LIFE AFTER SNOWBALL: THE MISTAKEN POINT BIOTA AND THE ORIGIN OF ANIMAL ECOSYSTEMS

The Mistaken Point biota of eastern Newfoundland (575-560 Ma) represents the oldest large and architecturally complex organisms in Earth history. These Ediacaran fossils are preserved on more than 100 large bedding surfaces spanning nearly 4 km of section, each surface littered with tens to thousands of fossil specimens that died in place when they were smothered beneath beds of volcanic ash. The Mistaken Point biota lived in a deep-slope environment below both storm wave base and the photic zone. The oldest Mistaken Point fossils postdate the glacial diamictites and cap carbonate of the Gaskiers Formation (580 Ma) by only 5 million years, implying a causal relationship between the meltdown of the "Snowball" glaciers and the proliferation of animal life. However, these fossils of the Drook assemblage are large and complex, including Charnia fronds nearly two meters long, implying either extremely rapid rates of evolution or a pre-Snowball origin of the Ediacara biota. Fossil assemblages become increasingly diverse upward through the section, reaching a maximum of 15 taxa in the Mistaken Point Formation (565 Ma). The biota consists entirely of cm- to m-scale, soft-bodied, sessile, benthic organisms/colonies, many of them showing a similar fractal architecture. Trace fossils and bilaterian body fossils are conspicuously absent from the Mistaken Point biota. Mistaken Point fossil assemblages occur as untransported census populations that are readily amenable to study using modern methods in spatial ecology. Tiering above the sea floor showed patterns strikingly similar to those of Phanerozoic and modern suspension-feeding animal communities. Multispecies interactions were rarer than in modern oceans, but species richness, abundance, and diversity values, as well levels of intraspecific interaction, all fall within the typical range observed in modern slope communities. These studies show that ecological processes present in these early Ediacaran communities were strikingly similar to the processes that operate in modern deep-sea animal communities.