NEW DATA ON THE MORPHOLOGY OF PLESIOMORPHIC ISOROPHID EDRIOASTEROIDS

Numerous specimens, including a complete ontogeny, of new a Middle Cambrian isorophid edrioasteroid from the Kaili Biota (China) shed new light on the plesiomorphic isorophid condition. This species has several features that unite it with later isorophids including a well defined peripheral rim, imbricate interambulacral plating, unplated ventral surface, and strongly curved ambulacra. However, many of the features of the ambulacral and interambulacral plating, details of the oral frame, and hydropore/gonopore complex are unlike any other edrioasteroid taxon. The ambulacral cover plates are formed into a multi-tiered arrangement seen in imbricate eocrinoids, helicoplacoids, and cinctans rather than simple biserial or multiserial patterns of more derived isorophids. The floor plate system is quadraserial with two biseries of podial pores rather than uniserial lacking pores as in derived isorophids, or a simple biseries with one biseries of podial pores as in edrioasterids. Although the plating of the interambulacra look similar to later forms, in the new Cambrian taxon, plates are inserted in the middle of the plate fields rather than restricted to the adradial edges. The oral frame includes both floor plate homologues and interradial elements. The position and number of these elements calls into question the homology of various frame elements for plesiomorphic echinoderms in general. The presence of these interradial elements is similar to the condition seen in edrioasterids and edrioblastoids potentially providing the first synapomorphy for Edrioasteroidea sensu Bell (1976). Interestingly, this character is lost in all of the familiar post Cambrian isorophids. The hydropore and gonopore are expressed as two separate, but adjacent pyramids in the proximal right CD interambulacral area. Although the position is not unusual, the separate openings and the lack of incorporation of these structures into the oral frame are reminiscent of eocrinoid grade echinoderms rather than isorophids.