Trilobite fossil. Fossilised Calymenella sp. trilobite, rolled into a ball. The trilobites are an extinct group of marine arthropods, with a hard, segmented shell that is divided into 3 sections. The Calymenella group was common from the Lower Silurian to the Middle Devonian period (around 435-360 million years ago). It had a wide geographical range, occurring over regions that are now the land masses of Europe, Australia and North and South America. Many trilobites could roll themselves up into a ball (as seen here) when threatened. Assuming this position ensured that only the shell was exposed, defending the animal from attack.
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