Dunkleosteus: The Armoured Giant Fish of the Devonian Seas

Dunkleosteus was a large armoured fish that lived in the seas of the Late Devonian, roughly 382 to 358 million years ago. A member of the extinct group known as placoderms, it had a heavily plated head and jaws lined with self-sharpening bony blades, making it one of the top predators of its time. Its best-known fossils, from the Cleveland Shale of Ohio, have made it a symbol of the “Age of Fishes”.

Background: the Age of Fishes

The Devonian Period, about 419 to 359 million years ago, is often called the Age of Fishes because fish diversified so dramatically. Jawless fish, early sharks, spiny acanthodians, ray-finned fish and lobe-finned fish all shared the seas. Lobe-finned fish of this time included the ancestors of land vertebrates, such as Tiktaalik.

Placoderms, meaning “plated skins”, were among the most successful Devonian fish. Their heads and the front of their bodies were covered by bony plates joined at a hinge. They are important to scientists because they include some of the earliest vertebrates with jaws.

Dunkleosteus belonged to the arthrodires, the largest group of placoderms. The name means “jointed neck”, referring to the joint between the head shield and the trunk shield that allowed the head to tilt upward as the mouth opened.

Discovery and naming

Fossils of the animal were collected from the Cleveland Shale in northern Ohio in the 19th century. One important collector was Jay Terrell, a local amateur, and in 1873 the geologist John Strong Newberry named the species Dinichthys terrelli, meaning “Terrell’s terrible fish”.

In 1956 the French palaeontologist Jean-Pierre Lehman moved the species into a new genus, Dunkleosteus, honouring David Dunkle of the Cleveland Museum of Natural History. The name combines Dunkle’s surname with the Greek word osteon, meaning bone.

Excavations during road and construction projects in the Cleveland area, especially in the 20th century, yielded many more specimens. The Cleveland Museum of Natural History now holds one of the world’s largest collections of the fish.

Anatomy and feeding

Only the armoured front part of Dunkleosteus was preserved as bone. The rest of the skeleton was made of cartilage, which rarely fossilised, so the shape of its body and tail is reconstructed from better-preserved relatives.

Its jaws carried no true teeth. Instead the bony plates of the upper and lower jaws formed sharp cutting edges and pointed fangs at the front. As the jaws closed, the edges sheared past each other and wore themselves sharp.

In 2007 Philip Anderson and Mark Westneat produced a biomechanical model of the skull. They estimated a bite force of roughly 4,400 newtons at the front fangs and about 5,300 newtons at the rear blades, among the strongest of any fish. Their model suggested the jaws could open in around 20 milliseconds, fast enough to create suction that would pull prey into the mouth.

Fossil evidence suggests Dunkleosteus fed on other fish, including other placoderms and early sharks. Masses of partly digested fish bones found in the Cleveland Shale are thought by some researchers to be material regurgitated by large arthrodires.

How big was it?

The size of Dunkleosteus has been revised several times. Older popular accounts often described lengths of 6 to 10 metres, with weights of several tonnes. These estimates came largely from comparisons with modern sharks, whose body proportions differ from those of arthrodires.

In 2023 Russell Engelman proposed a new method based on the size of the orbit and head relative to the body in better-preserved arthrodires. He estimated that typical adult Dunkleosteus terrelli were about 3.4 metres long, with the largest perhaps reaching about 4.1 metres, and weighed roughly a tonne. Other researchers continue to discuss the figures, but most now favour lengths well below the older extremes.

Species and distribution

The best-known species is Dunkleosteus terrelli from the Late Famennian Cleveland Shale, about 360 million years old. Other species have been assigned to the genus, including Dunkleosteus marsaisi from Morocco and further species from North America and Europe, though the classification of several of them is debated.

During the Late Devonian, the region that is now Ohio lay near the equator at the edge of the continent Laurussia, covered by a shallow, often oxygen-poor sea. The muddy bottom waters helped preserve the bones of fish that sank to the sea floor.

Extinction

Placoderms dominated many Devonian ecosystems but did not survive into the Carboniferous. The Late Devonian extinction, especially its final Hangenberg event about 359 million years ago, wiped out the remaining placoderms, including Dunkleosteus. Sharks, ray-finned fish and lobe-finned fish took over the ecological roles they left behind.

Placoderms and the origin of jaws

Jaws were one of the most important innovations in vertebrate history. Before they evolved, fish were jawless filter feeders or grazers. Jaws allowed vertebrates to grasp, bite and process food, and opened up new predatory lifestyles.

Placoderms are central to understanding how jaws evolved. For a long time they were treated as a single side branch, but discoveries in China changed that view. Entelognathus primordialis, described in 2013 from late Silurian rocks about 419 million years old, was a placoderm with jaw bones resembling those of bony fish. It suggested that placoderms are not a single natural group but a series of early jawed lineages, some closer to the ancestors of modern jawed vertebrates than others.

Placoderms also provide early evidence of internal fertilisation. A fossil of the placoderm Materpiscis from the Gogo Formation of Western Australia, described in 2008, contained an embryo, showing that some placoderms gave birth to live young.

In museums and popular culture

Dunkleosteus is a fixture of natural history museums. Casts of its head armour are displayed in many institutions, including the Cleveland Museum of Natural History and the Field Museum in Chicago. Full-body models, often based on older shark-like reconstructions, also appear in exhibitions.

The fish has featured in television documentaries and many books on prehistoric life. As size estimates have changed, newer reconstructions have given it a shorter, deeper body with a larger head relative to its length, closer to the proportions of its better-known relatives. The debate over its size has itself attracted wide public interest, showing how much attention this ancient fish still receives.

Timeline

• c. 419 million years ago: The Devonian Period begins.
• c. 382 million years ago: The earliest fossils assigned to Dunkleosteus appear.
• c. 372 million years ago: The Kellwasser event strikes marine life.
• c. 360 million years ago: Dunkleosteus terrelli lives in the sea that deposited the Cleveland Shale.
• c. 359 million years ago: The Hangenberg event; placoderms die out.
• 1873: John Strong Newberry names Dinichthys terrelli.
• 1956: Jean-Pierre Lehman creates the genus Dunkleosteus.
• 2007: A biomechanical study estimates its bite force.
• 2023: A new study substantially reduces its estimated length.

Why it matters

Dunkleosteus is one of the best-studied early jawed vertebrates and a key example of how jaws transformed vertebrates into powerful predators. Its changing size estimates also show how palaeontologists refine reconstructions as methods improve, separating popular legend from what the fossils can support.