When Life Got Big: the Ediacara Biota and the Origin of Animals  

The Ediacara biota (578–543 Ma) represents a fundamental watershed in the history of life on Earth – the appearance of abundant large eukaryotes after 3 billion years of mostly microbial evolution.   The biota comprises cm- to m-scale, soft-bodied organisms preserved as impressions under rapidly deposited beds of sandstone or volcanic ash.  Ediacara-type fossils are known from about 40 localities worldwide, and immediately preceded the shelly fossils of the Cambrian. 

Early Ediacaran fossils (578–558 Ma) are known exclusively from deep-water deposits in Newfoundland, England, and NW Canada, implying that large and complex eukaryotes evolved in this environment.  The oldest-known Ediacaran megafossils, metre-long fronds and other fossils from deep-water turbidites in Newfoundland, immediately followed the final Neoproterozoic ice age (Gaskiers glaciation; 580 Ma) during a period of massive oxygenation of the deep sea.  Most early Ediacarans are rangeomorphs, an extinct clade (“failed experiment”) utilizing self-similar branching elements that were used as modules to construct an array of taxa with highly divergent shapes (e.g. Fractofusus, Bradgatia, Pectinifrons).  Rangeomorph communities were similar to those of modern suspension-feeding animals, implying that rangeomorphs may have been stem-group animals, but there is no evidence for guts, brains, or mobility among any taxa. 

Younger assemblages of Ediacara-type fossils (558–543 Ma) are known mainly from shallow-water settings, including the Flinders Ranges in Australia, the White Sea and Ukraine in Europe, and Namibia in southern Africa, with a few deeper-water occurrences in Siberia and western Canada.   Rangeomorphs are present but much reduced in importance and mostly found in deeper-water environments.  Shallow-water assemblages are dominated by petalonamids, an extinct clade that utilized straw-like elements as modules to build a variety of growth forms.  Possible sponges have been reported but, despite early reconstructions that showed the Ediacara biota dominated by jellyfish and soft corals, no unequivocal cnidarians have been described.  Abundant peristaltic burrows attest to the presence of bilaterians and some intriguing taxa such as Kimberella and Spriggina may represent bilaterian body fossils.  Calcareous shells of Cloudina and Namacalathus represent the oldest evidence of skeletonization.  Other ecological innovations included mobile grazing on the microbial mats that covered much of the sea floor, but there is no evidence of macrophagous predation or scavenging among any of the soft-bodied organisms of the Ediacara biota. 

The youngest-known Ediacaran megafossils are 543 Ma petalonamids (Pteridinium and Swartpuntia) immediately below the base of the Cambrian in Namibia.  A few possible Ediacaran survivors have been reported from the Cambrian but are controversial.  Earlier attempts to explain the disappearance of the Ediacara biota as reflecting a taphonomic loss are seemingly contradicted by their apparent absence from Cambrian lagerstätten such as the Burgess Shale.  Latest Neoproterozoic anoxia and/or predation and competition from newly evolving Cambrian animals represent more likely explanations for the abrupt demise of the Ediacara biota, an early experiment in the evolution of large eukaryotes.