What Was Dunkleosteus? The Armoured Fish With Jaws Like Shears
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Three hundred and sixty million years ago, before there were trees of any size on land and long before the dinosaurs, the seas that covered what is now Ohio held a fish whose head and shoulders were encased in plates of bone and whose mouth held not teeth but two pairs of self-sharpening bony blades that closed with the force of a crocodile's bite. Dunkleosteus was the largest of the placoderms, the first vertebrates with jaws, and for a few million years it was the largest predator that had ever lived; its whole family then vanished in one of the great extinctions, leaving nothing behind but the jaw itself, which every fish, and every reader, has inherited.
The armoured fish
The placoderms, plated skins, were the dominant fish of the Devonian, the period from 419 to 359 million years ago sometimes called the age of fishes, and their distinguishing feature was a head and trunk shield of bony plates, hinged at the neck so that the head could tilt up while the lower jaw dropped, giving a wide gape fast. They were among the first animals with jaws, which evolved from the front gill arches of jawless fish about 430 million years ago, and the jaw is the reason fish, and later everything with a backbone on land, could grasp, bite and eat things larger than could be sucked in. Placoderms ranged from a few centimetres to the size of Dunkleosteus, and one of them, Materpiscis, is the oldest animal known to have given birth to live young, found with an embryo and an umbilical cord.
How big and how it bit
Dunkleosteus terrelli is known almost entirely from its armour, since the rest of its skeleton was cartilage that did not fossilise, and estimating its length from the head has produced figures from six to ten metres; a 2023 analysis based on the proportions of living fish with similar heads argued for about four metres and a much stockier body, a tuna's shape rather than a shark's, which is still the largest animal of its time. The bite is better known:
- •No teeth: the jaws carried two pairs of bony plates whose edges ground against each other as the mouth closed and stayed sharp by wearing, like a rodent's incisors
- •A bite force of about 5,000 newtons at the front and over 7,000 at the back, calculated in 2006 from the skull's mechanics, the strongest of any fish then known and enough to cut through the armour of other placoderms, which is what the bite marks on their plates show it did
- •A gape that opened in about a fiftieth of a second, sucking in prey as it opened and shearing it as it closed
- •Regurgitated pellets of fish bones and armour, found beside the fossils, showing it swallowed prey whole and spat out what it could not digest
Its world
The Late Devonian seas were warm and shallow over much of the continents, full of reefs built by sponges and corals, and the fish in them were placoderms, early sharks, the lobe-finned fish whose descendants were about to walk onto land, and the ray-finned fish that would inherit the water. Dunkleosteus was the apex predator of the American Midwest sea, found in the Cleveland Shale along with sharks it appears to have eaten, and the marks of its jaws on the plates of other Dunkleosteus suggest it ate its own kind. It lived at the top of a food web that was, for the first time in Earth's history, arranged around large animals with jaws hunting other large animals with jaws.
The end of the placoderms
The Late Devonian extinction, spread over several pulses between about 372 and 359 million years ago, killed off the reefs, most of the marine animals of the shallow seas and every placoderm, in a crisis whose cause is argued between low oxygen in the oceans, the spread of the first forests on land drawing down carbon dioxide and cooling the planet, volcanism and, in the most recent proposals, ultraviolet radiation from a depleted ozone layer. The placoderms had no descendants; the sharks and the ray-finned fish survived and took the seas, and the lobe-finned lineage that had already produced the first four-legged animals took the land. The armour that had made Dunkleosteus unbreakable was not the reason it lasted or the reason it died, and its family's total disappearance is one of the reminders that success in one age is no protection in the next.
The takeaway
Dunkleosteus was the largest of the placoderms, the armoured fish that were the first vertebrates with jaws and the rulers of the Devonian seas, a predator of about four metres or more with a bony head shield and self-sharpening bone blades that closed with a force of several thousand newtons and cut through the armour of its prey. It hunted sharks and its own kind in the shallow seas over Ohio 360 million years ago, and its entire family vanished in the Late Devonian extinction, leaving the jaw as its legacy.