The Eocrinoidea are an extinct class of

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...

s that lived between the Early Cambrian and Late Silurian periods. They are the earliest known group of stalked, arm-bearing echinoderms, and were the most common echinoderms during the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

. Eocrinoids were a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group that may have been ancestral to six other classes:

Rhombifera The Eocrinoidea are an extinct class of echinoderms that lived between the Early Cambrian and Late Silurian periods. They are the earliest known group of stalked, arm-bearing echinoderms, and were the most common echinoderms during the Cambrian. ...

Diploporita Diploporita is an extinct class of blastozoan that ranged from the Ordovician to the Devonian. These echinoderms are identified by a specialized respiratory structure, called diplopores. Diplopores are a double pore system that sit within a depr ...

, Coronoidea, Blastoidea, Parablastoidea, and Paracrinoidea. They may also be the progenitors of the

cystoids Cystoidea is a class of extinct crinozoan echinoderms, termed cystoids, that lived attached to the sea floor by stalks. They existed during the Paleozoic Era, in the Middle Ordovician and Silurian Periods, until their extinction in the Devonian ...

, who are believed to be ancestral to modern crinoids. The earliest genera had a short holdfast and irregularly structured plates. Later forms had a fully developed stalk with regular rows of plates. They were benthic suspension feeders, with five ambulacra on the upper surface, surrounding the mouth and extending into a number of narrow arms. An unusual Ordovician form was the conical ''

Bolboporites ''Bolboporites'' is an extinct genus of conical echinoderms that lived in the Ordovician of Europe and North America. They are interpreted to have lived on the seafloor with the pointed end of the cone down in the sediment and the broad end upwa ...

'' with its single brachiole.Rozhnov, S.V. and Kushlina, V.B. 1994. Interpretation of new data on ''Bolboporites'' Pander, 1830 (Echinodermata; Ordovician), p. 179-180, in David, B., Guille, A., Féral, J.-P. & Roux, M. (eds.), Echinoderms through time (Balkema, Rotterdam). See also

List of echinodermata orders This List of echinoderm orders concerns the various classes and orders into which taxonomists categorize the roughly 7000 extant species as well as the extinct species of the exclusively marine phylum Echinodermata. Subphylum Crinozoa Cla ...

References

External links

Palaeos on EocrinoidsEvolutionary palaeoecology of early epifaunal echinodermsCambrian explosion with Eocrinoid information
Blastozoa Paleozoic echinoderms Cambrian echinoderms Silurian echinoderms Cambrian first appearances Silurian extinctions {{paleo-echinoderm-stub