Edrioasteroidea

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Edrioasteroids
Fossil range: Ediacaran - Permian

An unidentified edrioasteroid

Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Pelmatozoa
Class:	Edrioasteroidea

Genera

?Arkarua
- ?A. adami
Edrioaster (type genus)
- E. bigsbyi
Streptaster
- S. vorticellatus
Cystaster
Carneyella
- C. pilea
- C. faberi
- C. ulrichi
Isorophus
- I. cincinnatiensis
Curvitriordo
Thresherodiscus
- T. ramosa (Foerste, 1914)
Edriophus
- E. levis
Cryptogoleus
- C. chapmani
Lispidecodus
- L. plinthotus (Kesling, 1967)

The Edrioasteroids are an extinct class of echinoderm that lived from the Ediacaran (if Arkarua was indeed an edrioaster) to the Permian periods of geologic time, about 600-300 million years ago. The animals usually consisted of a disk-like upper body made of many plates. The body plan for this class was simple: a main body (theca), composed of many small plates, a peripheral rim for attachment, and (in some species) a pedunculate zone for extension and retraction. Circling and sometimes attached to the body was a peripheral rim of plates. The main feature is the five arms called ambulacra that are contained in the body area radiating from the mouth in the center of the body outwards. The arms grow in a spiral pattern, sometimes in the same direction. The anus is situated under the mouth region and is made of small triangular plates to form a cone-shaped area. Edrioasteroid species are distinguished by differences in the ambulacral curvature and the plating covering the ambulacra and the mouth. The mode of life was sessile; they were often attached via a stalk made of small plates to a hard object such as a carbonate hardground or shell.

In the discocystinids, the area between the body and peripheral rim could be extended and retracted; in so doing the two were separated. The peripheral rim becomes the base of the stalk which was attached to a surface. Underneath the body is the recumbent zone about 12 mm wide in the genus Giganticlavus, followed by the pedunculate zone attached to the peripheral rim of 12 mm (Sumrall 1996).

[edit] References

All accessed on March 8, 2008.

http://www.ucmp.berkeley.edu/echinodermata/edrioasteroidea.html University of California, Berkeley.
http://drydredgers.org/edrio1.htm Compiled by Colin D. Sumrall.
http://www.tulane.edu/~csumral/Abstract Spiraclavus nacoensis, a New Species of Clavate Agelacrinitid Edrioasteroid from Central Arizona by Colin D. Sumrall.
http://www.science-art.com/image.asp?id=1357 Reconstruction by Emily Damstra.
http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_65113.htm Geological Society of America.
http://www.tulane.edu/~csumral/morph.html by Colin D. Sumrall

[edit] Taxonomy

Mikko's taxonomy

The Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky. The edrioasteroid is about 1.5 cm in diameter. In the background is the cyclostome bryozoan Corynotrypa.