Abstract

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.