New Lower and Middle Ordovician stelleroids (Echinodermata) and their bearing on the origins and early history of the stelleroid echinoderms

Abstract

Ophioxenikos langenheimi n. gen. and sp. (class Somasteroidea), Stibaraster ratcliffei n. gen. and sp., and Cnemidactis? macroadambulacralatus n. sp. (both class Asteroidea) are new stelleroid echinoderms described from Lower and Middle Ordovician strata of the western United States. Stibaraster clearly is at the asteroid grade of organization, although an early representative of the class. Ophioxenikos is the first fossil somasteroid recognized from beyond Europe. It is similar to Chinianaster and Villebrunaster, ambulacral characters of all three suggest affinities with ophiuroids. Cnemidactis? is recognized from North America; it is unusual in the presence of proportionately large marginal ossicles. An indeterminate species is unusual in its structural parallels with living taxa. In recent years, the possibility that edrioasteroids were ancestral to stelleroids has been revived. Supporting arguments for this hypothesis neglect important differences; ancestry of stelleroids remains uncertain.