Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution

The siliciclastic succession of the late Neoproterozoic Vendian Group in the White Sea area demonstrates a wide range of lithofacies, some recurring in a vertical succession. Significantly, each lithofacies contains a distinct assemblage of Ediacaran fossils that represents in situ benthic paleocommunities smothered in life position. These lithofacies define (1) a monospecific Inaria assemblage, restricted to the lower-shoreface muds; (2) a Charnia assemblage, within the middle-shoreface graded siltstone-shale couplets; (3) a Dickinsonia-Kimberella assemblage, confined to the interstratified sandstone and shale of prodelta; and (4) a Onegia-Rangea assemblage, preserved within channelized sandstone beds of the distributary-mouth bar. In the White Sea area a strong correlation exists between taxonomic composition, biostratinomic features, and paleoecological context of the Ediacaran fossil assemblages. Facies-controlled distribution is also evident in other Ediacaran localities, demonstrating the recurrence of similar facies relationships on a global scale. This pattern is interpreted as representing Ediacaran biofacies with Avalon-type biotas distributed in deep marine habitats, Ediacara-type biotas inhabiting microbial biofilms in shallow marine prodeltaic settings, and infaunal Nama-type biotas found in distributary-mouth bar shoals. This in turn reveals a marked degree of environmental sensitivity and ecological specialization. Correspondence between depositional environment and taxonomic composition speaks against any obvious biogeographic provinciality of the Ediacaran biotas, and also casts doubt on claims of substantial evolutionary change