THE MISTAKEN POINT BIOTA: ECOLOGY AND EVOLUTION OF THE EARLIEST EDIACARANS

The Mistaken Point biota of eastern Newfoundland represents the definite Ediacara-type fossils and thus the oldest large and architecturally complex organisms in Earth history (575-560 Ma). Mistaken Point organisms lived in a deep-slope environment below both storm wave base and the photic zone. Fossils in the Conception Group were preserved as census populations beneath beds of volcanic ash, whereas younger fossils of the Fermeuse Formation were mostly preserved beneath sandstone turbidites. An Ediacaran fossil Lagerstätten near Spaniard's Bay consists of numerous small fronds that were exquisitely preserved in three dimensions within concretions.

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.