Trilobites roamed the Earth for a long time. Trilobites flourished from the Cambrian period to the end of the Permian (521 to 252 million years ago). Over 20,000 known species of trilobites thrived throughout this time. These creatures evolved to behave differently from swimming in the oceans to crawling along the oceans floors. Sizing and features also experienced significant changes allowing them to adapt to there surrounding. Trilobites were exclusively marine (saltwater) creatures, and have been found in former oceans across the world. They are related to today's arthropods – insects, crabs, and similar animals.