Trilobites were highly successful sea creatures that appeared over 500 million years ago during the Cambrian Period. Their fossils have been found globally, and many come from Morocco. They belonged to a group of hard-shelled invertebrates called arthropods, which includes modern crustaceans, arachnids, insects, and horseshoe crabs. Trilobites evolved into over 15,000 species, making them the most diverse extinct animals ever found. They ranged from under an inch to almost three feet in length. Many trilobites had advanced calcite crystal eyes made up of multiple lenses. The last trilobites died out about 250 million years ago at the end of the Permian Period, when volcanic activity killed about 95% of species. After this mass extinction, the dinosaurs appeared.