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Atomodesma sp.
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Atomodesma is one of the more important bivalve species from the Permian, though its appearance doesn't always suggest that. Many of the specimens that have been found are damaged and distorted, and often all that remains are elusive fragments. As a consequence, naming specimens with any confidence is difficult. The geologist, J.B. Waterhouse, who studied the Permian rocks of New Zealand during the middle of the last century managed to identify ten or more taxa, and on the basis of that developed a suggested biostratigraphic sequence for the Permian in New Zealand. Doubts nevertheless remain about the reliability both of some of the identifications and the sequence, and most more recent finds have simply been ascribed to the genus, as Atomodesma sp. I've done the same with the specimens in this collection.
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Fragments of Atomodesma sp. from the Permian rocks (Tramway Formation) in the Upper Wairoa Valley, Nelson
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The key specimen shown here (top of page) is a good example of Atomodesma sp., from the Tramway Formation in the upper Wairoa valley. It's possibly A. trechmanni, which Waterhouse (1963) describes from this formation. It is in the order of 6 cm in length, and about half that in width. Note the hingeline, running along the top edge of the shell, which has opened up during fossilisation, and the beak which is just visible at its left-most end.
The further specimen from the same locality (above right) show the concentric wrinkles that are common on the shells. The images below show specimens from the Hackett Valley, likewise from the Tramway Formation, and thus safely Permian in age.
The further specimen from the same locality (above right) show the concentric wrinkles that are common on the shells. The images below show specimens from the Hackett Valley, likewise from the Tramway Formation, and thus safely Permian in age.
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Part of a sample of Atomodesma sp. in a siltstone block fom the Hackett Valley, showing the positive and negative impressions.
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A large part-specimen of Atomodesma from the Hackett Valley. Note the outline of the shell, narrowing to the right, and the large umbo at the right hand end.
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In many cases, finds of Atomodesma are even more enigmatic than the ones from the Hackett Valley, shown here: mere fragments that look like Atomodesma, but cannot be attributed to the gernus with any certainty. The best clue in these cases is the shell structure, which in Atomodesma comprises small hexagonal (or polysided) calcite prisms, lying normal to the surface, 0.025-0.05 mm in diameter. The best way to see these is with a thin section, viewed under a microscope. Often, the prisms are arranged in bands, corresponding to a growth period and delimited by a clear growth ring.
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Further Reading: Waterhouse, J.B. 1963 New Zealand specis of the Permian bivalve, Atomodesma Beryrich. Palantaeology 6(4), 699-717. |