Arkarua
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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 one species Arkarua adami is 3 to 10 mm 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 the Flinders Ranges in South Australia.
The name Arkarua comes from the name of a mythical giant snake of the local Aboriginal people. It was named by James G Gehling in 1987.
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 unequivocally identifiable echinoderm -- Helicoplacus -- is somewhat younger, in the early Cambrian. Helicoplacus is not radially symmetric and has three fold symmetry, instead of the typical five fold symmetry. This suggests that Arkarua may not have been an echinoderm. It is more likely to be a cnidarian or algae. McMenamin/Sielacher have proposed Arkarua as a conventional mainline member of the Vendazoa, with five fold symmetry and non-iterated cell families.
[edit] 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.
