(WHTM) — October 11 is National Fossil Day when paleontologists, students, and educators hold fossil-related events across the county celebrating and teaching the importance of paleontology.

This is the 14th anniversary of National Fossil Day, according the National Park Service.

Thousands of plant and animal fossils have been found in Pennsylvania, as well as more than 275 minerals according to the state’s Department of Conservation and Natural Resources. But there is only one fossil that has claimed the status of the state’s official fossil.

Phacops rana, Pennsylvania’s state fossil, is a type of trilobite. According to the department, trilobites are an extinct category of joint-legged animals or arthropods related to crabs, lobsters, shrimps, spiders, and insects.

The department adds that trilobites were among the most complex organisms without backbones to ever have existed. They had well-developed nervous systems, large antennae, and many appendages for swimming, walking, or feeding.

More, trilobites were also characterized by their hard outer skeleton and large eyes, having the most ancient vision system known to scientists, according to the department.

Phacops rana. Photo Courtesy Pennsylvania Department of Conservation and Natural Resources

These fossils are common in many of the early to middle Paleozoic rocks of central Pennsylvania, according to the department.

Complete trilobite fossils, though, are rare to find today because their rigid outer skeletal segments were held together by flexible connections that decayed after death. Currents and scavengers often then separated the skeletal parts.

Scientists say that these fossils parts are still common because the animals would molt and lose their outer skeletons when they grew. According to the department, a single trilobite probably produced 10 to 12 potentially preservable skeletons in its lifetime.

Phacops rana are found in Pennsylvania’s Devonian-age rocks, which range from 541 to 359 million years old.

Act of Dec. 5, 1988, named the Phacops rana as Pennsylvania’s official fossil.

The Act reads,

“Phacops rana is a specific type of trilobite, a small sea creature. Trilobites were rulers of the sea during the Cambrian Period, 515 to 600 million years ago. Trilobites are so named because their bodies are divided lengthwise into three parts or “lobes.” Phacops rana means “frog eyes” because of the large holes for eyes on the fossil. Fossils of Phacops rana are found in many parts of Pennsylvania, and, therefore, the Phacops rana is selected, designated and adopted as the official State fossil of the Commonwealth of Pennsylvania.”

Act of Dec. 5, 1988, P.L. 1113, No. 138