Trilobite

The trilobite (Phacops rana) is Pennsylvania's official state fossil, designated by the General Assembly in 1988. It is an extinct marine invertebrate — a distant relative of modern horseshoe crabs and spiders — that lived in shallow seas covering what is now Pennsylvania during the Devonian period. Pennsylvania's Devonian rock formations are among the richest trilobite-bearing deposits in North America.

Phacops rana is recognizable by its large, distinctive eyes. Each eye is made up of individual calcite lenses, a design found in no living animal today. Enrolled specimens — curled into a tight ball like a pill bug — are one of the most common forms found in Pennsylvania shale.

Phacops rana was a small, armored sea creature about 1 to 3 inches long. Its body was divided into three parts from front to back: a rounded head shield, a segmented middle section, and a short tail plate. Its most striking feature was its pair of schizochroal eyes — large, raised compound eyes with individual calcite lenses spaced apart, giving the animal a wide field of view in the ancient sea.

The trilobite lived in warm, shallow marine environments and likely fed on organic debris and small organisms on the seafloor. When threatened, it could roll into a tight ball, protecting its soft underside with its hard outer shell. Phacops rana went extinct about 359 million years ago at the end of the Devonian period, during one of the largest mass extinction events in Earth's history.

Pennsylvania designated Phacops rana as its state fossil in 1988. The push came largely from schoolchildren across the state, who petitioned the General Assembly to give Pennsylvania an official fossil to match the state's exceptional Devonian rock record. The trilobite was a natural choice: its fossils are found in nearly every county underlain by Devonian shale, and Pennsylvania has been a center of trilobite research since the nineteenth century.

Scientists had been describing Pennsylvania trilobites since the 1830s, when geologists first mapped the state's extensive Hamilton Group formations. By the time of the 1988 designation, Phacops rana was already one of the best-documented fossil species in the state, with specimens in museums and university collections across Pennsylvania.

The Mahantango Formation, a Middle Devonian marine shale that stretches across central Pennsylvania, is one of the most productive trilobite localities in the state. Exposures in Perry, Juniata, and Snyder counties have produced complete enrolled Phacops rana specimens as well as partial exoskeletons (molted shells left behind as the animal grew). These outcrops are the most studied trilobite sites in the state.

Devonian shale also outcrops in western Pennsylvania, in Crawford, Mercer, and Erie counties, where road cuts and stream banks regularly expose trilobite fossils. Collectors have found Phacops rana at dozens of informal sites across these counties for over a century. In eastern Pennsylvania, the Hamilton Group appears in Monroe and Pike counties along the ridge and valley zone.