Because of ecdysis we have a good fossil record of the moult stages of many fossil arthropods. The three primary divisions of the body are set at right angles to the unique three-lobed longitudinal division from which the group takes its name. A single plate made up of several fused plates. Like the head, a single plate made up of several fused plates. All trilobites were marine .
Trilobites first appear in rocks of lower Cambrian age.
They are especially abundant in the Cambrian and Ordovician and serve as important index fossils .