Trilobites (meaning "three lobes") are a fossil group of extinct marine arthropods that form the class Trilobita. Trilobites also form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the still unnamed Cambrian Series 2 (about 521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except...
