Helicoplacoidea is an extinct

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

within the

Echinodermata 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 li ...

. All known taxa were discovered in sediments dating back to the Cambrian.Fossilworks: Helicoplacoidea
''fossilworks.org''. Retrieved 2021-01-13.

Description

Helicoplacoid skeletons consist of small platelets arranged like those of

asteroids An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

, lacking fused skeletons as in

echinoids Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard s ...

. Little is known about their ecology, besides that they were most likely sessile suspension feeders. Members of the group were able to expand and contract their bodies, and some have speculated that they may have largely fed at night.University of Berkeley: Helicoplacoidea
''ucmp.berkeley.edu.'' Retrieved 2021-01-15. Others say that they were adapted to the more stratified sediment of the Cambrian and that changes in the composition of this sediment resulted in their extinction. Most likely the extending and contracting played a role in gas exchange and nutrition intake as it generated a water current.
/ref> They seem to be one of the oldest groups of echinoderms, and also one of the first to go extinct, as they first appeared some 530 million years ago and would go extinct around 516 million years ago.

Fossils and taxonomy

Most of the fossils are from White Mountains, Nevada. The rarity of fossils and general poor preservation of them has made it hard to determine any kind of phylogeny for them, though three genera ('' Helicoplacus'', '' Polyplacus'' and '' Waucobella'') are currently recognized.Paleobiology Database: Helicoplacoidea.
''paleobiodb.org''. Retrieved 2021-01-15. Also due to this, it is difficult to establish a conclusive taxonomic classification of the Helicoplacoidea. A list of helicoplacoid genera, as recognized by the Paleobiology Database, is as follows: ::Family †Helicoplacidae :::†'' Helicoplacus'' Durham & Caster, 1963 :::†'' Polyplacus'' Durham, 1967 :::†'' Waucobella'' Durham, 1967 ''Helicoplacus guthi'', one of two species of '' Helicoplacus'', was named after the fossil's discoverer, Peter Guth. However, Wilbur (2006) suggests that this specimen is actually a poorly preserved specimen of ''Helicoplacus gilberti''. This fossil measured 3.3 cm, and comes from the Cambrian formation of Poleta, California.

References

Prehistoric Echinozoa Cambrian echinoderms {{paleo-echinoderm-stub