Trilobites dominated the waters around the world for millions of years. They were the first free-swimming creatures of any size, the first to have an external skeleton for armor, and the first to have jointed legs. The petrified bodies and casts, or imprint, of Trilobite remains tells us of the creatures’ existence. Their three lobes earned them the name trilobite. Trilobites appeared in the seas about 570 million years ago and were prolific by the time they vanished 350 million years later. Trilobite fossils are a great aid in identifying the age of sedimentary formations. Trilobite fossils have been found around the world.