Trilobites are extinct, marine arthropods that were were one of the dominant life forms in our oceans for nearly 250 million years. They possessed a segmented body with a hard exoskeleton and jointed appendages. Many trilobites could roll themselves up into a ball for protection, much like modern pill bugs. There are tens of thousands of described species of trilobites. They filled many different environmental niches, some were predators, some ate algae, some crawled along the sea floor while some burrowed in the soft mud. Trilobites first appeared in the fossil nearly 540 million years ago, and disappeared during the mass extinction at the end of the Paleozoic era, 252 million years ago. Given their abundance and wide distribution in the fossil record, they are often used as “index fossils” to help date rock formations.