Trilobite fossil, coloured scanning electron micrograph (SEM). Trilobites were arthropods, the shelled segmented invertebrates that include crabs and insects and spiders. Their exoskeleton covered them on top (seen here) as they fed on the seabed. The body is divided into three parts: the cephalon (head), the thorax (numerous segments) and in some trilobites, a pygidium (tail). The eyes (top left and top right) were a compound collection of tiny lenses (the holes seen here). Uniquely, trilobite eye lenses were made from calcite, the transparent crystal form of calcium carbonate. Trilobites varied in length from 0.1 to 70 centimetres. They lived between 500 and 300 million years ago.
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