Fossil trilobite preserved in siltstone, from the Burgess Shale area. Trilobites are arthropods that lived in the sea during the Cambrian Era more than 300 million years ago. A fossil is created when an animal is quickly buried in sand, mud or volcanic ash after death, and the normal decay processes do not occur. The minerals of the hard parts of the animal, such as shell or bone, are gradually replaced with rock. The trilobite has a head (or cephalon, top), segmented thorax (centre) and a tail (or pygidium, bottom). All trilobites have three lobes which run from head to tail. This specimen is from the Mount Stephen trilobite beds (Ogygopsis shale) in British Columbia, Canada.

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