Pennsylvania: Phacops rana (state fossil)

The state fossil of Pennsylvania is a trilobite common in the state's Devonian Period sediments. These rocks were laid down on the floor of a shallow sea that covered much of North America during the Devonian, some 405 to 365 million years ago. Many of the best specimens of Phacops come from the Mahantango Formation in the central and eastern part of the state.

Trilobites were arthropods -- relatives of modern insects and crustaceans -- and as such, had a segmented body, jointed appendages, and an exoskeleton. Like modern arthropods, they had to periodically molt their shells to grow, so one trilobite could have left a dozen or more shed carapaces to fossilize. They were a very successful group throughout the Paleozoic Era, and went extinct at the close of that time in an event that wiped out over 90% of the species on earth.

Phacops rana can be recognized by its large eyes (which remind some of a frog's -- the specific name 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. This rolling technique may have offered protection for the soft underbelly of the creature.

Because of its abundance and popularity with collectors, Phacops rana was designated the Pennsylvania state fossil by the General Assembly on December 5, 1988.

For further information:

Pennsylvania symbols

The State Fossil