Author : Thomas Edgar Guensburg
Publisher :
ISBN 13 : 9780877104582
Total Pages : 43 pages
Book Rating : 4.1/5 (45 download)
Book Synopsis The Oldest Known Crinoids (early Ordovician, Utah) and a New Crinoid Plate Homology System by : Thomas Edgar Guensburg
Download or read book The Oldest Known Crinoids (early Ordovician, Utah) and a New Crinoid Plate Homology System written by Thomas Edgar Guensburg and published by . This book was released on 2003 with total page 43 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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, 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. Evidence from ontogeny, cup-stem and radial orientation, and now, early morphologic transitions necessitates revision of long used skeletal terminology. Dual ray and cup-base references for plate homologies, rather than the traditional ray-only model, are consistent with this evidence. Consequently, infrabasals are redefined as the cup-base circlet of both dicyclic and monocyclic crinoids. The use of basals is restricted to plates between infrabasals and radials of dicyclic (and tricyclic) crinoids. Traditional classifications emphasizing cup plate circlet number is unreliable early in the crinoid record; instead, posterior and interray cup morphology provides the most consistent phylogenetic information.