DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Fossil trilobites (Proetus bohemicus) preserved in Morrocan rock from the Devonian Period of 400-360 million years ago. Trilobites, and other segmented invertebrates like crabs, spiders and insects, are classified as arthropods. They lived in the sea between 500 and 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. Trilobites have a head (or cephalon, bottom), segmented thorax (centre) and a tail (or pygidium, not clearly seen). These trilobites are 2.5 centimetres long.
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