Arkarua
Encyclopedia
Arkarua is a small, Precambrian
disk-like fossil
with a raised center, a number of radial ridges on the rim, and a five-pointed central depression marked with radial lines of 5 small dots from the middle of the disk center. The only known species, Arkarua adami, is 3 to 10 mm in diameter.
Arkarua is known only from the Ediacaran
beds of the Flinders Ranges in South Australia
, and takes its name from a mythical giant snake of the local Aboriginal people (Gehling 1987).
Echinodermata. Because of its flattened disk- or button-shape, coupled with its pentamerous symmetry, some claim that it can be further classified into the Edrioasteroidea, a subgroup of the echinoderms.
However, it lacked a feature present in later echinoderms: the stereom
, a unique crystalline form of calcium carbonate
from which their skeletons are built.
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
disk-like fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
with a raised center, a number of radial ridges on the rim, and a five-pointed central depression marked with radial lines of 5 small dots from the middle of the disk center. The only known species, Arkarua adami, is 3 to 10 mm in diameter.
Arkarua is known only from the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
beds of the Flinders Ranges in South Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and takes its name from a mythical giant snake of the local Aboriginal people (Gehling 1987).
Classification
All known specimens of Arkarua are casts that give no clue to the internal structure, making classification problematic. Because of Arkaruas pentamerous symmetry, it is tentatively placed within phylumPhylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Echinodermata. Because of its flattened disk- or button-shape, coupled with its pentamerous symmetry, some claim that it can be further classified into the Edrioasteroidea, a subgroup of the echinoderms.
However, it lacked a feature present in later echinoderms: the stereom
Stereom
Stereom is a calcium carbonate material that makes up the internal skeletons found in sea urchins, and all other 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 being a matrix of calcite...
, a unique crystalline form of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
from which their skeletons are built.
External links
- Vendian animals: Arkarua from the Ediacara Hills of Australia, from the University of California Berkeley Museum of Paleontology. (pictures)
- Gehling, J.G. 1987. Earliest known echinoderm — a new Ediacaran fossil from the Pound Subgroup of South Australia. Alcheringa 11:337-345.