an opposite extreme, as indicated by the cap-carbonates immediately 

 above the glacial sediments. 



The period following the last of these glacials, starting at about 

 600 million years ago, saw two remarkable radiations of animal life 

 (Conway Morris, 1993). The first of these is known as the Ediacaran 

 radiation and involves soft-bodied organisms whose remains are typ- 

 ically preserved in sandstones. The first evidence of these turned up 

 in the Nama basin of southern Namibia as early as 1908, and was 

 described by Giirich in 1933. These organisms were typically flat 

 or leaflike with a very characteristic quilted structure, reminiscent 

 of an air mattress, but there were also circular, medusoid-like struc- 

 tures. A similar fauna came to light in 1946 in the Ediacara Hills 

 of South Australia and it is from this locality that the radiation 

 gained its name. Since then, similar fossils have become known 

 from at least 30 localities on 5 continents (Narbonne, 1998). 



The remarkable structure shown by these organisms prompted 

 Seilacher (1992) to create a new Kingdom, the Vendobionta, for 

 them and he wrote that they were, 



immobile foliate organisms of diverse geometries that were only a few millimetres 

 thick, but reached several decimetres in size. A shared characteristic is the serial 

 or fractal quilting of the flexible body wall, which stabilised shape, maximised 

 external surface and compartmentalised the living content. Since no organs can be 

 recognised, this content is thought to be a plasmodial fluid rather than multicellular 

 tissue. (Seilacher, 1992) 



In a later development of this concept, Buss and Seilacher (1994) 

 put forward the hypothesis that the Vendobionta should be regarded 

 as a phylum, rather than a Kingdom, constituting a monophyletic 

 sister-group of the Eumetazoa. They speculated that the Vendobionta 

 were cnidarian-like organisms that lacked stinging cells, or cnidae, 

 and that cnidarians arose later through acquisition of cnidae by sym- 

 biosis with microsporidians. In this view, Ediacaran fossils are not 

 thought to have been ancestors of living coelenterates, but rather, 

 represented a failed experiment in the history of animal life. 



The affinities of Ediacaran organisms have been the subject of 

 vigorous debate in the last 20 years, but whatever they actually were, 

 it is now clear that this fauna existed for about 55 million years, 

 showing maximum diversity during the last 20 million (Narbonne, 



20 



