What Was Dunkleosteus? The Armoured Fish With Jaws Like Shears

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.

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

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 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.

For most of the twentieth century placoderms were treated as a tidy group, a side branch of early jawed fish that left no descendants. Discoveries over the past two decades have made that picture less certain. In 2013 researchers described Entelognathus, a fish from the Silurian of China around 420 million years old, which had a placoderm-style armoured body but also the outer jaw bones found in bony fish, the same bones that became part of your own upper and lower jaw. It looked like a link between the armoured fish and the lineage that led to us.

Many palaeontologists now think placoderms were not one natural family but a series of early branches, with sharks and bony fish arising from within them. On that view the statement that placoderms had no descendants depends on where you draw the group's boundary: Dunkleosteus's own branch died out, but the wider placoderm grade may include your ancestors. The details are still argued, because the evolutionary trees change as new fossils are found and as scientists weigh different features. What is agreed is that the armoured fish of the Silurian and Devonian sit very close to the origin of all jawed vertebrates alive today.

The species name terrelli honours Jay Terrell, an amateur collector who found armour plates in the shale of northern Ohio in the 1860s. They were described in 1873 by the geologist John Strong Newberry, who placed the fish in a genus called Dinichthys, meaning terrible fish. As better specimens accumulated it became clear that this fish was different enough to need a genus of its own, and in 1956 it was renamed Dunkleosteus, Dunkle's bone, after David Dunkle, a palaeontologist at the Cleveland Museum of Natural History, which still holds one of the largest collections of the animal anywhere.

The Cleveland Shale that yields these fossils is a black, finely layered rock made from mud that settled on a sea floor starved of oxygen. Scavengers and the microbes of decay struggled there, so carcasses that sank into it were often left undisturbed, and many plates are found inside hard rounded lumps called concretions that formed around them as the mud turned to stone. The same conditions occasionally preserved far more than bone: early sharks such as Cladoselache from these rocks show traces of muscle and even the outlines of internal organs, which makes this stretch of Ohio one of the richest windows on Devonian sea life.

Dunkleosteus wore plates of a tissue called dermal bone, bone that forms directly in the skin rather than from cartilage. Placoderms and other early armoured fish were covered in it, and although later fish and land animals lost most of the armour, the tissue did not disappear. It was put to new uses.

Several bones in your own body are dermal bones inherited from that armoured past, including the flat bones of the roof of the skull and parts of the jaw, and the collarbone. They still form in the distinctive way the old armour plates did, directly from connective tissue. In that sense, the protective shell of fish like Dunkleosteus survives in a reduced form inside every person's head.

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.