Arkarua
|
|
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 species Arkarua adami is 3 to 10mm across. Because of the five-fold symmetry, Arkarua has been proposed as a possible precursor to the Echinoderms. Some claim it is not only an echinoderm, but is specifically an Edrioasteroid. A few Edrioasteroids are reported from the Lower Cambrian of California and Newfoundland as well as the middle Cambrian of British Columbia in Canada. Arkarua is known only from the Ediacaran beds of South Australia.
All known specimens of Arkarua are casts that give no clue to the internal structure. There is no sign of the calcium carbonate stereoms (plates) that are diagnostic of echinoderms. Neither can a mouth be identified. The earliest, well-studied echinoderm -- Helioplacus -- is somewhat younger. It is not radially symmetric and does not possess five fold symmetry. McMenamin/Sielacher have proposed Arkarua as a conventional mainline member of the Vendazoa, with five fold symmetry and non-iterated cell families.
External links
- Vendian animals: Arkarua from the Ediacara Hills of Australia (http://www.ucmp.berkeley.edu/vendian/arkarua.html), from the University of California Berkeley Museum of Paleontology. (pictures)