NEW FLATTENED, ASYMMETRICAL, RHIPIDOCYSTID ECHINODERM FROM THE BROMIDE FORMATION (SANDBIAN) OF SOUTHERN OKLAHOMA, AND A REVISED CLASSIFICATION OF EOCRINOIDS, RHIPIDOCYSTIDS, AND PARACRINOIDS

Eocrinoids, the earliest class of brachiole-bearing blastozoan echinoderms, had a small radiation in the Cambrian, then underwent a much larger radiation in the Early to Late Ordovician, producing many new classes and several advanced orders and families, including flattened rhipidocystids. When monographed in 1982, the echinoderm fauna in the Bromide Formation at the top of the Simpson Group (Sandbian) in Oklahoma had one archaic eocrinoid genus and six paracrinoid genera, but no rhipidocystids. However, a new rhipidocystid genus was subsequently described from the Oil Creek Formation, three units lower in the Simpson Group (Whiterockian).

A new and unusual rhipidocystid(?) genus and species has now been collected from a shaly interval in the upper Mountain Lake Member of the Bromide Formation in the Criner Hills of southern Oklahoma. Three complete and 11 partial specimens of this new genus have a moderately flattened theca with 7-8 thicker marginals around the edges, three basals, and 3-4 thinner central plates on each side. The aboral side is thinner and more domed than the oral side and often has missing centrals. An offset and highly tilted ambulacral system has an elliptical mouth surrounded by 3-4 short radiating ambulacra which then branch off the theca to become uniserial(?) erect appendages. The stem facet is fairly large, slightly elliptical, and highly oblique. No columnals picked from the matrix have been identified as belonging to this rhipidocystid, but small domed holdfasts attached to strophomenid brachiopods occur in the same bed.

This new echinoderm resembles strongly flattened rhipidocystids having relatively few marginals, three basals, and large flat centrals, along with short ambulacra on a slightly tilted summit bearing erect biserial or uniserial brachioles. It also resembles some lens-shaped paracrinoids having more thecal plates, but also three basals, along with recumbent uniserial ambulacra bearing erect uniserial brachioles. Because of these similarities, rhipidocystids and this new Bromide genus should be removed from the eocrinoids and grouped together in the Family Rhipidocystidae, Order Digitata, containing six Early to Late Ordovician genera, in the enlarged Class Paracrinoidea.