GIANT, SPIRALED, SPINE-BEARING EDRIOASTEROID FROM THE UPPER ORDOVICIAN BROMIDE FORMATION OF OKLAHOMA

Very large thecal fragments, up to 45 mm wide, of a new edrioasteroid with spiraled ambulacra showing unusual divergent morphology have been collected from the upper Bromide Formation (Sandbian, Upper Ordovician) in southern Oklahoma. Specimens have long, fairly wide, flat ambulacra separated (and slightly overlapped) by slightly wider, laterally imbricate interambulacra. Thecal shape could be domal with ambulacra spiraling around a moderately domed upper surface, globular with ambulacra spiraling down the side of a rounded theca, or more elongate with ambulacra extending down the sides of a cylindrical theca. No attachment surface (peripheral rim?) at the thecal base has been observed, perhaps implying that a globular or elongate thecal shape (with a small rim) is most likely. Also, the oral area, hydropore, and periproct are still unknown.

The distinctive ambulacra have both flat, biserial, floor plates exposed laterally, and a second much smaller set of internal, uniserial, floor plates medially. The biserial floor plates form a narrow, U-shaped, food groove medially protected by an alternating set of small, diamond-shaped, cover plates, have small thin spines attached to a row of tiny bosses along each external edge, and have large, elliptical, sutural pores internally. Although inconspicuous, the uniserial floor plates usually indicate isorophid affinities, and within the isorophids, only pyrgocystids (such as Pyrgocystis and Streptaster) have much-reduced, uniserial, floor plates and ambulacral spines. However, this new edrioasteroid is much larger and has longer flat ambulacra than any pyrgocystids, which typically have a small domal or turret-shaped theca either with short, straight, petalloid ambulacra, or medium-length curved ambulacra, both with highly domed cover plates. These ambulacral differences may indicate that this new Bromide edrioasteroid is only distantly related to pyrgocystids and other known isorophids.