Trilobites were a very diverse group of marine arthropods, first appearing in the fossil record 521 million years ago during the Early Cambrian. They were one of the most successful early animals on our planet with over 25,000 described species that filled nearly every evolutionary niche. Trilobites were one of the many lifeforms lost during the Permian mass extinction event 250 million years ago, and have left behind an excellent fossil record for us to study.