''Helicoplacus'' (often misspelled ''Helioplacus'') is the earliest well-studied

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

. Fossil plates are known from several regions. Complete specimens were found in Lower

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

of the White Mountains of

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. The animal was a cigar-shaped creature up to long that stood upright on one end. Unlike more typical echinoderms such as

sea star Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...

s, ''Helicoplacus'' does not have fivefold symmetry. Instead, there is a spiral food groove on the outside along which food was moved to a mouth that is thought to be on the side. The respiratory system appears to be primitive. Although the animal does not look like a typical echinoderm, the plates are composed of the characteristic calcareous plates known as ''

stereom Stereom is a calcium carbonate material that makes up the internal skeletons found in all echinoderms, both living and fossilized forms. It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest be ...

'' that are common to all echinoderms. The ambulacrum is similar to that of the Edrioasteroidea; as a result, ''Helicoplacus'' may belong to Pelmatozoa. 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 An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...

echinoderm because of its fivefold symmetry, though it appears to lack stereoms and a mouth. ''Helicoplacus'' is thought to have been a suspension feeder living at moderate depths in highly oxygenated water with strong enough currents to ensure a steady food supply. It is typically found in greenish

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s and, rarely found in shallow water

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s and

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

s. The helically spiraling rows of plates radiating from the base, which in life probably was anchored in the muddy substrate.

Stratigraphic distribution

''Helicoplacus'' is one of the earliest mineralizing echinoderms, appearing alongside close relatives ''Polyplacus'' and ''Waucobella'' in the Poleta formation, among strata notable for an exceptional abundance of echinoderm fossils (including also edrioasteroid fragments).Zamora, S. ''et al.'' (2013). Cambrian echinoderm diversity and palaeobiogeography. In: Harper, D. A. T. & Servais, T. (eds). ''Early Palaeozoic Biogeography and Palaeogeography''. Geological Society, London, Memoirs, 38: 157-173. The Poleta Formation is correlated with the as-yet unratified Cambrian Stage 3 (Series 2), giving it a provisional date of ~.

References

External links

* Pictures of 'Helicoplacus' can be found at http://www.usna.edu/Users/oceano/pguth/website/pl00001.htm Helicoplacoidea Prehistoric Echinozoa genera Cambrian echinoderms Paleozoic echinoderms of North America Cambrian genus extinctions {{paleo-echinoderm-stub

Stratigraphic distribution

See also

References

External links