The trilobite Calymene celebra is Wisconsin’s state fossil.
Trilobites were a group of crab-like animals with hard exoskeletons (outer skeletons) similar to those of modern insects. Trilobites have a three-lobed body (B): Two grooves divide the body lengthwise into three sections. The middle section is the axial lobe; the other two sections are the pleural lobes. The body can also be divided into three sections from head to tail: the front section is the head, or cephalon, the middle section is the thorax, and the tail section is known as the pygidium. These marine animals had a series of small, bilobate (two-pronged) legs beneath their exoskeleton (A). One part of the leg, the exopodite, was used for walking; the other, the endopodite, was used for gas exchange or “breathing.”
On the cephalon, a series of lines, or facial sutures, are present. These sutures opened when trilobites molted their skeletons. The trilobite pushed itself out of its old skeleton and grew a new one; most trilobite fossils are instars, or molted skeletons. The region of the cephalon between the eyes and bounded by the sutures is called the glabella.
Trilobites had compound eyes, much like those of insects. Some trilobites were adapted to mud-burrowing, and possessed vestigial, or nonfunctional, eyes. Mud-burrowing trilobites had smooth, streamlined bodies. (To see trilobite burrow fossils, see our trace fossils gallery.)
Trilobites first appeared in the Cambrian, but after a diversification they dwindled and eventually became extinct during the Permian. Trilobite fossils are not common in Wisconsin. Generally, they form a large component of Cambrian fossil assemblages, and Ordovician rock commonly contains trilobite fragments. In Silurian rock, trilobites may be associated with reef-like habitats. Devonian trilobites are not particularly abundant. Fossil tracks of trilobites can be found on rock that was once the Paleozoic seafloor.