The oldest known crinoids (Early Ordovician, Utah) and a new crinoid plate homology system - BAP #364
Six new primitive genera from the Early Ordovician of western Utah significantly advance our understanding of early crinoid history. Two of the new genera are the oldest crinoids (early Ibexian- early Tremadoc) and represent a previously unknown protocrinoid evolutionary grade, differing from other crinoids in having a nonstandardized dorsal cup and arms bearing ambulacral floor plates. Titanocrinus sumralli, new genus and species, has hundreds of unorganized mid-cup and interbrachial plates separating the cup-base basal circlet and the ray plates. Glenocrinus globularis, new genus and species, has similar plating with a more compact globular cup and fewer mid-cup plates, and is near the origin of diplobathrid camerates. A third taxon, Eknomocrinus wahwahensis, new genus and species, from the same lower stratigraphic interval, is a stem-group monocyclic camerate crinoid with standardized but irregular cup plating and other emergent crinoid traits shared with protocrinoids. Three new camerates from higher in the section are more derived. Cnemecrinus fillmorensis, new genus and species, middle Ibexian (latest Tremadoc), is a reteocrinid camerate, and Adelphicrinus fortuitus, new genus and species, and Habrotecrinus ibexensis, new genus and species, both latest Ibexian (middle Arenig), are monocyclic camerates. The latter taxon has unique accessory plates and major cup plate shapes. The fossil record indicates rapid diversification of disparid, cladid, and camerate crinoids by the end of the middle Ibexian (late Tremadoc), each arising independently from the protocrinoid stem group.
T.E. Guensburg, J. Sprinkle
Pages: 43, 9 pls., 13 text-figs., 5 tables
Issue: BAP 364
Year published: 2003