Origin of Echinoderms in the Paleozoic Evolutionary Fauna: The Role of Substrates

Abstract

Comparison of echinoderm faunas from two partly coeval Early Ordovician units, the Fillmore Formation of western Utah and the Ninemile Shale of central and southern Nevada, indicates that the distribution of echinoderms during the initial radiation of the Paleozoic Evolutionary Fauna was substrate-controlled. Attached echinoderms, such as crinoids and edrioasteroids, are much more common on hard substrates in the shallow-water Fillmore, whereas vagile forms, such as rhombiferans and mitrate stylophorans, are found on soft substrates in both the shallow-water Fillmore and deeper-water Ninemile. We found few intermediates or holdovers among these new Early Ordovician echinoderms, implying that the Cambrian and Paleozoic Evolutionary Faunas were most likely discrete and real and that the Ordovician Radiation was already well under way by the middle of the Early Ordovician.