BASAL ECHINODERMS: THEIR PLEISIOMORPHIC CHARACTERS AND AND WHAT SETS THEM APART FROM MOST MODERN CROWN GROUPS?

Basal and crown group echinoderms are part of the Cambrian Fauna. Many basal groups are bilateral or nearly bilaterally symmetrical (Homalozoa) and all have skeletal plates of (bone-like in structure) stereom, composed of high Mg, calcite. Most basal forms are flattened, had a recumbent life style, and possessed some locomotor capabilities (enabled by a stele or flexible feeding structure). Thecae tend to be composed of small platelets (tessellated) with fused platelets forming well defined, taxonomically significant, bilaterally to nearly bilaterally arranged, marginal plates (single series of plates: e.g., Stylophora, Homoiostelea, Homostelea, double series of plates: most Ctenocystoidea. As in virtually all echinoderms, a hydropore opens into the water vascular system that is apparently restricted to the theca and does not extend into the ambulacra. Terminus of the gut in basal forms is marked by an elaborate anal pyramid or clam shell-like valve and coupled with the commonly tessellated body; morphology suggests a hydro-skeletonized theca to aid in respiration. Ambulacral systems can be exothecal or epithecal (being recumbent on the sides of the marginals). Brachiole-like feeding structures are common; a food groove roofed over by tall narrow cover plates. Food capture is assumed to have been by ciliary-mucoid tissues on the inner surface of the cover plates and areas adjacent to the food groove. Exothecal brachiolar morphology is variously structurally supported by: 1, brachiolar plates with a lumen running through them and continuous with the body cavity to produce, in part, a partial hydrostatic support system (Homoiostelea); 2, a uniserial aulacophore (Stylophora); or 3, by a simple biserial brachioles (closely related stem group gogiids and all other blastozoans). The variety of support structures indicates exothecal feeding arose multiple times with the basic feeding mechanism being extended by various extrusions of plates from the oral area of the theca. If this is correct, cover plates, which probably originated as mouth covers and may be homologous in these stem groups. Cambrian crown group(s): Edrioasteroidea, are commonly characterized by pentamery and the probable incorporation of the water vascular system into the ambulacral system; they may be ancestral to all modern groups.