Edrioasteroids are a clade of primitive discoidal echinoderms that have a worldwide distribution and a geological range that spans the Paleozoic. Their body, or theca, is low and domal and they feed via five “arms,” called ambulacra, that radiate from a centrally located mouth on the dorsal surface. In this study, twenty-eight specimens of the straight-armed species Neoisorophusella lanei from the late Mississippian Kinkaid Formation, southern Illinois, were analyzed, and measured for the total ambulacral area and thecal diameter. Intense crowding on the surface (>15,000 individuals per square meter) results in many specimens having distorted, polygonal outlines. For this study, only circular specimens were selected for measurement because this distortion affected volume calculations. Specimens used in this study ranged in diameter between 1.9 mm and 11.5 mm and represent stages 3 through 7 of the Bell (1976) ontogenetic series. The length of the A ambulacrum was measured from the junction of the primary oral cover plates to the distal ambulacral tip and the B - E ambulacral lengths were measured between the tip of the lateral bifurcation plates and the tip of the ambulacra. Ambulacral widths were measured at the midpoint. Volume was calculated assuming the thecal shape was a spherical section.

A linear relationship occurs between the ambulacral area and the specimen volume. We interpret this correlation to represent a constant metabolic need regardless of the ontogenetic stage of the individual, i.e. nutrient need per unit volume is constant. Larger species of isorophinid edrioasteroids have curved ambulacra, and in some large species, very mature specimens have meandering, spiraling or branching ambulacra. We interpret these late ontogenetic modifications as strategies for maintaining a constant ratio between the surface area of the ambulacra and thecal volume.