVE TO PHANEROZOIC AND MODERN BENTHIC ASSEMBLAGES

The Ediacara biota are a morphologically diverse, globally distributed set of benthic taxa generally interpreted as stem metazoans. In South Australia more than 20 fossiliferous beds have been excavated from the Rawnsley Quartzite, providing an unprecedented window into the structure of Ediacaran ecosystems. A striking feature of these beds is marked differences in diversity-abundance structure between even immediately superposed beds. To investigate whether such variability is in fact anomalous, we compared Rawnsley Quartzite assemblages to similarly densely sampled and lithologically homogeneous Phanerozoic fossil assemblages. We use multiplicative diversity partitioning to decompose within-assemblage diversity into within-bed (alpha) and between-bed (beta) components. The alpha diversity of the Rawnley Quartzite assemblage is slightly below the Phanerozoic median, but its beta diversity exceeds that of 98% of Phanerozoic assemblages. Although only a handful of bedding planes can be compared, the older Ediacaran assemblages of the Mistaken Pt. Formation, Newfoundland, are also above the 90th percentile of beta diversity for Phanerozoic assemblages. To test the hypothesis that the high beta diversity of Ediacaran assemblages is due to their exceptional preservation as ecological “snapshots” with minimal spatiotemporal averaging, we compiled datasets that sampled modern benthic invertebrates in homogenous substrates over a limited distance and depth range. These assemblages show a range of beta diversity values similar to that of Phanerozoic fossil assemblages, implying that the unusually high beta diversity of Ediacaran assemblages is unlikely to be entirely attributable to lack of reworking and time-averaging. Distributions of pairwise Bray-Curtis distances between beds in Phanerozoic assemblages are similar to those observed in modern benthic habitats over m to km distances, whereas those in Ediacaran assemblages are similar to those observed in modern benthic habitats over 10-100 km distances. We conclude that the exceptionally high variability of Ediacaran assemblages is a reflection of their ecology, and suggest that it may be a consequence of low motility and larval dispersal, lack of sediment disturbance by bioturbators, and a high degree of niche overlap.