Details: Trilobites are primitive marine Arthropods. They appeared in the Cambrian, about 570 million years ago, and became extinct in the Permian, about 240 million years ago. Their shape diversified over time. The name Tri-lob-ite derives from the division of the external skeleton into three parts,or lobes. Fossilized remains of these creatures have been found on every continent. They had thin antennae and numerous feet. They inhabited the sea bottoms, where they would find their food. Today they are guide fossils which help mark specific geologic eras.