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Devonian Shales

Devonian Shales Fossils:

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Location: Lucas County, Ohio

Time: 417 to 354 million years ago, during the Devonian period

About this Famous Find: Throughout the Devonian, shallow seas moved in and out over much of North America, including Ohio. Life flourished in these warm, tropical seas, and marine animals (specifically fish) diversified to the degree that the Devonian has been referred to as “The Age of Fishes.” Plants and tetrapods began to thrive on land. Much of our knowledge about this interesting time in geologic history comes from the abundant and spectacular fossils collected from Devonian rocks in Ohio.

The oldest of the Devonian rocks in Ohio, the Holland Quarry Shale near Toledo, formed in the brackish waters of a shallow bay. A wide variety of marine animals and plants are preserved there, including many different kinds of fish such as agnathans, heterostracans, and arthrodires. Eurypterids (“sea scorpions”), which could grow to nearly three meters long, preyed on many of these animals.

Middle Devonian rocks of the region reveal the spread of a shallow sea. They are noted for their abundant and well-preserved invertebrate fossils. One deposit, known as the Silica Shale, is known worldwide for producing beautifully preserved invertebrate fossils, including brachiopods, corals, echinoderms, and trilobites. Other deposits of this age have yielded corals (some more than a meter in diameter) and well-preserved fish fossils, including skulls of an arthrodire, Macropetalichthys, and the sharp-toothed jaws of a ray-finned fish, Onychodus sigmoides. Small teeth and scales record a great diversity of fish.

Later Devonian deposits of the Ohio Shale hint at environmental changes in the shallow sea. By this time, there was a stagnant, low-oxygen environment in the deepest waters. Few organisms could survive there, not even the bacteria that help to decompose animal and plant remains. As animals died their remains were left intact and buried in mud on the sea floor. Over time, they became exquisitely preserved fossils. Some of the most remarkable are fish fossils: armored arthrodires, such as Dunkleosteus, and early sharks, like Cladoselache. Plant fossils are also abundant, including the remains of Protosalvinia, an alga that floated in the sea, and parts of terrestrial plants like Archaeopteris that were washed out to sea and buried in the mud.