DETAILED ANALYSIS OF STYLOPHORAN APPENDAGE PLATES - LATE ORDOVICIAN, OHIO

Stylophorans are now universally recognized as echinoderms, but some controversy persists into the nature of the appendage (aulocophore) being interpreted either as a feeding appendage or a stalk. The stylophoran Enoploura popei is found in the Richmondian (Upper Ordovician) Liberty Formation in Cincinnati, Ohio. Locally, large numbers of isolated thecal plates can be washed from shales allowing aulocophore elements to be studied in isolation. Elements were picked by hand from washed sediment residues and sorted by plate type. The best preserved plates were studied using an SEM allowing us to see morphological details and ultra-structure of different plate areas. The stylophoran aulacophore is divided into three regions. Proximally, there are tectals and inferolaterals positioned dorsally and ventrally respectively. These elements are boomerang-shaped with faceted, imbricate sutures allowing movement, and bear a shelf internally for soft tissue attachment. The central portion of the aulocophore is the stylocone. This element bears a large cup-shaped process on the proximal side that opens dorsally and a circular flange forming the distal face. The distal face is similar to those of the ossicle elements to which the stylocone attaches. The dorsal surface has a deep, V-shaped groove that is similar to a pair of ossicles fused together. Distally the aulocophore bears two sets of elements, V-shaped ossicles and cover plates ventrally and dorsally respectively. The ossicles vary in size and shape, being larger, higher and more strongly curved proximally. On the upper articular surface lie a pair of apophyses that are concave and convex on the proximal and distal surfaces respectively. A vertical groove runs the height of the articular surface on both faces with a slightly convex swelling at the lower extremity proximally and a deep circular depression in the same position distally. The sidewalls of the ossicles bear a double-lobed process along the distal margin for attachment of soft tissues. The cover plates are trapezoidal in lateral view with a basal lip that articulates with the underlying ossicles. The opposite edge of the plate is very thin suggesting no strong articulation between the tops of the plates. Proximal and distal sutures are imbricate, consistent with cover plate overlap.