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Ohio's State Fossil Fish

Dunkleosteus artist rendering

Illustration by Madison Perry

Dunkleosteus terrelli: Terror of the Devonian Period

Dunkleosteus terrelli (pronounced DUNK-ul-AH-stee-us TARE-rell-eye) was the top predator near the end of the Devonian Period, known as the “Age of Fishes.” Nicknamed the “Dunk,” it belonged to a now-extinct group of fishes called placoderms (PLA-kuh-durms), or “plate skin” fish. Like other placoderms, a portion of the Dunk’s body was covered with bony plates, like a layer of armor. The Dunk cruised the surface of a sea that once covered Ohio. It had a strong, fast bite and might have eaten early sharks, bony fish, smaller placoderms, and ammonoids (animals related to squids that had coiled shells).

Dinichthys jaw bone in Ohio Shale concretionWhen a Dunk died, its body eventually sank to the bottom of the ancient sea, where it was buried in soft, black mud. Over millions of years, these fine, muddy sediments piled up and solidified to form a sedimentary rock called shale. The Dunk’s hard armor was preserved as fossils within the shale.

Dunkleosteus fossils are known from North America (including Ohio), Europe, and northern Africa. Many of the world’s best fossils of Dunkleosteus were collected in northern Ohio from layers of black shale that geologists call the Cleveland Shale. Some of these Dunk fossils are on display in the Cleveland Museum of Natural History. Fossil remains of  Dunkleosteus and other placoderms also occur inside spherical rocks called concretions (see example of Dunk relative Dinichthys herzeri lower jaw bone in image above).

Fun Facts

SCIENTIFIC NAME: Dunkleosteus terrelli
HOW TO SAY IT: DUNK-ul-AH-stee-us TARE-rell-eye
NICKNAME: “Dunk”
AGE: Late Devonian Period
TIME: About 360 million years ago (130 million years before the first dinosaurs)
LENGTH: Up to 8.8 meters or 9.6 yards (29 feet)
WEIGHT: Up to 4 metric tons (4,000 kilograms) or 4.4 U.S. tons (8,800 pounds)
STATUS: Extinct (none are living today)

A Top Predator without Teeth

Dunk vs great white jaw comparisonJaws of the modern great white shark (left) have many rows of teeth that fall out easily. Older, worn teeth are replaced with newer, sharp teeth.

The Dunk’s jaws (right) had no teeth at all! Instead, exposed bone on the upper and lower jaws came together like self-sharpening scissors for shearing through bones and shells of its prey.

How big? How do we know?

Dunk comparison to great white and human diverThe largest Dunks were longer and heavier than any living great white sharks (see image of Dunk, great white shark, and human diver). Because only the armor on the head and thorax of Dunkleosteus was preserved as fossils, we do not know exactly how long or how heavy they were.

However, by assuming that the Dunk had similar body proportions to modern predatory fish, paleontologists can estimate Dunk’s length and weight by comparing the size of the largest skulls and jaws to those of large fish living today. The length and weight given in the Fun Facts above are based in part on a large lower jaw fossil in the collections of the Cleveland Museum of Natural History.

Dunk Fossils

Some placoderms, like the Dunk, had bony armor only on the front part of the body. The Dunk’s armor plates covered its head and part of its thorax. The hard parts of an animal’s body are more likely to survive as fossils. Parts of the skeleton made of cartilage and soft parts, such as muscle, fat, and skin, are more likely to decompose after animals die. Bones contain minerals that make them hard and rigid. These minerals helped to preserve the Dunk’s armor plates as the fossils we see today.

VIDEO


Division of Geological Survey geologist Erika Danielsen explains what led scientists to the fish fossil 360 million years after its existence.

Downloads

Dunkleosteus terrelli poster, 18 x 24 inches (pdf)

GeoFacts 35: Dunkleosteus terrelli—Ohio's State Fossil Fish (pdf)

Ohio Rocks! Activity Book (pdf)