THE MISTAKEN POINT BIOTA: ECOLOGY AND EVOLUTION OF THE EARLIEST EDIACARANS
The oldest Mistaken Point fossils postdate the glacial diamictites and cap carbonate of the Gaskiers Formation (580 Ma) by less than 5 million years, implying a causal relationship between the termination of Neoproterozoic glaciation and the proliferation of animal life. The cosmopolitan Ediacaran frond Charniodiscus, widely regarded as a stem-group cnidarian, occurs commonly in the Mistaken Point biota, but bilaterian body fossils and trace trace fossils are conspicuously. More than 85% of all specimens and species in the Mistaken Point biota are rangeomorphs, a group of colonial organisms that exhibited a modular construction of similar, highly fractal elements. These elements were combined as modules to construct frond-, spindle-, comb-, leek-, or bush-shaped colonies. Rangeomorphs appear to represent an extinct high-level clade of early radial animals, perhaps intermediate between the poriferans and the cnidarians. Rangeomorphs characterized the first 15 million years of Ediacaran evolution, perhaps because their fractal growth and modular construction required less genetic complexity than was required by other animal phyla. Rangeomorphs were unable to compete with later, more highly evolved animals, and occur only rarely in younger Ediacaran assemblages and are not known from any Cambrian or younger assemblages.