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Somasteroidea are an extinct subclass of asterozoan echinoderms (star-shaped echinoderms), known to have lived during the early Paleozoic (Spencer, 1951). Somasteroids first appear in the fossil record in the Early Ordovician (Tremadocian, ~510-493 Ma), just prior to the appearance of the other two asterozoan classes, Asteroidea (sea stars) and Ophiuroidea (brittle stars). While the asteroids and ophiuroids have subsequently undergone extensive radiations and represent the two most successful extant echinoderm classes, somasteroid forms do not appear in the fossil record after the late Devonian and are therefore thought to have been extinct since then (Spencer, 1951; Blake, 1982).
Somasteroids are flattened, fivefold radially symmetrical echinoderms that range in shape from that of a pentagon to that of a star with protruding arms. It appears that somasteroids were benthic marine animals that were filter- and/or detritus-feeders. Despite their relatively rigid arm structure, somasteroids are thought to have been somewhat mobile and able to scavenge along the ocean floor (Spencer, 1951).