JIM AMOS / SCIENCE PHOTO LIBRARY JIM AMOS / SCIENCE PHOTO LIBRARY
Trilobite. Close-up of a well-preserved fossilised trilobite. The dorsal exoskeleton (outer shell) of the trilobite is clearly visible as ridges running across the top of the fossil. This is divided into three lobes by longitudinal furrows, hence the name trilobite. The head (at bottom) is the site of the animal's eyes. The trilobites are an extinct group of once common marine arthropods. They existed from the Cambrian to the Permian geological eras (from 590 to 286 million years ago). Most species are thought to have lived on the sea floor like modern day crabs. Their flexible backs allowed them to roll up into a tight ball when being attacked by predators.
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