By James John
Jay Terrell ran a hotel in Sheffield Lake, Ohio, and in his spare time he walked the shale cliffs above Lake Erie looking for fossils. Most of what turned up there was unremarkable: trilobites, brachiopods, the odd shell. In 1867, something else came out of the rock, dense bone fragments unlike anything the local collectors had seen. Terrell carried them to John Strong Newberry, Ohio’s state geologist, a man used to sorting through amateur finds that rarely amounted to much. This one did. By 1873, Newberry had described the fossil formally and given it a name, Dinichthys terrelli, in honor of the man who found it.
Terrell never saw the finished monster his fossil belonged to. Neither did Newberry.
Why the Shale Kept Its Secrets So Well
Geology, more than luck, is what handed Terrell his find. Roughly 365 million years ago, a shallow sea covered what’s now northeastern Ohio, and rivers draining the rising land to the east dumped mud into it in heavy pulses. That mud buried dead animals fast, and the resulting muck held so little oxygen that the usual armies of scavengers and decay bacteria couldn’t get to the bodies before they were sealed in. Paleontologist Glenn Storrs of the Cleveland Museum of Natural History has described the result as close to ideal fossil preservation, with bones, body outlines, and even skin texture all locked into black rock instead of rotting away.
That’s the reason the Cleveland Shale kept producing Dunkleosteus specimens long after Terrell’s initial find, one after another, for the century and a half that followed.
A Genus of Its Own
For most of a century, terrelli sat filed under Dinichthys, a genus Newberry had already used for a handful of other large armored fish turning up in the same black shale. Only in 1956 did paleontologist Jean-Pierre Lehman look closely enough at the skull architecture to conclude that terrelli didn’t belong there at all. He split it into its own genus and named that genus after the man who had spent the 1940s hauling Cleveland Shale specimens into museum drawers: David Dunkle, curator of vertebrate paleontology at the Cleveland Museum of Natural History.
Dunkle had a Harvard doctorate, a research focus on ancient fish, and, by most accounts, no idea a fish would eventually be named after him mid-career. He also has a stranger footnote to his résumé. In 1942, on a fossil-collecting trip to Montana, he pulled a small tyrannosaur skull out of the ground that decades later would touch off one of paleontology’s longest-running arguments: whether it belonged to a distinct pygmy tyrannosaur called Nanotyrannus or to a juvenile Tyrannosaurus rex. Dunkleosteus is the tidier legacy. Nobody argues about whether it’s a real genus.
A Legend Sized by Guesswork
What Dunkleosteus terrelli looked like in life took much longer to pin down than what to call it, and the guesses only got bigger with time. Complete skeletons don’t exist, because placoderms like Dunkleosteus armored only their heads and the front third of their bodies in bone. Everything behind that was cartilage, and cartilage almost never survives 360 million years. Paleontologists were left reconstructing an entire animal from a skull, and the standard shortcut was to assume a body plan like a modern shark’s, long and torpedo-shaped, gradually tapering toward the tail. Run that assumption forward from a skull as large as Dunkleosteus’s, and the extrapolations ran generous: length estimates crept as high as 10 meters, on par with an orca, a figure repeated in documentaries and museum placards for years.
In 2023, paleontologist Russell Engelman took a different approach. Rather than assuming Dunkleosteus was built like a shark, long and slender, he measured the ratio between head length and body length in dozens of living fish and in smaller placoderm relatives of Dunkleosteus that happen to survive as complete skeletons. Short-headed fish, it turns out, are reliably short-bodied fish, and Dunkleosteus’s skull runs about 61 centimeters. Run that math and the old 10-meter estimate collapses. Engelman’s revised figure: something closer to 3.3 to 3.7 meters, 11 to 13 feet, roughly a third of the legend.
The correction doesn’t make Dunkleosteus any less formidable. Engelman’s own comparison lands the point: a 3.3-meter Dunkleosteus carried roughly the same body mass as a 4.6-meter great white shark. It was built dense and thick through the trunk, closer in proportions to a tuna than to a long, lean shark, and for an ambush predator patrolling shallow Devonian seas that bulk may have mattered more than raw length ever did.
The Bite the Size Correction Couldn’t Touch
Whatever Dunkleosteus’s true length turned out to be, its jaws needed no revision. Instead of teeth, the animal grew hardened bony plates along the edges of its jaw, shaped into points and blades. Every time the jaw closed, the upper and lower plates sheared against each other and re-sharpened their own cutting edges, a self-maintaining weapon no living animal still carries in quite the same form.
Biomechanics researchers Philip Anderson and Mark Westneat built a model of that jaw’s mechanics and calculated the forces it could generate: roughly 4,400 newtons at the rear cusps, climbing to 5,300 newtons at the cutting tip, translating to pressures around 21,000 pounds per square inch at the blade’s edge. That’s in the range of a modern crocodile’s bite, delivered by an animal whose actual body length had just been cut by two thirds. Shrinking the fish didn’t shrink what it could do to whatever it caught.
The speed mattered as much as the force. Researchers studying the joints between Dunkleosteus’s skull, neck armor, and jaw have estimated the whole mechanism could snap open in as little as 20 milliseconds, fast enough to generate a burst of suction that helped pull prey toward those self-sharpening plates rather than waiting for prey to swim within reach. A three-meter animal with a bite that opens in a fraction of a second and closes with crocodile-grade force doesn’t need to be orca-sized to dominate a Devonian reef.
An Ending Nobody Has Fully Explained
For all the progress made on Dunkleosteus’s size and its bite, one basic question about it remains open: why it disappeared at all. Placoderms as a group were wiped out by extinction pulses near the end of the Devonian, but that broad pattern doesn’t explain why an animal built like an armored battleship, with a bite that could shear through nearly anything else in its ecosystem, lost out over the long run to sleeker, cartilage-skeletoned sharks that share the seas today.
William Hlavin, another paleontologist who has spent decades working the Cleveland Shale collections, has floated one possibility without treating it as settled: that heavily armored fish like Dunkleosteus may simply have been too top-heavy, metabolically and structurally, to keep competing once faster, lighter-bodied predators became common. It’s an informed guess, not a verdict. The rock record that solved the mystery of the animal’s size and the mechanics of its jaw hasn’t yet solved the mystery of its exit.
Ohio Keeps the Title
Dunkleosteus terrelli fossils kept surfacing around Cleveland long after Terrell’s first find, through decades of amateur collecting and, in the 1960s, a wave of specimens uncovered during construction of Interstate 71 through the Rocky River valley. In 2020, the state made the association official: Ohio named Dunkleosteus terrelli its state fossil fish, only the second fossil animal, after the trilobite Isotelus, to receive that kind of formal recognition.
It’s a fitting emblem for an animal whose public image has been rewritten at least twice: once when it got its own genus, and again when its size got taken down a peg by better math. Terrell found bone fragments in a shale cliff in 1867 with no way of knowing what shape the finished animal would eventually take, in the fossil record or in the retelling. A century and a half later, that shape is still being adjusted.
Dunkleosteus terrelli is usually introduced as one entry in a longer roll call, the standout example in a chapter about the origin of jaws in vertebrates generally. Told on its own, its story is less about jaw evolution as an abstract process and more about how much a single fossil skull can keep teaching people, more than 150 years after a hotel owner in Sheffield Lake noticed something strange sticking out of a black cliff face.
