The Ediacaran Biotas in Space and Time

Abstract

The "Ediacaran organisms," which preceded and overlapped the Cambrian radiation of metazoans, include many fossils whose systematic positions remain contentious after over fifty years of study. It might seem that nothing particularly useful can be learned from a biota full of oddballs. However, analyses of the distribution of the Ediacaran organisms in time and space can be carried out without having to guess at the systematic position of the organisms. Combining these results with data on paleotectonics, paleoenvironmental parameters, and the ages of various assemblages sheds light on the origins, ecology, and even the systematic positions of the Ediacaran organisms. Parsimony Analysis of Endemism (PAE) confirms earlier studies in grouping Ediacaran biotas into three major clusters: the Avalon, White Sea, and Nama Assemblages. The available radiometric and stratigraphic data suggest that the Avalon is the oldest, the White Sea is next oldest, and the Nama extends to the base of the Cambrian. The "frondlike" Ediacaran taxa, and to a lesser extent the "medusoids," collectively show significantly longer stratigraphic ranges, broader geographical and paleoenvironmental ranges, and less provinciality than "bilaterian" and tubular taxa. Almost all tubular Ediacarans appear to be confined to equatorial areas, whereas other Ediacaran organisms show weak or no latitudinal diversity gradients. I conclude that the Ediacaran organisms show a diverse range of responses to various environmental parameters. There is no basis for classifying them all as having a single body plan and mode of life, as has often been done in the past.