Trilobites are an extinct group of marine arthropods which existed during the Paleozoic era, from the Cambrian period to the Permian period - about 540-245 million years ago. For more information about geologic time periods, check out our Geologic Time Chart.
Complete trilobite fossils first appear in Early Cambrian period, these rocks formed about 540 million years ago. By the end of the Cambrian period (about 500 million years ago), trilobites appear to have been very common and had probably reached their peak in terms of diversity. More than 10,000 different species developed over the 300 million years they roamed the ocean floors.
During the Ordovician and Silurian periods, the diversity of trilobites began to diminish. During the Carboniferous period, they were quickly (at least in terms of geologic time) becoming an endangered species. Extinction of the trilobites (all 10,000 species) was complete by the end of the Permian period - about 245 million years ago.
Trilobites are so named (tri-lob-ite) because their bodies are composed of three distinct lobes; a central (axial) lobe and two side (pleural) lobes. Most lived in fairly shallow water and were benthic which means that they walked on the bottom.
The head (cephalon) is typically crescent-shaped and often bears two compond eyes (although some trilobites were blind). The swelling in the middle of the cephalon - between the eyes - is the glabella. The free cheeks, or librigenae (also sometimes referred to as the "head shield"), are located to the sides of the eyes and are separated from the rest of the cephalon by facial sutures. The pleural lobes are made up of overlapping pleurae, which covered the delicate legs.