Trilobite (Phacops rana) was designated the state fossil of Pennsylvania in 1988. An elementary school science class campaigned for this invertebrate that lived in Pennsylvania more than 250 million years ago .

Related to crabs, lobsters, shrimp, spiders, and insects, trilobite is a common fossil found in many of the early to middle Paleozoic rocks of central Pennsylvania. Like modern arthropods, trilobites had to periodically molt their shells to grow, so one trilobite could have left a dozen or more shed carapaces to fossilize.

Pennsylvania's state fossil can be recognized by its large, frog-like eyes ("rana" is a reference to a common frog), its fairly large size (up to 6 inches long), and its habit of rolling up into a ball like a pill bug.

Source:

Pennsylvania: Phacops rana: State Fossils.com

Links:

Prehistoric Pennsylvania: The State Museum
Pennsylvania - Symbols: PA Historical & Museum Comm.
State Symbols: Hello Pennsylvania
State Fossils & Dinosaurs - 50 States