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Echinoderms were a major enduring component of Paleozoic marine faunas beginning with the rapid diversification of crinoids and other classes in the Early Ordovician. This diversification was triggered by increased availability of habitable areas accompanying sea-level rise and changed sedimentation styles. Newly collected Early Ordovician echinoderms facilitate determination in detail of life modes and important adaptations during this transition interval. Substrate consistency and stability were crucial spatial limiting factors that resulted in heterogeneous distribution patterns for echinoderm classes. Crinoids were overwhelmingly large-food-groove suspension feeders with relatively open filtration fans attached by small discs to hard-ground surfaces in shallow water. They were preadapted for this life mode. Rhombiferans were mobile small-food-groove suspension feeders; they lived on soft substrates at various depths. Several important crinoid clades underwent a second phase of diversification during the Middle Ordovician by developing holdfasts adapted to soft substrates and densely pinnulate arms with small food grooves, leading to dominance of this class among Paleozoic echinoderms. Our findings are in basic agreement with the overall onshore diversification pattern of most Paleozoic benthic invertebrates, but we argue for an extrinsic environmental control (substrate availability) for the observed echinoderm distribution.

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. Keywords: crinoid origin, Early Ordovician, western United States, Ordovician diversification.