Trilobites were an important group of marine invertebrates during the Paleozoic era. Because old forms were constantly dying out while new ones evolved, many species of trilobites serve as index fossils defining the time periods in which they lived. As a group, trilobites were part of a mass extinction that occurred at the end of the Paleozoic, during the Permian, 250 to 260 million years ago. That sounds like a long time ago, doesn't it?