Similar to the horseshoe crab, Trilobites - meaning 3 lobes-lived from 550 to 200 million years ago in Cambrian, Paleozoic, and Devonian seas. Trilobites are most closely related to the chelicerates, which include the horseshoe crabs and spiders. They are now all extinct, but their fossil remains are found in many places in the U.S. and worldwide. Some, in the Canadian Rockies, have been found as large as 2½ feet in length.