Ausich, William I. School of Earth Sciences, Ohio State University, Columbus, Ohio.
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- Evolutionary paleoecology
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The only surviving subclass of the echinoderm class Crinoidea. The Articulata include all living members of the echinoderm class Crinoidea and are generally regarded as the post-Paleozoic crinoids. Crinoids are exclusively marine organisms, as are all members of the phylum Echinodermata. More than 600 crinoid species live in today's oceans, and it is probable that many more are undescribed because crinoids typically live in remote habitats. The Articulata are defined by the co-occurrence of numerous characters, including a dicyclic or cryptodicyclic calyx (calcareous plates of the calyx comprising two circlets of plates, with the basal plates below and the radial plates above; note that the basal plates may be hidden); no anal plates (calyx comprising only radial plates and/or basal plates); a mouth exposed on the oral surface; axial nerves present in a canal that penetrates the basal, radial, and arm plates; arm plates held together with both muscular and ligamentary tissues; and the arms being pinnulate (fine branches from every arm plate, alternating along the arm) [Fig. 1]. Living crinoids include the sea lilies (stalked crinoids) [Fig. 2] and feather stars (unstalked crinoids) [Fig. 3]. Ecologically, crinoids (in particular, comatulids) may be locally very abundant in several ocean-floor habitats. See also: Crinoidea; Echinodermata
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