The Ediacaran biota has been informally divided into three ecostratigraphic units: the lowermost deep marine Avalon Assemblage, the overlying shallow-marine White Sea Assemblage, and the uppermost Nama Assemblage. While there are some shared taxa among all three, they have been considered relatively robust, informal, biostratigraphic entities. We document a rich new fossil site that includes most of the Avalon Assemblage biota from the EM Coombs Surface at Inner Meadow, Newfoundland, Canada, and date it to 550.78 ± 0.60 Ma, equivalent to the youngest White Sea Assemblage faunas. This demonstrates that the Avalon and White Sea assemblages are contemporaneous, that the differences between them are ecological, not stratigraphic, and that the Avalon Assemblage biotas were also affected by the 550 Ma Kotlin Crisis that has hitherto been considered to only affect the White Sea Assemblage. The impoverished post-extinction Nama Assemblage succeeds the Kotlin Crisis, which our data reveal to be more profound than previously known. The Kotlin Crisis is considered to be the first significant extinction event experienced by the Eumetazoa, ∼10 m.y. before the Cambrian radiation.
McIlroy et al. (Thu,) studied this question.