The Ediacara

50 years ago the impressions of what seemed to be soft bodied animals were found in the Upper Proterozoic rocks of the Ediacara hills in southern Australia. Since then representatives of the Ediacaran fauna have been found on every continent, and in rocks dated at up to 600 million years old. More than 100 species have been described, all of them soft-bodied with high surface area to volume ratios and marked radial or bilateral symmetry. None of them seem to have been confined to a particular geographic region, indeed some have a worldwide distribution. The Ediacara have been the subject of continuous debate since their discovery for two reasons. Firstly, what place do they have in the 'explosion' scenario, how are they relationed to the more familiar early Cambrian faunas; and secondly, why are they preserved at all when they are exclusively soft-bodied organisms? The Ediacaran frond fossil Charniodscus oppositus. From A review of the frond-like fossils of the Ediacaran assemblage. Records of the South Australian Museum, 17(23).

The first question is one which has split palaeontologists into two groups. One group see at least some of the Ediacara as members of known phyla and therefore potential ancestors of the lower Cambrian metazoans, for example the Ediacaran frond-like fossils are possible cnidarians, and some of the bilaterally symmetrical segmented organisms could be ancestral to the annelids and arthropods. (references Waggoner. 1996). The opposing view is that the Ediacara are not on the main evolutionary line to later metazoans, they represent rather a 'failed experiment' in multicellular design, and perhaps are not really animals at all (references Buss & Seilacher. 1994).

This latter view also quite neatly explains the preservation conundrum. If the Ediacara are unique and unlike any biota alive since, it is possible their body walls were contructed from a substance tougher than conventional soft-bodied organisms, and therefore with greater fossilization potential. Another view is that the lack of, or restriction of burrowing and scavenging organisms in the Vendian favoured the preservation of the Ediacara.