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Sea shell evolution keeps out predators

An EU-funded study has tested different hypotheses of what controls the microstructures found in mollusc shells and how the shells evolved to protect their owners from predators.
Sea shell evolution keeps out predators
The aim of the MOLLUSC EVOLUTION (Changes in mollusc shell microstructure through time) project was to fill gaps in our knowledge regarding the early history of the mollusc shell and its relationship to evolutionary improvements in early oceanic predators.

Researchers used scanning electron microscopy to take detailed photographs of 50 remarkably preserved molluscs, some of which dated back to the early Cambrian period around 540 million years ago. A database of shell microstructures in Palaeozoic molluscs was also created in order to better determine the patterns in shell features.

The images and database enabled scientists to test hypotheses regarding control over biomineralisation in early molluscs. One example is whether predation or seawater exerted greater control over the nature and organisation of minerals within the shell.

New data indicated that most early and middle Cambrian molluscs had extensive calcite in their shells, unlike modern shells where aragonite is much more common. This is because fossil species lived during the time of calcite seas when calcite was the main inorganic marine calcium carbonate precipitate.

These findings agreed with the hypothesis that saltwater chemistry exerted a major influence on the minerals used in constructing the earliest mollusc shells. Furthermore, molluscs from the Cambrian period tended to have thicker organic components to the shell. The result was a more flexible shell than that of modern molluscs, possibly because predators at this time lacked hard claws or jaws.

Fossils from the subsequent Ordovician period (around 450 million years ago) were found to have shells with a relatively thick mineral layer. In addition, some lineages possessed nacre (also called mother-of-pearl). This was consistent with the hypothesis that mollusc strength increased during the Great Ordovician Biodiversification Event as the intensity of predation increased significantly.

Detailed preservation of nacre in Ordovician fossils enabled its comparison with different fossil groups that possessed it, including gastropods, cephalopods, bivalves and monoplacophorans. Such comparisons helped to test the hypothesis that nacre originated independently in the different groups of molluscs, suggesting that increasing predation during the Cambrian-Ordovician.

MOLLUSC EVOLUTION findings will enable researchers in the future to develop and test new hypotheses related to the controls and evolution within the shells of various lineages of ancient molluscs.

Related information

Keywords

Predators, mollusc shells, shell microstructure, Cambrian, biomineralisation, calcite, nacre
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