The Edrioasteroidea

Growing on this Ordovician brachiopod, at the lower right, is a round organism with five rays, looking suspiciously like a starfish. It is in fact Isorophus, a member of the extinct echinoderm class Edrioasteroidea. Although superficially like starfish, edrioasteroids were sessile - attached to a substrate by a short thick stalk covered with plates. Their ambulacra - the five radiating feeding grooves in the upper part of the organism, covered by large plates - grew in a curved, often spiral or nearly spiral pattern.