Trilobites were some of the most successful creatures to ever live on Earth—and finding one of their fossils is one of the joys of modern fossil collecting. These hard-shelled, segmented animals lived for hundreds of millions of years in the oceans; they are called “three-lobed” because of their head, thorax, and tail. They were one of the first arthropods (animals with jointed legs), comprised over 15,000 different species, and ranged in size from about a half inch (13 millimeters) to 2.3 feet (70 centimeters) long.

Trilobites first appeared in the Early Cambrian (during what is called the “Cambrian Explosion”); they increased in diversity in the Devonian period; and their numbers decreased in the Silurian (probably due to the appearance of sharks and other predators). They became extinct after the Permian mass extinction. Although they did not exist during the time of the dinosaurs, scientists respect this species for its tenacity and abundance.