A trilobite is an extinct, small arthropod of the subphylum Trilobita that lived during the Paleozoic Era and are extremely common as fossils. Trilobites had a hard outer covering divided into three lengthwise and three widthwise sections (segmented exoskeleton divided into three parts). Their heads had two prominent compound eyes similar in structure to those of modern insects. They had a flattened, oval body varying in length from 1 inch or less to 2 feet. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left behind, with some 17, known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics. Photograph originally captioned: A trilobite, 5 million years old.