Helicoplacus

Helicoplacus (often misspelt Heiloplacus) is the earliest well-studied fossil echinoderm. Fossil plates are known from several regions. Complete specimens were found in Lower Cambrian strata 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, helicoplacus 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 side. 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 common to all echinoderms.

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

Helicoplacoids 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.