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