Somasteroidea

The Somasteroidea, or Stomasteroidea, is an extinct order of asterozoan echinoderms first defined in 1951 by W. K. Spencer.^[1] Their first appearance in the fossil record was in the Early Ordovician (Tremadocian) and they had become extinct by the Late Devonian (Famennian). 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.^[6]

Somasteroidea Temporal range: Early Ordovician–Late Devonian PreꞒ Ꞓ O S D C P T J K Pg N Tremadocian to Famennian
Villebrunaster thorali, a basal member of the Somasteroidea
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Asterozoa
Order:	†Somasteroidea Spencer, 1951[1]
Families
†Archegonasteridae Spencer, 1951[1] †Archophiactinidae? Spencer, 1927[2] †Catervaparmasteridae Blake, 2000[3] †Chinianasteridae Spencer, 1951[1] †Helianthasteridae? Owen, 1965[4] †Villebrunasteridae Fell, 1963[5]