edrioasteroid

Edrioasteroidea is an extinct class of echinoderms. The living animal would have resembled a pentamerously symmetrical disc or cushion. They were obligate encrusters and attached themselves to inorganic or biologic hard substrates (frequently hardgrounds or brachiopods). A 507 million years old species, Totiglobus spencensis, is actually the first known echinoderm adapted to live on a hard surface after the soft microbial mats that covered the seafloor were destroyed in the Cambrian substrate revolution.

The oldest undisputed fossils of Edrioasteroidea are known from Cambrian ( Stage 3, about 515-520 Ma ago) of Laurentia and are among the oldest known fossils of echinoderms. Some authors propose that an enigmatic Ediacaran (about 600 Ma) organism '' Arkarua'' is also an edrioasteroid, but this interpretation did not gain wide acceptance. Last edrioasteroids are known from Permian (Late Kungurian, about 270-280 Ma).

Anatomy

The body plan for this class was simple: a main body (theca), composed of many small plates, with 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 consisted of five arms, or ambulacra, in the body wall radiating outwards from the central mouth. The ambulacra grew either curved or straight. When curved, they may all turn in the same direction or else one or two on the right side will curve opposite the others. The ambulacra are built of underlying floor plates that form the food groove and protective cover plates that roof the food groove. The anus was under the mouth region and was made of small triangular plates to form a cone-shaped area. The bottom surface of the theca is unplated.

Edrioasteroid species are distinguished by differences in the ambulacral curvature, the relationships of the cover plates, and ornamentation. 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. Several examples of epibiotic attachment have also been noted.

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 became the base of the stalk which was attached to a surface. Underneath the body was a ''recumbent zone'', which was about wide in the genus ''Giganticlavus'', followed by the pedunculate zone attached to the peripheral rim of .Sumrall 1996

Taxonomy

List of genera

A very incomplete list of some genera.

*?'' Arkarua''
**?''A. adami'' (may be a Trilobozoan with pentamerous symmetry, and not an echinoderm at all)
*''Walcottidiscus'' (oldest undisputed edrioasteroid, from the Middle Cambrian Burgess Shale community)
**''W. typicalus''
*''Kailidiscus''
**''K. chinensis''
*'' Edrioaster'' (type genus)
**''E. bigsbyi''
**''E. priscus''
*'' Edriophus''
**''E. levis''
*'' Paredriophus''
**''P. elongatus''
*'' Totiglobus''
**''T. nimius''
**''T. lloydi''
*'' Lebedodiscus''
*'' Foerstediscus''
**''F. grandi''
**''F. splendens''
*'' Cystaster''
**''C. stellatus''
*'' Cryptogoleus''
**''C. chapmani''
*'' Bellochthus''
*'' Streptaster''
**''S. vorticellatus''
*'' Cryptogoleus''
**''C. chapmani''
*'' Carneyella''
**''C. pilea''
**''C. faberi''
**''C. ulrichi''
*'' Isorophus''
**''I. cincinnatiensis''
*'' Isorophusella''
*'' Camptostroma''
*''**C. roddyi''

*'' Rectitriordo''
*'' Agelacrinites''
*'' Krama''
**''K. devonicum'' (Bassler), 1936
*'' Parakrama''
*'' Hemicystites''
**''H. bohemica''
**''H. chapmani''
**''H. devonicus''
*'' Neoisorophusella''
**''N. lanei''
**''N. berryi''
**''N. maslennikovi''
**''N. whitesidei''
*'' Curvitriordo''
*'' Thresherodiscus''
**''T. ramosa'' (Foerste, 1914)
*'' Postibulla''
**''P. westergaardi''
*'' Parapostibulla''
**''P. belli''
**''P. graysoni''
*'' Eopostibulla''
*'' Pyrgopostibulla''
*'' Yorkicystis''
*'' Torquerisediscus''
*'' Cooperidiscus''
*'' Dynocystis''
*'' Stalticodiscus''
*'' Ulrichidiscus''
*'' Clavidiscus''
*'' Discocystis''
*'' Hypsiclavus''
*'' Spiraclavus''
*'' Giganticlavus''
*'' Lispidecodus''
**''L. plinthotus'' (Kesling, 1967)

References

External links

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.
*https://web.archive.org/web/20060830083438/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.
*https://web.archive.org/web/20060830083430/http://www.tulane.edu/~csumral/morph.html by Colin D. Sumrall

Taxonomy

Mikko's taxonomy

Gallery

File:Edrioasteroid_Smithsonian.JPG, ''Foerstediscus splendens''
Image:OrdovicianEdrio.jpg, The Upper Ordovician edrioasteroid ''Cystaster stellatus'' on a cobble from the Kope Formation in northern Kentucky. In the background is the cyclostome bryozoan
File:Edrioasteroid from the Milwaukee Formation.jpg, Two edrioasteroids, possibly ''Krama'' or ''Agelacrinites'', with dissociated ambulacral plates, from the Devonian of Wisconsin.

{{Taxonbar, from=Q137569

Cambrian first appearances
Permian extinctions