Putting Numbers to Assemblages of the Ediacara Biota and the Significance of Novel Morphologies and Ecologies

Most fossil assemblages of the Ediacara Biota are not conducive to the use of standard comparative quantitative paleoecological techniques. The White Sea and Mistaken Point assemblages are notable exceptions (e.g. Grazhdankin, 2004, Clapham et al., 2003); however, neither of these localites provide the opportunity to examine successive beds.

The Ediacara Member of the Rawnsley Quartzite cropping out south of Ediacara in South Australia provides an opportunity to excavate successive beds at several intervals including many deposited under similar depositional conditions as well as several representing complimentary environments. A total of over 150 square meters are exposed in 15 bedding surfaces. In addition, fossils are preserved in 3-D and as bioclasts. While rarified species richness based on described species from the Ediacara Member is comparable to that of Mistaken Point and the White Sea assemblages, many of Ediacara Member fossils are yet to be described including some that represent novel constructional morphologies. Among these are numerous tubular organisms as well as a variety of more simple morphologies with few definable characters. Most striking is a fossil with unusual bed top preservation (rather than standard hyporelief) preserved consistently in both positive and negative relief within individual specimens. The fossils are interpreted to represent an organism with an encrusting lifestyle. The fossil morphology consists of a central raised portion (positive relief) that increases in size as the overall organism increases in size and grooves which radiate and diverge from this central portion. These fossils have similarities in size distribution, taphonomy and many aspects of gross morphology to fungi but encrusting algae cannot be ruled out.

The Ediacara beds show unpredictably very high values of evenness with each of 11 of the beds dominated by a different genus and 4 of the beds dominated by Aspidella and its taphonomic variants. -->