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Helioplacus is the earliest well studied echinoderm. Plates are known from several regions. Complete specimens were found in the early Cambrian of the White Mountains of California. The animal turned out to be a cigar shaped creature up to 7 cm long that stood upright on one end. Unlike more typical echinoderms such as starfish, helioplacus does not have five fold symmetry. Instead, there is a spiral food groove on the outside along which food was moved to a mouth that is thought to be located on the top. The respiratory system appears to be primitive. Although the animal does not look like a typical echinoderm, the plates are the characteristic calcaerous plates called stereoms that are found in all echinoderms.

Other contemporaneous echinoderms are known to have existed from their dissociated plates, but other than a few possible edrioasteroids, helioplacus is the earliest echinoderm that is well enough preserved to analyze its characteristics. One much earlier form called Arkarua has been hypothecated as an ancestoral echinoderm because of its five fold symmetry. But Arkarua appears to lack both stereoms and a mouth. Helioplacids seem to have lasted for about 15 million years in the Lower Cambrian.

Helioplacids are thought to have been suspension feeders living at moderate depths in highly oxygenated water with strong enough currents to ensure a steady food supply. They are typically found in greenish shales and are rarely found in shallow water sandstones and limestones.

Note that the spelling helicoplacus is about as common as helioplacus. It would be a good bet that since I settled on the latter spelling, the former is probably correct.

One species is Helicoplacus guthi Durham 1993 from the White Mountains of California

Pictures of 'Helioplacus' can be found at http://www.usna.edu/Users/oceano/pguth/website/pl00001.htm and http://astrobiology.ucla.edu/ESS116/L14/L14

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