Beige-colored trilobite fossil on a black background.
Calymene celebra trilobite in the UW Geology Museum, Madison, WI.

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.

Two grayscale diagrams showing trilobite anatomy.
Trilobite anatomy. A: Bilobate appendages of a living arthropod, similar to that of trilobites. B: Exoskeleton of trilobite.

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 page.)

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.

Trilobite fossil. The trilobite protrudes from a chunk of medium-tan rock that sits on a textured black background.
A flattish piece of light-tan rock with a trilobite on the right side. The trilobite has been circled in black marker.
A sharp-cornered piece of gray rock with a small trilobite fossil visible in the lower left. Photo includes a metric ruler at the bottom for scale.
Several light brown trilobite fossils in a single chunk of whitish-gray rock.
A small piece of grayish-white stone with a cream-colored trilobite fossil partially sticking out. Metric ruler at the bottom for scale; the trilobite is about 5-6mm wide.

(Photos courtesy the Milwaukee Public Museum.)

Grayscale scientific illustrations of trilobite fossils.
Trilobite fossils. A: Cephalon of Illaenus, a common trilobite in Ordovician rock. The cephalon is similar in shape to a strophomenid brachiopod [2.5 cm]. B: Complete specimen of the Ordovician trilobite Isotelus. It has a more triangular cephalon and pygidium than Bumastus, and a more pronounced axial lobe [5 cm]. C: Bumastus, an Ordovician mud-burrowing trilobite with a broad, rounded cephalon and pygidium [6.5 cm]. D: Large cephalon of Bumastus [4 cm]. E and F: Stubby, rounded pygidium and disarticulated cephalon of the Cambrian trilobite Conaspis. The glabella is more rounded than that of Crepicephalus (shown on this plate as H) [3 cm]. G: Large, egg- shaped pygidium of the upper Cambrian trilobite Dikelocephalus [6 cm].The pygidium looks similar to a fish tail. H: Molted parts of the Cambrian trilobite Crepicephalus. The glabella is more squared than that of Conaspis. Extensions of the pygidium make it look similar to a swallowtail butterfly wing [3 cm]. I: The Ordovician trilobite Ceraurus, showing the characteristic tail spines and pronounced “ribs” of the pleural lobes [4 cm].
Grayscale scientific illustrations of trilobite fossils.
Additional trilobite fossils. A: Cephalon and pygidium of the Cambrian trilobite Wilburnia. The glabella is similar to that of Conaspis (F in the previous set of images), but with a pronounced flared lip along the outer margin [1.5 cm]. B: Articulated specimen of Cedaria, found in Cambrian rock. It is similar to Wilburnia, but smaller and with a narrower pygidium and lip [3 cm]. C: Cephalon of Ptychaspis, a Cambrian trilobite. It is similar to Cedaria, but much larger and with more widely spaced eyes [3 cm]. D: Dalmanites, a Silurian trilobite possessing bumps on the cephalon [4.5 cm].