Trilobites were extinct arthropods that form the class Trilobita. When Trilobites appear in the fossil record of the Lower Cambrian they are already highly diverse and geographically dispersed. Trilobites appeared in the Middle Cambrian period, they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction. During the Late Devonian extinction, all Trilobite orders, with the exception of Proetida, died out.
The last family of trilobites disappeared in the mass extinction at the end of the Permian period about 250 million years ago. All the other families died out in the devonian extinction.