Aspidella

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Aspidella is also a synonym for the mushroom genus Amanita.
Aspidella
Fossil range: Ediacaran
Cyclomedusa specimens. Large Aspidella look essentially identical, but have either concentric rings or rays.
Cyclomedusa specimens. Large Aspidella look essentially identical, but have either concentric rings or rays.
Scientific classification
Kingdom: incertae sedis
Genus: Aspidella
Billings, 1872
Species: A. terranovica
Binomial name
Aspidella terranovica
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 610 to 555 million years ago. The diameter of circular Aspidella varies from 1 to 180 mm.[1] 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.[original research?] 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.[2]

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.

[edit] Systematics 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.

[edit] References

  1. ^ Peterson. P. 131
  2. ^ Cyclomedusa, Paliella and Ediacaria — see Grazhdankin, D. (in press): Lethaia. doi:10.1111/j.1502-3931.2007.00025.x
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