Earliest crinoids: New evidence for the origin of the dominant Paleozoic echinoderms

Abstract

The oldest crinoids have been discovered in Early Ordovician strata of the western United States. A set of emergent crinoid traits based on these and other early crinoids enables reinterpretation of crinoid origins and early history. The new fossils retain primitive echinoderm characteristics, including ambulacral floor plates and largely unorganized cup plating, a first for crinoids. They lack shared derived characteristics linking them to other stalked echinoderms, including blastozoans. Contrary to current widespread opinion, crinoids originated as an independent group during the Cambrian, apparently from an edrioasteroid ancestor. All four major Paleozoic crinoid clades had evolved by the early Ibexian (Tremadocian), and this initial diversification slightly preceded those of most other Paleozoic evolutionary fauna components. These earliest crinoids attached to carbonate hardgrounds developed on sponge-algal mounds, intraformational conglomerates, and grainstones.