Trilobite Fossil, measures 2.75"H x 4.5"W, Trilobites (meaning "three lobes") are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years. Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton.