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A natural mould and matching plaster cast of Lima georgii-boehmi (LPB8_123) from Highfield Farm
One of the more uncommon but significant fossils from the Triassic rocks around Nelson is this bivalve. As the pictures show, it's characterised by a finely ribbed shell, with clear but somewhat delicate growth-lines and a slightly hunched umbo that curves slightly to one side. The shape is sub-oval (i.e. not quite round) and the ribs are seen t have a wavy appearance towards the outer edge of the shell. Zooming in on the photo also shows that each rib, and the depression between, is composed of a number of fine, parallel lines, and that between the majopr growth lines there are similarly finer markings running around the shell. This gives the surface a textured appearanance, almost like corduroy.
From comparisons with other material - e.g. the specimens illustrated by Marwick (1953) and described by Wilckens (1927) - I am fairly confident that these fossils belong to the species Lima georgii-boehmi. The description by Wilckens, for example, is shown on the right. The species is also recognised by the National Biodiversity Inventory, where it is attributed to the Kaihikuan-Oretian-Otamitan stages of the Triassic. At Highfield Farm, where this specimen was found, it appears to be focused mainly (if not quite entirely) in the Oretian stage, for which it is a useful marker. Nevertheless, there are some doubts about its classification. Marwick (1953) grumbles that the definition of the genus Lima has to be 'used in a very wide sense to accommodate this shell'. Perhaps for this reason, several of the published papers on the Triassic fail to mention it and refer only to Lima sp. |
Lima georgii-boehmi
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