POPULATION STRUCTURE IN THE EDIACARA BIOTA FROM MISTAKEN POINT, NEWFOUNDLAND

The presumed affinities of the enigmatic Ediacara biota have been much debated. However, even in the absence of concrete phylogenetic affinity, numerical paleoecological approaches can be effectively used to make inferences about organismal biology, the nature of biotic interactions, and life history. One aspect of the Ediacara biota that has not yet received much attention is in-depth analysis of their population structure. The Mistaken Point Ecological Reserve in Newfoundland, Canada contains the oldest Ediacaran communities (579 ~ 560 Ma), with thousands of fossils preserved in situ on the upper surfaces of large, decameter-scale bedding planes. These organisms were originally soft-bodied and have been exceptionally preserved by overlying volcanic ash in a Pompeii-style death mask, representing census assemblages ideally suited for size-frequency distribution studies. We used data compiled by Clapham et al. (2003) focusing on three rangeomorph taxa (Fractofusus, Beothukis, Pectinofrons), and one non-rangeomorph taxon (Thectardis). We used univariate and multivariate Bayesian Information Criterion (BIC) in the statistical environment R to compare population structure models. Size-frequency histograms reveal positive (right) skewed unimodal distributions with large variance, with relatively few individuals in the smallest size classes, followed by a peak in the lower end of the distribution, and decreasing numbers in larger size classes. The best-supported population structure using BIC resolves communities of all studied Ediacaran taxa at Mistaken Point as single cohorts. That conclusion can be explained by one (or more) the following hypotheses: 1) all populations represent single age-classes, indicating a single highly synchronous reproductive event; 2) the Ediacaran organisms possessed slow growth rates relative to rates of reproduction and/or recruitment thus size-frequency distributions appear continuous; 3) the studied organisms reproduced aseasonally (=continuously), so that size modes are absent. Numerical paleoecological studies offer an innovative means of evaluating population structure in extinct organisms, and ultimately may provide an alternative means of inferring phylogenetic affinities in enigmatic groups.