Trilobites, which were early invertebrates were crustaceans, or sea creatures with hard shells.
Trilobites were abundant in the Cambrian period. Most were no bigger than 4 inches (10 centimeters) long, although fossils found near Boston, Massachusetts, of one species of trilobite measure 18 inches (45 centimeters) long.
All trilobites had bodies made up of three sections. Trilobite fossils are abundant in rocks from the Cambrian period. This is partly because there were so many of them and because they had hard shells.
You can buy trilobite fossils in almost every fossil, rock, and nature store today.