Trilobite eye fossil, coloured scanning electron micrograph (SEM). Trilobites were arthropods, the shelled segmented invertebrates that include crabs and insects and spiders. Like insects, their eyes were compound, consisting of a collection of tiny lenses (the holes seen here). Uniquely, trilobite eye lenses were made from calcite, the transparent crystal form of calcium carbonate. Trilobites were marine animals, feeding on the seabed. They ranged in length from 0.1 to 70 centimetres. They lived between 500 and 300 million years ago. Formation of fossils occurs when an object is buried under a rock-forming material, like mud, which preserves structural details.

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