Tracing the evolution of the holothurian body plan through stem‐group fossils

The fossil echinoderm Palaeocucumaria, from the early Devonian Hunsrück Slate of southwestern Germany, has been studied using both traditional techniques and X‐ray microtomography, and its anatomy clarified. Phylogenetic analysis shows that it is a stem‐group holothurian with a combination of characters that help understand how the modern (crown‐group) holothurian body plan developed. Echinoids and holothurians have evolved along different paths, by differential growth of the larval‐ and rudment‐derived body regions. Palaeocucumaria shows that late stem‐group holothurians had a water vascular organization with a single external madreporite and calcified stone canal leading to the aboral end of the peripharyngeal coelom, and five primary radial water vessels that gave rise to tentacle‐like tube‐feet. This fossil data, in combination with a molecular phylogeny based on 18 s‐like rRNA gene sequence data, is used to order evolutionary steps in the making of the crown‐group holothurian body plan. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 670–681.