ABSTRACT
Fossil evidence suggests that the water vascular system of echinoderms evolved before stereom which, in turn, evolved before pentamery, Cladistic analysis of Lower Cambrian genera reveals that the carpoid/echinoderm dichotomy was the first evolutionary divergence. Helicoplacoids are stem-group echinoderms with multi-layered spiral plating and three biserial ambulacra with tube feet and multiplated cover plate sheets, which radiated from a lateral mouth. Camptostroma is the most primitive pentameral echinoderm and probably ancestral to all living groups. It had multi-layered plating, five ambulacra in a 2–1-2 arrangement and the anus, gonopore and hydropore in the CD inter-ray. The position of the latter is fundamental in orientating echinoderms. Camptostroma gave rise to the Eleutherozoa via Stromatocystites and the Pelmatozoa via Kinzercystis, Stromatocystites had a flat theca with single-layered, tesselate plating. Kinzercystis had an elongate stalk and biserial brachioles derived from aligned cover plates in Camptostroma. Lepidocystis had both brachioles and free terminal portions of its ambulacra as true arms. Lepidocystis probably gave rise to Echmatocrinus with multiple, free, uniserial arms and thence to the crinoids. Kinzercystis probably gave rise to Gogia and the systoids s.l. (= blastozoans).
