Cambrian fossils illuminate the ambulacrarian common ancestor

The origins of ambulacrarians, which comprise the morphologically disparate echinoderms (sea stars and their relatives) and the worm-like hemichordates, are poorly constrained, obscuring the assembly of their exceptionally disparate body plans. We describe Yujingia glutenotunica, an ambulacrarian from the early Cambrian Chengjiang biota (518 Ma). Yujingia combines a simple cylindrical body plan with a crown of specialized appendages, including pinnate arms bearing filter-feeding grooves and alternating side-branches. Phylogenetic analyses recover Yujingia within a grade of tentaculate fossils immediately basal to the last common ancestor of all modern ambulacrarians. These results suggest that ambulacrarians evolved from semi-sessile organisms with complex suspension-feeding adaptations, otherwise only found in crown-group echinoderms and tentaculate hemichordates (pterobranchs). Yujingia fills a key gap in the early deuterostome fossil record, thus challenging the conventional worm-like, pharyngeal filter-feeder models for the ambulacrarian last common ancestor and supporting unexpectedly deep origins for the echinoderm and pterobranch body plans.