Helicoplacoid skeletons consist of small platelets arranged like those of asteroids, lacking fused skeletons as in echinoids. 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.[3] 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.[3][4] 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.[1]
Fossils and taxonomyedit
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.[5] 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:[5]
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.[6]
Referencesedit
^ abcDurham, J. W.; Caster, K. E. (17 May 1963). "Helicoplacoidea: A New Class of Echinoderms". Science. 140 (3568): 820–822. Bibcode:1963Sci...140..820D. doi:10.1126/science.140.3568.820. PMID 17746435. S2CID 33443660.
^Wilbur, Bryan C. (1 August 2006). "Reduction in the number of Early Cambrian helicoplacoid species". Palaeoworld. 15 (3): 283–293. doi:10.1016/j.palwor.2006.10.009.
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helicoplacoidea, extinct, class, within, echinodermata, known, taxa, were, discovered, sediments, dating, back, cambrian, temporal, range, middle, cambrian, late, cambrian, preꞒ, nthe, holotype, specimen, helicoplacus, guthi, scientific, classificationkingdom,. Helicoplacoidea is an extinct class within the Echinodermata All known taxa were discovered in sediments dating back to the Cambrian 1 2 HelicoplacoideaTemporal range Middle Cambrian Late Cambrian PreꞒ Ꞓ O S D C P T J K Pg NThe holotype specimen of Helicoplacus guthi 1 Scientific classificationKingdom AnimaliaPhylum EchinodermataClass Helicoplacoidea Helicoplacus Polyplacus WaucobellaDescription editHelicoplacoid skeletons consist of small platelets arranged like those of asteroids lacking fused skeletons as in echinoids 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 3 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 3 4 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 1 Fossils and taxonomy editMost 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 5 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 5 Family Helicoplacidae Helicoplacus Durham amp Caster 1963 Polyplacus Durham 1967 Waucobella Durham 1967 dd dd 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 6 References edit a b c Durham J W Caster K E 17 May 1963 Helicoplacoidea A New Class of Echinoderms Science 140 3568 820 822 Bibcode 1963Sci 140 820D doi 10 1126 science 140 3568 820 PMID 17746435 S2CID 33443660 Fossilworks Helicoplacoidea fossilworks org Retrieved 2021 01 13 a b University of Berkeley Helicoplacoidea ucmp berkeley edu Retrieved 2021 01 15 Palaeos Helicoplacoidea Armor Plated Spirals a b Paleobiology Database Helicoplacoidea paleobiodb org Retrieved 2021 01 15 Wilbur Bryan C 1 August 2006 Reduction in the number of Early Cambrian helicoplacoid species Palaeoworld 15 3 283 293 doi 10 1016 j palwor 2006 10 009 nbsp This prehistoric echinoderm related article is a stub You can help Wikipedia by expanding it vte Retrieved from https en wikipedia org w index php title Helicoplacoidea amp oldid 1140717938, wikipedia, wiki, book, books, library,
