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The pre‐radial history of echinoderms

Gene sequence data now identify a robust phylogeny of deuterostomes and provide a framework within which the evolution of echinoderms can be interpreted. The topology of the molecular tree makes a number of important predictions about the morphological characters of the earliest echinoderm at its split from hemichordates: it possessed gill slits (but not a notochord), had a bilaterally symmetrical body plan in the adult and, less certainly, underwent torsion during development. Carpoids, a highly contentious group of extinct deuterostomes with a plated calcite skeleton that have variously been interpreted as stem‐ and crown‐group chordates, stem‐group echinoderms or stem‐ and crown‐group echinoderms, display many of these basal characters and provide critical evidence for how the latest common ancestor of hemichordates and echinoderms was transformed into a pentaradiate crown‐group echinoderm.