EVOLUTIONARY AND ECOLOGICAL IMPLICATIONS OF BODY SIZE DYNAMICS OF EDIACARA ORGANISMS

The peculiar biota that inhabited the oceans during the Ediacaran Period have few, if any, modern analogs from which to infer behavior and interactions among individuals, populations, and the environment. Given the considerable amount of published data on Ediacaran fossils, even with the enigmatic phylogenetic nature of these organisms, it is possible to derive ecological information by analyzing their body size distributions through time. Body size is a fundamental character of organisms and is often correlated with a number of ecological variables including population size, geographic range size, metabolic and reproductive rates, and risk of extinction, among many others. Over-dispersion of body size among taxa has been used as an indicator of niche partitioning. Fluctuations in variation through time within a group may signify a change in selective pressures, where previously occupied niches are made available and new taxa can diversify and fill the void. Using this approach, applied to data collected from an extensive literature review of Ediacaran fossil occurrences, we can better piece together the story of the rise and fall of the Ediacara biota. A meta-analysis of body size measurements of approximately 190 species occurrences from 20 formations, representing over 5000 individuals, in addition to an analysis restricted to samples of ten or more specimens were performed. Both analyses yielded a common trend of decreasing variability of body size for circular, oval, and spindle body plans, followed by an increase in size variability of tubular body plans. Along with changes in size variation, we see, a reduction in size of the circular, oval, and spindle forms coinciding with the rise of the tubular forms, which display an increase in body size and taxonomic diversity, around 550-545 Ma. The relative timing of increases and decreases in body size values and variation of the diverse Ediacaran baupläne provide potential clues as to the evolutionary ecology and community structure during their acme and subsequent decline.