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.