Trilobite, any member of a group of extinct fossil arthropods easily recognized by their distinctive three-lobed, three-segmented form. Trilobites, exclusively marine animals, first appeared at the beginning of the Cambrian Period, about 542 million years ago, when they dominated the seas. 

Trilobite looks like little hard-shelled insects and is often nicknamed "bugs" by fossil collectors, but they are not related to insects. Trilobites have no known direct descendants. Their closest living relatives would be the chelicerates. Though horseshoe crabs are often cited as their closest living relatives, they are no closer evolutionarily than other chelicerates.

Trilobites have been extinct since before the age of Dinosaurs (about 251 million years ago), but some living creatures bear such close superficial resemblance to trilobites that they cause great excitement when encountered. 

They died out at the end of the Permian, 251 million years ago, killed by the end Permian mass extinction event that removed over 90% of all species on Earth. They were very diverse for much of the Palaeozoic, and today trilobite fossils are found all over the world. 

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