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Sciadiocrinus, Convergence on the Family Pirasocrinidae (Crinoidea: Echinodermata)

Ronald D. Lewis and Harrell L. Strimple
Journal of Paleontology
Vol. 64, No. 2 (Mar., 1990), pp. 293-300
Published by: Paleontological Society
Stable URL: http://www.jstor.org/stable/1305573
Page Count: 8
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Sciadiocrinus, Convergence on the Family Pirasocrinidae (Crinoidea: Echinodermata)
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Abstract

Sciadiocrinus Moore and Plummer is synonymized with Schistocrinus Moore and Plummer, with the name Sciadiocrinus retained by the Law of Priority. This synonymy is based on a reexamination of the type material, study of a previously undescribed ontogenetic sequence in S. tegillum Strimple and Moore, and close inspection of separate radial plates and anal-sac spines. The holotype specimen of the type species of Sciadiocrinus was incorrectly illustrated in the original description of the genus, and appears to be immature. The basal plates of this specimen are shown to be separated from each other by the junction of the radial plates with the infrabasals--a principal character in the original concept of Schistocrinus. The Sciadiocrinus tegillum hypodigm now includes specimens which also have relatively large radial--infrabasal sutures as well as individuals with little or no contact between these plates. Examination of this collection and previously described material shows that mode of arm branching, details of radial-plate articular facets, and some features of the anal-sac spines are also equivalent in the two genera. Sciadiocrinus, presently classified in the family Pirasocrinidae, is distinct from all known pirasocrinid taxa in a number of morphological characters. We believe the genus should be removed from the Pirasocrinidae, and recommend its assignment to the family Anobasicrinidae based primarily on similarities in arm structure. The remarkable convergence between Sciadiocrinus and the Pirasocrinidae emphasizes the adaptive significance of the spinose, umbrella-like anal-sac roof, which apparently served as a deterrent to predators.

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