Extinct marine arthropods. Name is in reference to the three longitudinal lobes (right pleural lobe, axial lobe, left pleural lobe). One of the earliest-known arthropod groups. High diversity. Most were benthic as predators, scavengers or filter feeders, although some swam and fed off of plankton. Three major sections of the body include the cephalon (head), thorax and pygidiam (tail end). Typically had complex, compound eyes. Generally, trilobite fossils are of the hard, exoskeletal pieces. Fossils of soft parts are rare. Trilobite tracks (trace fossils) are sometimes found. Early Cambrian to Devonian except for Proetida which went extinct at the end of the Permian (mass extinction).