Somasteroidea

The Somasteroidea, or Stomasteroidea, is an extinct order of asterozoan echinoderms first defined in 1951 by W. K. Spencer. Their first appearance in the fossil record was in the Early Ordovician and they had become extinct by the Late Devonian. They are similar to the asteroids in that their bodies are flattened dorsoventrally and they have five petaloid arms with broad bases. The ambulacral plates in somasteroids are simple and unspecialized, and the arms were thought to be not flexible and were unable to assist in feeding, but the oral mouth parts were more complex.

Relationship to other Asterozoan classes

Most authors consider Somasteroidea to be the basal stock from which the other three classes evolved, but an argument in favor of monophyly and a position closer to stenuroids and ophiuroids than to asteroids has also been made.
Somasteroids are "more or less petaloid," with arm shape reflecting virgal series lengths. Somasteroids have been described as being more rigid in shape than derived asterozoans, although this apparent structure could be exaggerated by tissue changes at the time of death.