Sprinkle, James Department of Geological Sciences, University of Texas, Austin, Texas.
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A medium-sized class of primitive, brachiole-bearing, blastozoan echinoderms of the class Crinozoa that ranged from the Early Cambrian to the Middle Silurian, although few eocrinoids survived past the Middle Ordovician. About 32 eocrinoid genera have been described from North America, Europe, North Africa, and Australia; other occurrences of distinctive plates that may belong to eocrinoids have also been noted. Eocrinoids are the most diverse class of echinoderms known from the Cambrian with about 15 genera, and different members appear to have been ancestral to nearly all of the more advanced brachiole-bearing echinoderm classes that appeared in the Early or Middle Ordovician, such as rhombiferans, parablastoids, and coronoids. Eocrinoids have a globular, conical, or flattened theca or body, with many irregularly arranged to partly organized, imbricate or adjacent plates (see illustration). Most Cambrian genera have sutural pores on the plate margins, apparently for respiration. Many early eocrinoids have a multiplated, cylindrical to slightly inflated holdfast for attaching the theca to objects lying on the sea floor. Holdfasts apparently evolved into a true columnal-bearing stem in late Middle Cambrian eocrinoids, and most later genera have that advanced type of attachment structure. Eocrinoids typically have two to five short ambulacral grooves radiating from the mouth on the summit to many long, erect, biserial brachioles that were used for feeding. Most eocrinoids were attached, low- to medium-level suspension feeders that used the brachioles to collect small food particles drifting by the theca. Most researchers have argued that eocrinoids are a valid class containing genera that did not develop the foldlike respiratory structures found in more advanced blastozoan classes. However, others have recently proposed that eocrinoids are a paraphyletic stem group that should be discarded, with the included genera reassigned to other classes. See also: Echinodermata
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