The Helicoplacoidea is a small group of fossil echinoderms known only from the Lower Cambrian. In life, they were shaped somwhat like a slender football or a fat cigar, and were able to extend or contract the length of their bodies. Their "skin" was covered in spirals of overlapping ossicles that functioned like armor; their "mouth" was a long groove that also spiralled around their body.
Not much is known about the biology of this group; fossils are rare, and there are no living species. It is thought that helicoplacoids lived in burrows, extending their bodies outward to feed. In any case, the helicoplacoids are among the oldest groups of echinoderms to appear in the fossil record, along with eocrinoids and another short-lived echinoderm group, the Camptostromatoidea. At left is a fossil of Helicoplacus from the Lower Cambrian strata of the White Mountains in California. This is the only place in the world that complete fossils of this group of animals have ever been found.
Source: D. Nichols, 1979. "Echinodermata", pp.277-286 in R. W. Fairbridge & D. Jablonski, The Encyclopedia of Paleontology (New York, Academic Press).