COOPERIDISCUS, DINOCYSTIS AND THE ORIGIN OF DISCOCYSTINID EDRIOASTEROIDS

Edrioasteroid echinoderms were common components of Mississippian and Pennsylvanian shallow marine faunas, largely because of the origination and radiation of discocystinids. These unusual edrioasteroids were mushroom-shaped with a highly inflatable and retractable pedunculate zone. With thecal diameters of up to 90 mm, they include the largest of all edrioasteroids. It has long been known that discocystinids arose among derived agelacrinitid edrioasteroids, but transitional taxa were not forthcoming. Two poorly known and problematic Late Devonian taxa, Cooperidiscus alleganius and Dinocystis barroisi bear on this problem and are redescribed.

Cooperidiscus from the Upper Devonian Chemung Group of New York, Pennsylvania and associated rock in West Virginia is a large agelacrinitid. It has been incompletely described from specimens preserving an oral side not bounded by a peripheral rim. The aboral surface, originally described as Xenocystites carteri, represents an incompletely preserved, irregularly plated pedunculate zone. Some specimens show the oral frame interior through the open, unplated center of the poorly preserved peripheral rim. The aboral surface is consistent with other transitional discocystinids such as Torquerisediscus kypsi, but the ambital plates are uniquely developed into spines suggesting protective contact with the substrate.

Dinocystis from the from the Upper Devonian Evieux Formation of the Condroz Group, Belgium was originally described as an edrioasterid because it bears a globular theca, but the morphologies of this taxon were poorly documented. A new investigation of the type material consisting of internal molds shows it to be a transitional discocystinid. Few details are evident on the oral surface other than the long curved ambulacra, tessellate interambulacral plating and the position of the periproct. However, the molds preserve internal details of the oral frame and floor plates that have vertical sutures and the lateral extensions of discocystinids. The aboral surface bears a well-developed pedunculate zone with plates organized into columns, lacks a recumbent zone as in early discocystinids and bears a small peripheral rim forming a holdfast. This taxon is the oldest known discocystinid and the only member of the clade outside North America.