Relatively large and diverse body fossils and traces of the Ediacaran assemblage must have been preceded by the evolution of simpler, probably smaller, metazoans. The ancestral benthic burrowing metazoans may not have been fossilized but should be recognizable by bioturbate fabric in the mud they inhabited. An examination of the shale of the 1.3-eon (1.3 × 109 yr)-old Belt Supergroup disclosed no hint of biologic reworking, indicating either that the Belt basin was a large anaerobic seaway or that Belt sedimentation predated the evolution of burrowing metazoans. The lack of bioturbation, even in what were probably shallow shelf environments, supports the latter conclusion that metazoans capable of bioturbation evolved later than 1.3 eons B.P.

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