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Trilobites are extinct arthropods that form the class Trilobita. They appeared at the start of the Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Late Devonian extinction, all trilobite orders, with the sole exception of Proetida, died out. The last of the trilobites disappeared in the mass extinction at the end of the Permian 250 million years ago (m.y.a.).
Different trilobites made their living in different ways. Some led a benthic life as predators, scavengers or filter feeders. Some swam in order to feed on plankton. Still others are thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.
The bodies of trilobites are divided into three parts (tagmata): a cephalon (head), composed of the two preoral and first four postoral segments completely fused together; a thorax composed of freely articulating segments; and a pygidium (tail) composed of the last few segments fused together with the telson. The pygidia are still fairly rudimentary in the most primitive trilobites. The thorax is fairly flexible—fossilised trilobites are often found curled up like modern woodlice for protection.
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