EXTINCTION OF THE EDIACARA BIOTA

The Ediacaran-Cambrian boundary signals a drastic change in diversity and in the structure of ecosystems, which, coupled with a strong negative C isotope anomaly, has been interpreted as evidence for a mass extinction. The Ediacara biota, consisting of stem group animals in addition to extinct higher-order clades, shifts to the familiar (and not so familiar) Cambrian and Paleozoic faunas. Although metazoans are demonstrably present in the Ediacaran, their ecological contribution is dwarfed by Ediacaran-type clades such as the Rangeomorpha and Erniettomorpha, while Ediacaran-type constructions such as fronds, flat-lying-recliners, mat-stickers, and mat-scratchers are virtually non-existent in younger assemblages. To evaluate the likelihood of a terminal Proterozoic mass-extinction, we explored temporal and biogeographic distributions of Ediacaran taxa combined with evaluations of morphospace ranges and ecosystem construction throughout the Ediacaran. The paucity of temporally-resolved localities with diverse Ediacaran assemblages, combined with difficulties associated with differences in taphonomic regimes before and after the transition hinders the evaluation of a proposed Ediacaran mass-extinction. However, the demonstration of geographic and morphometric range changes offers a novel means of assessing the downfall of Ediacara-type taxa at the hands of emerging metazoans. Ultimately, the combination of studies on morphospace occupation, ecosystem construction, biostratigraphy, and biogeography showcases the severity of the end Ediacaran extinction on the early evolution of macroscopic life.