The trilobites flourished in Cambrian and Ordovician seas, but they disappeared in the great Permian extinction at the close of the Paleozoic era (251 million years ago). Because they had heavy exoskeletons that readily fossilized, they left behind an abundant record of their existence (Figure 31.19). About 10,000 species have been described.

The trilobites are the earliest known arthropods to have had jointed appendages. The body segmentation and appendages of trilobites followed a relatively simple, repetitive plan, but some of their appendages were modified for different functions. This specialization of appendages is a theme in the continuing evolution of the arthropods.