Aspidella terranovica
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Aspidella Temporal range: Ediacaran |
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| 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 | |
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Aspidella terranovica Billings, 1872 |
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| Synonyms | |
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Possibly numerous, see text. |
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Possibly numerous, see text.
Aspidella is an Ediacaran disk-shaped fossil. It was most likely the imprint of the holdfast of a stalked organism, such as an erniettomorph or rangeomorph.
Aspidella consists of disk-shaped fossils, with concentric rings and/or centripetal rays. The diameter of circular Aspidella varies from 1 to 180 mm. Most individuals are between 4 and 10 mm, but smaller animals would presumably have decayed before they could fossilize. 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.
The rarity of large individuals probably indicates that Aspidella were r-strategists, producing numerous offspring of which most died young. It is most common in deep-water sediments, but is a constituent of most Ediacaran fossil assemblages, including those deposited above storm wave-base. The organisms can reach densities of 3000 m−2.
Just like Ediacaria (see also below), Aspidella has initially been considered a scyphozoan jellyfish. This initial designation has been refuted; some specimens have been shown to be 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); whereas others represent microbial colonies.
Peterson, et al. have proposed that Aspidella (and the similar Charnia and Charniodiscus) were analogous to fungi.
The upper and lower surfaces of the fossils have a distinct elemental composition that resembles that of fossilized biofilms. The sediment within the fossils also has a distinct composition, being enriched in certain elements with respect to the rock matrix. Since it is difficult to account for such a distribution of elements by post-mortem diagenetic processes, it would appear that the elements (and thus the sediment) were incorporated into the organism whilst it was alive.
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