MORPHOLOGICAL SATURATION AND CONSTRAINTS IN EDIACARA FOSSILS

Ediacara organisms show three stages of evolution, each of which is represented by a fossil assemblage of distinct taxonomic composition. These three assemblages are, in geochronological order, the Avalon, White Sea, and Nama assemblages. Compared to the Avalon and Nama assemblages, the White Sea assemblage consists of a much greater number of Ediacara taxa. In this study, we analyzed the morphological diversity (disparity) of Ediacara fossils in order to further characterize the evolution of Ediacara fossils. We collected morphological data of nearly 200 Ediacara taxa from more than 10 assemblages that were described in 20 monographs. We coded 49 morphological characters describing overall body shape, symmetry, segmentation, and many other features believed to be biological rather than taphonomic in origin.

Our preliminary analysis of these data using dissimilarity analysis and multidimensional scaling method suggests that the three Ediacara assemblages have similar morphological disparity and occupy largely the same area in the Ediacara morphospace. This pattern in disparity and morphospace utilization suggests that, to the extent of coded characters, Ediacara morphospace was saturated since the Avalon assemblage. The subsequent taxonomic diversification without morphospace expansion in the White Sea assemblage implies possible morphological constraints on Ediacara evolution.