Trilobites are a fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago). They were hard-shelled, segmented creatures that lived in Earth's ancient seas. They are considered to be one of our planet's earliest complex life-forms and are one of the key signature creatures of the Paleozoic Era. Trilobites went extinct before dinosaurs ever existed.