Pennsylvania Fossil

Trilobite

(Phacops rana)

Adopted on December 5, 1988.

The Trilobite, Phacops rana, was adopted on December 5, 1988.  A water animal measuring just one to four inches, the Phacops Rana lived and left its mark Pennsylvania more than 250 million years ago. A science class of elementary school students brought this tiny invertebrate to the attention of the House of Representatives.

Phacops rana is a fossil organism known as a trilobite (pronounced "tri-lobe-ite"). Trilobites are an extinct category of jointed-legged animals related to crabs, lobsters, shrimp, spiders, insects, and so on. This group of creatures, called arthropods, are among the most complex of all the animals without backbones and trilobites are no exception. They had well-developed nervous systems and large antennae. Trilobites had many appendages for swimming, walking, or feeding. Although these appendages are relatively rare in most groups of trilobite fossils. Phacops is one of four genera for which they are fairly well known and studied. Trilobites also had a hard outer skeleton composed of chitin, a complex organic protein, and the mineral apatite (calcium phosphate).

Many trilobites had large eyes. They are, in fact, the first organisms on earth known to have eyes. The trilobites had compound eyes, composed of many individual lenses, like those of insects. Trilobites possess the most ancient visual system known to scientists and thus they provided some of the best direct evidence of eye evolution.

Trilobites are a common fossil in many of the early to middle Paleozoic rocks of central Pennsylvania, i.e., rocks that are between 570 and 365 million years old. Complete fossil specimens are rare because the animals were composed of rigid outer skeletal segments joined by flexible organic connections that decayed on the death of the animal. Currents, scavengers, and molting all served to separate skeletal parts, which comprise the most common trilobite fossils in Pennsylvania. This common abundance of trilobite parts in the fossil record, in fact, was enhanced by the fact that the animals grew by casting off their outer skeleton in a series of molt stages. One animal probably produced ten to twelve potentially preservable skeletons in its lifetime.

An interest in trilobites is not restricted to scientists and geological dilettantes. They are prized by jewelry and curio collectors. This interest is a long standing one. Trilobites were found on necklaces belonging to the prehistoric inhabitants of 15,000 year old rock shelters of Europe. The Ute Indians of the western United States fashioned trilobites into amulets. The Ute name for these fossils was, "timpe khanitza pachavee" which means "little water but like stone house in."

Phacops rana is found in Pennsylvania's Devonian-age rocks (rocks between 405 and 365 million years old). A publically available location for finding fossil specimens of this fascinating creature are described in Pennsylvania Trail of Geology, Park Guide #16.