Aspidella

:Aspidella" is also a synonym for the mushroom genus "Amanita.

Taxobox

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image_width = 250px
image_caption = "Cyclomedusa" specimens. Large "Aspidella" look essentially identical, but have "either" concentric rings "or" rays.
fossil_range = Ediacaran
regnum = "incertae sedis"
genus = "Aspidella"
genus_authority = Billings, 1872
species = "A. terranovica"
binomial = "Aspidella terranovica"
binomial_authority = Billings, 1872
synonyms =Possibly numerous, see text.

"Aspidella" is an Ediacaran disk-shaped fossil.

In 1872, Elkanah Billings described "Aspidella terranovica" fossils from Duckworth Street, St. John's, Newfoundland. They were in a Precambrian outcrop of black shale. Billings was the head paleontologist with the Geological Survey of Canada at the time. Even so, his findings were disputed by Charles Doolittle Walcott, who claimed that the shapes in the rocks were concretions formed inorganically. Other explanations were that the circles were gas escape bubbles, or fakes planted by God to lure those with little faith into error. They were the first Ediacaran(Vendian) fossils described by a scientist.

For decades "Aspidella" and its partner fossils were not considered to be Precambrian life forms. This lasted until the work of Reg Sprigg who discovered the Ediacara Hills fossils. Fossils were found in many other parts of the world in rocks of about the same age and became accepted as genuine remains of life forms.

Other places where "Aspidella" are found is the Bonavista Peninsula and Mistaken Point in Newfoundland, and the Twitya formation in British Columbia; see also below.

The time range of "Aspidella" ranges from Ma|610 |555. The diameter of circular "Aspidella" varies from 1 to 180 mm. [Peterson. P. 131] Most individuals are between 4 and 10 mm, however; smaller animals would presumably have decayed before they could fossilize. That large individuals are rare, probably indicates that "Aspidella" were r-strategists, producing numerous offspring of which most died young.Or|date=January 2008 Other "Aspidella" take the form of ellipses, 3-8 cm long and 1-4 cm wide. Most have a central pimple. The rim of all specimens is made up by ridge-edged rays and/or concentric rings.

Just like "Ediacaria" (see also below), "Aspidella" has initially been considered a scyphozoan jellyfish. It appears that it was rather the holdfast of some organism, the main body of which extended into the open water but broke off before fossilization. A few specimens bearing stubs of stalks opposed to the central pimple support this.

However, similar fossils, often considered synonymous, have recently been conclusively designated microbial colonies, which raises the possibility that "Aspidella" is itself a microbial colony. ["Cyclomedusa, Paliella and Ediacaria" — see aut|Grazhdankin, D. (in press): Lethaia. doi|10.1111/j.1502-3931.2007.00025.x]

Some research is currently being done defining "Aspidella" in South Australia.

Peterson, et al. have proposed that "Aspidella" (and the similar "Charnia" and "Charniodiscus") were analogous to fungi.

ystematics and taxonomy

"Aspidella" is derived from the Latin diminutive of Ancient Greek "aspis" (Ασπις, a round shield). "terranovica" is Latin for "from the new land" (ie Newfoundland).

Different morphological forms have been called "Ediacaria" or "Spriggina wadea". The differences between the small elliptical "typical" "Aspidella", the flat, ringed "Spriggina" wadea" and the large, pimpled and rayd "Ediacaria" seem to be due to different taphonomical conditions however. For example, "S." wadea" and "Ediacaria" appear to be remains of the same animals, only that the former was fossilized in more compact, fine-grained clay whereas the latter is known from rocks that originally were predomnantly sandy sediment.

Numerous other taxa may also be junior synonyms of "Aspidella":
*"Beltanella"
*"Cyclomedusa"
*"Glaessneria"
*"Irridinitus"
*"Jampolium"
*"Madigania"
*"Medusinites"
*"Paliella"
*"Paramedusium"
*"Planomedusites"
*"Protodipleurosoma"
*"Tateana"
*"Tirasiana"
*"Vendella"

Due to its nondescriptness, "Aspidella" might be considered a form taxon, an artificial assemblage of similar-looking and similar-living organisms without indicating of a phylogenetic relationship. In this case, some presumed synonyms (such as "Ediacaria" or "Cyclomedusa") would remain valid. The type specimens were of the ellipsoid type (they are lost, but a cast remains). Thus, if "Aspidella" in the loose sense turns out to be an assemblage of more or less related taxa, the genus name would apply only to the smallish ellipsoid specimens.

References

* (2000): The first named Ediacaran body fossil, "Aspidella terranovica". "Palaeontology" 43(3): 427-456. doi|10.1111/j.0031-0239.2000.00134.x [http://www.blackwell-synergy.com/doi/pdf/10.1111/j.0031-0239.2000.00134.x PDF fulltext]
*aut|Peterson, Kevin J., Ben Waggoner and James W. Hagadorn. (2003): "A Fungal Analog for Newfoundland Ediacaran Fossils?" "Integrative and Comparative Biology", 43:127–136. Found at [http://icb.oxfordjournals.org/cgi/reprint/43/1/127.pdf] - Retrieved July 19, 2007.