Dunkleosteus was a fish in armour. Its head and the front of its body were covered in bony plates, and instead of teeth it had sharp bony blades that sliced against each other like shears. It hunted in the seas of the Late Devonian, more than 360 million years ago, and for a long time it was described as a ten-metre monster. Recent studies have cut it down to about 3.4 m for a typical adult, still a formidable predator, but a very different animal from the one in older books.
Armour and blades
Dunkleosteus belonged to the placoderms, a group of armoured fish that flourished in the Devonian. Most of what survives is that armour: the bony head shield and the plates around the front of the body. The plate at the back of the head was 5 to 8 cm thick; the rest of the armour was thinner, about 1 to 2.5 cm. Its surface was smooth and unornamented.
Its jaws worked through a four-part linkage of bones. Models published in 2007 and 2009 found it could snap its mouth open in about 20 milliseconds and estimated bite forces of up to about 7 500 newtons at the blades. A 2025 study found that significantly more of the skull was made of cartilage than had been assumed, and argued that those bite forces, and the idea that it sucked in prey as it opened its mouth, need to be re-examined.
How big, really?
Because only the armoured front of the body fossilises well, its total length has always been an estimate. Older figures of 5 to 10 m often gave no method at all, and the largest estimate with a stated method, from 2017, compared the size of its mouth with the mouths of sharks. In 2023 the palaeontologist Engelman showed that arthrodires, the group of placoderms it belonged to, had proportionally larger mouths than sharks, so such methods overestimated the length of complete arthrodire fossils at least twice. Using a new measure, the distance from the eye to the back of the gill cover across nearly a thousand species of fish, he estimated about 3.4 m for a typical adult and about 4.1 m for the largest known.
What it lost in length it gained in bulk. Engelman’s 2024 reconstruction gives it a short, very deep, almost cylindrical body, a head making up about 18 per cent of its length, and a crescent-shaped tail like a tuna’s, built for fast swimming. Its eyes, often drawn small, were in fact fairly large, about 8 cm across in one specimen. A typical adult would have weighed about a tonne.
Life in a Devonian sea
Most fossils of Dunkleosteus terrelli come from the Cleveland Shale of Ohio, rock laid down in an offshore sea whose surface waters held oxygen but whose bottom had none, and no animals living on it. Dunkleosteus was a hunter of the open water. Teeth thought to belong to Orodus, a fast-swimming relative of sharks, have been found with its remains, and masses of fish bone suggest it coughed up the hard parts of its meals.
The end of the armoured fish
Dunkleosteus, and every other placoderm, disappeared during the crises at the end of the Devonian, ending with the Hangenberg event about 359 million years ago. No placoderm has been confirmed from the period that followed. Why the crisis happened is still debated: candidates include a lack of oxygen in the oceans, glaciation and falling sea levels, volcanic eruptions and damage to the ozone layer.
Placoderms as a group are gone, but they were probably not a dead end. They are thought to sit at the base of the family tree of all jawed animals, with sharks, bony fish and the four-limbed animals, including humans, arising from among them, although one 2016 analysis kept them as a separate branch.
