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The class Somasteroidea (Echinodermata, Asterozoa): morphology and occurrence

Published online by Cambridge University Press:  01 October 2015

Daniel B. Blake  and
Thomas E. Guensburg
Daniel B. Blake
Affiliation:
Department of Geology, University of Illinois, 506 W. Springfield, Champaign, IL 61820, USA 〈dblake@illinois.edu〉
Thomas E. Guensburg
Affiliation:
Sciences Division, Rock Valley College, 3301 N. Mulford Road, Rockford IL, 61114 USA 〈t.guensburg@rockvalleycollege.edu〉
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Abstract

The class Somasteroidea Spencer, 1951, is basal within the subphylum Asterozoa. Members are most readily recognized by presence of series of rod-like so-called virgal ossicles extending laterally from each ambulacral ossicle. Five somasteroid genera are recognized and assigned to two families. Four genera are Gondwanan, three of these (Chinianaster, Thoralaster, Villebrunaster) from the Lower Ordovician Tremadocian of France and one (Archegonaster) from the Middle Ordovician upper Darriwilian of the Czech Republic. The fifth genus, Ophioxenikos, is Laurentian from the Floian of Nevada. Catervaparmaster, previously assigned to the Somasteroidea, is left in open nomenclature; absence of virgal-series ossicles favors a lineage apart from the principal asterozoan clades. Asterozoan fossils are readily separated from fossils of other echinoderm groups. The subphylum therefore is thought to be monophyletic, its ancestry unknown. Skeletonized representatives of the four major asterozoan clades first occur through a relatively narrow Early Ordovician stratigraphic interval. Robust skeletons therefore are thought to have evolved after a time of unknown duration including only lineages that were no more than weakly calcified. The French occurrences are from a relatively deep distal shelf setting on soft substrates whereas the Nevada occurrence was in a shallower, active setting. Differences document early ecologic diversification.

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Journal of Paleontology , Volume 89 , Issue 3 , May 2015 , pp. 465 - 486
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Copyright © 2015, The Paleontological Society 

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