BIVALVE AND GASTROPOD SHELLS IN THE RED CRAG
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BIVALVE AND GASTROPOD SHELLS IN THE RED CRAG – FURTHER DISCUSSION HOWARD MOTTRAM Opening remarks In last year’s journal I reviewed the types of calcium carbonate that occur in commonly found Red Crag bivalve and gastropod shells (Mottram, 2018). It may have seemed that the types of calcium carbonate varied somewhat randomly between and within taxonomic groups. However, when grouped by lifestyles, there was a logic as to how these molluscs exploited the different types of calcium carbonate relative to their needs. Shell fabrication - aragonite or calcite? We tend to think that aragonite is less resilient than calcite because it is unstable over time and it dissolves more readily, especially in very cold water and under high pressure. These physico-chemical limitations of aragonite were not problematic to the bivalves and gastropods because they had short life spans (usually less than 15 years) and, when the Red Crag sediments were deposited, the bivalves and gastropods were living within temperate latitudes in a shallow North Sea (i.e. not arctic and the water pressures at the sea-bed would have been relatively low). Aragonite is actually slightly harder than calcite but what was important to the development and maintenance of the shells was how these different types of calcium carbonate behaved in their respective layers in the shell walls. Each layer was composed of either calcite or aragonite, never a heterogeneous mix, and the layers were built up from very thin sheets in which the crystals were laid down in particular styles from a range of possible crystal shapes and orientations. If a crack occurred in a shell then it was most likely to propagate ‘along, but not across, the grain’. In a simple situation, if elongate crystals in a sheet were aligned along the length of the sheet and in the next sheet they were aligned across the length of the sheet, the effects of cracks would have been minimised: this is the principle that makes plywood so effective. The arrangements of the sheets within the layers, helped by the tiny proportion of protein between the crystals, also gave the shell structure a little flexibility, i.e. reduced brittleness. Materials scientists have tested shells and found that the ways in which the shells have been built up makes shells over 1,000 times better at resisting crushing or stretching than single flawless crystals of calcite or aragonite. In practice, the possible shapes and orientations of aragonite crystals would have created even stronger sheets of building material than calcite would have done, e.g. Zhang et al., 2011, p.8683. When I refer to aragonite sheets as having been stronger than calcite sheets, this was in terms of resisting both squeezing (compression) and stretching (tension). Calcite may not have seemed the best option in respect of strength, but deposition of it would have been quicker and less demanding of an individual’s resources (Vermeij, 1993b, p.52). Therefore, calcite was also used, especially when the objective was to thicken the shell walls.
Trans. Suffolk Nat. Soc. 55 (2019)