THE EDIACARA BIOTA: NEOPROTEROZOIC ORIGIN OF ANIMALS AND THEIR ECOSYSTEMS

Although different molecular phylogenetic clocks suggest that the Metazoa originated sometime between 1200-600 Ma, all putative fossil animals older than 575 Ma are equivocal or have been refuted. Reduced oxygen levels and nearly continually cold conditions through the Cryogenian and early Ediacaran Periods (ca. 740-580 Ma) may have restricted the size and abundance of early animals.

The oldest reasonably unequivocal fossil animals are represented by the Ediacara biota (630?, 575-542 Ma), an assemblage of cm- to m-scale, soft-bodied animals that occur as impressions on the bases of sandstone event beds or beneath beds of volcanic ash. Ediacaran assemblages 575 Ma from the Avalon Zone of Newfoundland include fronds up to 2 m long and other large fossils that postdate the youngest Neoproterozoic glacial deposits of the Gaskiers glaciation by less than 5 myr. These may be coeval with the appearanceof large spiny acritarchs in Australia and the Doushantou animal embryos in China, implying that the release of the pressures of the so-called “snowball” Neoproterozoic glaciations was a major event in eukaryote evolution. Early Ediacarans (575-560 Ma) were mainly rangeomorphs, an extinct phylum(?) of highly fractal organisms that constructed modular frond-, bush-, comb-, or feather-shaped colonies that probably functioned as filter-feeders. Rangeomorphs dominated earlier and deeper-water Ediacaran assemblages but gradually disappeared throughout the Ediacaran, possibly due to competition with evolving radial and bilaterian animals, and are not known from Cambrian or younger assemblages.

The advent of bilaterians ca. 560 Ma permitted widespread mobility, scavenging, and herbivory in the classic White Sea Ediacaran assemblages. The advent of calcified shells and macrophagous carnivory is known only from the final stages of the Ediacaran (Nama assemblages, <550-542 Ma). The Ediacara biota abruptly disappeared 542 Ma, perhaps due to oceanic anoxia or methane release (as marked by a short-lived C-isotope excursion to -10‰) or due to ecological escalation accompanying the Cambrian “explosion” of shells and brains.