Ancient deep ocean as a harbor of biotic innovation revealed by Carboniferous ophiuroid microfossils

Abstract

Fossil-informed molecular phylogenies have emerged as the most powerful tool for correlating biotic evolution and Earth history. The accuracy of these trees, however, depends on the completeness of fossil sampling. For most organismal clades, the available fossil record is insufficiently sampled. This is especially true for groups with a multi-element skeleton, such as echinoderms and vertebrates, where sampling efforts focus largely on rare finds of intact skeletons. For these groups, inconspicuous but informative skeletal fragments are commonly neglected. This sampling bias excludes the numerous paleoenvironments in which preservation of intact skeletons is extremely unlikely, in particular deep-water settings. We describe new finds of brittle-star, or ophiuroid, fossils retrieved from sieving residues of sediments deposited during the Atokan (Upper Carboniferous) on the deep shelf to upper slope of the Ardmore Basin in present-day southern Oklahoma, USA. Although preserved as disarticulated, microscopic ossicles, the pristine preservation of the skeletal microstructure allows for precise identification of the remains. Comparative anatomical and phylogenetic analyses confirm the presence of basal representatives of the extant ophiuroid orders Ophioscolecida and Amphilepidida. Our finds provide the first unambiguous fossil evidence that the early crown-group diversification of the Ophiuroidea was well under way long before the end-Permian mass extinction, and that a significant part of this diversification took place in deep-water settings, as previously predicted by molecular evidence.