ECOLOGICAL SUCCESSION IN THE NEOPROTEROZOIC

Ediacaran fossils at Mistaken Point, Newfoundland, are preserved beneath volcanic ash layers on large bedding surfaces, recording a "snapshot" of the benthic community as it appeared during life. This exceptional preservation as in situ census populations, with no spatial mixing or time averaging, allows precise description of the composition and ecological attributes of these deep-water Neoproterozoic communities. The seven studied Mistaken Point communities display significant between-community variation in species richness, fossil areal coverage (a proxy for biomass), diversity, and evenness values, as well as in the complexity of epifaunal tiering structures and spatial patterns. These observed differences are most consistent with the changes in community structure expected during ecological succession, suggesting that the fossil surfaces at Mistaken Point record "snapshots" of early, middle, and late successional communities. Ediacaran ecological succession at Mistaken Point is characterized by a progression from low diversity communities dominated by recumbent organisms ("spindles" or "pectinates") to higher diversity assemblages dominated by complexly tiered frondose taxa (Charnia, Charniodiscus, and "ostrich feathers"). These changes in species composition during a tolerance succession model have implications for the paleobiology of the constituent taxa, and suggest that the early successional forms may have required higher food supplies than those that dominated later stages. The presence of well-developed ecological succession trends in these oldest-known animal communities implies that they were governed by many of the same ecological processes characteristic of Phanerozoic and modern communities.