EARLY DIVERSIFICATION IN ECHINODERMATA

Phylogeny of early echinoderms is a hotly debated topic that centers on characters of symmetry, thecal morphology, respiratory structures and feeding structures. This reappraisal of classification focuses especially on feeding structures. All modern echinoderms have evolved from Lower Cambrian bipentameral forms with the ancestral 2-1-2 ambulacral pattern. Coevally, essentially flattened bilaterally symmetrical echinoderms evolved that have few synapomorphies with pentameral forms. Bilateral classes include forms that have: a) transverse sessile food grooves impressed into the thecal plates (Ctenocystoidea, M. Cam. and Homostelea, M. Cam.) or b) have a single uniquely constructed freestanding feeding appendage. These are the uniserial aulacophore (Stylophora, M.Cam. - L.Penn.) and the biserial brachiole-like brachitubulum, [new term] (Homoiostelea, M.Cam.- M. Dev.). The uniserial stylophoran aulacophore has been interpreted to have a water vascular system because of paired depressions (ampular pits) adjacent to the food groove. They appear to be limited to the cornutes and are here interpreted to be muscular depressions. The biserial brachiole-like brachitubulum in the Homoiostelea has a large lumen that opens into the theca and may have functioned hydrostatically. Both of these freestanding structures appear to have captured food in a manner similar to blastozoan brachioles (probable ciliary-mucoid food capture). The bilateral classes appear to lack the water vascular system in the feeding structures but in many cases, by virtue of having a hydropore, there is strong indication that a vascular system (water ring?) was present in the theca. Blastozoan brachioles are biserial structures without a lumen, or with a very small nerve canal that pierces the brachiolar plates, and probably are without a water vascular system running their length. In this subphylum and in classes where feeding structures are associated with the water vascular system (crinozoans and eleutherozoans) there is always the beginnings of, or fully developed, bipentameral symmetry. A major synapomorphy that defines extant echinoderms is the presence of the water vascular system incorporated in the pentameral feeding system. This synapomorphy is certainly present (in edrioasteroids) by the basal Middle Cambrian.