Blake, Daniel B. Department of Geology, University of Illinois, Urbana, Illinois.
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A Paleozoic (Ordovician and Devonian) order of sea stars of the class Asteroidea; only two genera have been recognized. Platyasteridans have relatively elongate flat arms; broad, transversely aligned ventral arm plates; and paxilliform aboral plates (see illustration). The morphology suggests living members of the Luidiidae; based on those similarities, some authors contend that luidiids are surviving platyasteridans. Fundamental differences in ambulacral system construction have been found between modern asteroids and all of the well-known early Paleozoic asteroids, including the platyasteridans. Similarities between platyasteridans and modern asteroids are atributable to evolutionary convergence. See also: Asteroidea; Echinodermata; Paxillosida
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