My, What a Big Mouth You Have

January 6, 2017
Blog by John Mangels
Science Communications Officer

An apex predator, as the term implies, is the king of the hill — the most capable killer in its particular environmental niche, presiding at the top of the food chain.

You can probably name some of the apex predators in today’s world: Lions. Crocodiles. Eagles. Great white sharks. Polar bears. Us.

Three hundred sixty million years ago, in the shallow subtropical sea that covered Ohio and much of Appalachia, one of the prehistoric world’s first known apex predators was a pickup truck-sized, armor-plated, iron-jawed fish called Dunkleosteus terrelli.

Two recent research projects by Cleveland Museum of Natural History scientists reveal some fascinating things about how Dunkleosteus (pronounced dun-kul-OS-tee-us) lived and grew, and possibly how it died. (Hint: The only thing apex predators have to fear are each other.) Both studies focused on the creature’s fearsome jaws, which were a mashup of scythe blades and railroad spikes and generated as much biting force as the biggest alligators.

“Dunkleosteus is going to take on anything that fits in its mouth,” says Curator of Vertebrate Paleontology Michael Ryan, PhD, who directed both projects.

“It could bite through you the way you’d bite through a marshmallow. Bam! That’s it,” adds Vertebrate Paleontology Preparator and Lab Manager Lee Hall, who examined evidence of the damage those bites could cause.

A Treasure Trove of Fossils

What researchers know about Dunkleosteus comes from its massive skull plates and jaw bones, which were entombed in the muddy ocean bottom when the creatures died and sank. Unfortunately, their backbones and soft-tissue body parts weren’t similarly preserved, so there are only educated guesses about what the lower two-thirds of Dunkleosteus looked like. Some scientists think it swam with a wavy sideways motion, like an eel.

The Museum has dozens of the world’s best-preserved Dunkleosteus specimens in its collection; in fact, the fish’s genus is named for former Vertebrate Paleontology Curator David Dunkle, who during the mid-1900s published numerous studies of it and other Devonian-era sea creatures.

Some of the Museum’s Dunkleosteus fossils were recovered from 1965 to 1967 when the Ohio Department of Transportation excavated the Big Creek Valley in southwestern Cleveland to build Interstate 71. The big dig exposed the black shale remnants of the ancient seabed and the fossilized aquatic lifeforms it contained. Other Dunkleosteus fossils are still being unearthed today from the shale outcrops along the Rocky River west of Cleveland, and from other area river and creek basins.

The Mechanics of a Fearsome Bite

The Devonian Period was the Age of Fishes, and from previous fossil studies we know that Dunkleosteus lived in an ocean ecosystem teeming with primitive sharks, bony fish and placoderms (Greek for “plate-skinned”), the broad category of large, heavily head-shielded fish to which Dunkleosteus belonged.

Its jaws made Dunkleosteus the most dangerous dude in the Devonian ocean. Evolution was experimenting with different jaw designs in the Devonian placoderm fishes and Dunkleosteus’ version was highly efficient.

The two lower jawbones, one on each side of the fish’s mouth, were as long as a man’s forearm. They were curved and sharp-edged, like a scimitar. At the end was a protruding, pointed cusp that looked like a big fang and helped puncture and grip prey.

Adult Dunkleosteus didn’t have traditional teeth, like we’re used to seeing in a shark; its mouth sliced quarry into chunks rather than chewing it. The exposed bony edges of the upper and lower jawbones functioned as cutting blades and were reinforced with crystalline dentine to make them extra durable. They worked like scissors, and the edges occluded, meaning that they rubbed against each other as the mouth opened and closed, creating a natural self-sharpening mechanism.

Even after hundreds of millions of years out of action, the jaws — tucked away in padded cabinet drawers in the Museum’s basement — still have a polished black sheen on their edges, a relic of that constant occlusive rubbing while the creatures fed.

The huge bony plates that made up Dunkleosteus’ head and upper body shield are often described and depicted as armor, though they actually were covered by skin. As thick as three inches in spots, the plates did indeed provide protection, but they also served as anchor points for the enormous muscles needed to work Dunk’s jaws.

That jaw action was surprisingly complex but brutally effective. For Dunkleosteus’ mouth to open, muscles contracted to lift its skull upward and back like a truck’s hood, raising the upper jaw along with it. Other muscles swung the hinged lower jaw downward and yanked it backward, increasing the mouth’s gape.

Computer modeling done several years ago by Chicago researchers using the Museum’s fossils showed that adult Dunkleosteus’ mouth could snap open as wide as 45 degrees in as few as 60 milliseconds. That’s so fast it would have created a vacuum strong enough to suck its prey into Dunk’s deadly maw. The bite force when the fish chomped down was only slightly less than that of a great white shark and some dinosaurs, the modeling showed.  

Additional bite analysis done by the Museum’s Dr. Ryan and colleagues in 2010 using juvenile Dunkleosteus jaws showed that the smaller fish, though not as powerful as their elders, could still pack a punch.

Here’s where the new research comes in.

Discovering a Shift in Jaw Functioning

During the past year, Dr. Ryan’s former Case Western Reserve University undergraduate student Jamie Boyle, working with his mentor and other colleagues including Museum trustee William Hlavin (who was part of the I-71 fossil recovery crew in the 1960s), studied 60 Dunkleosteus lower jaws. The specimens reflected all stages of the fish’s life, from a two-inch-long jaw that likely belonged to a baby all the way up to a two-foot whopper from the mouth of the biggest, oldest, fiercest adult.

The research team compared the measurements of seven features in each jaw. They wanted to understand how Dunkleosteus’ mouth changed as the fish matured, and what those changes might indicate about its behavior.

What they discovered is that, early in life, the jaw’s sharp cutting section grew more rapidly than the other jaw components did. Eventually, though, in adolescence the cutting segment’s growth spurt tapered off and the other components, particularly the spiky fang at the end of the jaw, caught up.

That change in growth rate and jaw shape, which occurred when the jaw was about 43 percent of its full, adult size, resulted in what scientists call an ontogenetic niche shift. As Dunkleosteus’ body size grew and its jaw matured, its feeding capabilities changed. When the fish was young and relatively small, its jaws functioned like scissors, allowing it to eat soft-bodied prey, such as bony fishes or small sharks. As the overall jaw lengthened and the fangs neared their adult length, Dunkleosteus’ mouth gained the ability to puncture and crush virtually anything, enabling it to attack other big, armored placoderms. The transformation vaulted Dunk to the top of the food chain, making it the king of the Devonian ocean.

Such niche shifts have been documented in other apex predators, but not in Dunkleosteus.

“We know that most vertebrates, as they grow up, move through various feeding niches,” Dr. Ryan says. “As a baby, you’re not going to feed on the same things you do as a teenager or adult. Your metabolism and dietary needs change, and your physical size changes, and that means you take on different sizes of prey. A good example is Tyrannosaurus rex. The adult has a skull almost as wide as it is deep, to accommodate massive jaw muscles that could have pushed its teeth through a thin steel plate. The teenage skull is thin, narrow and pointed and has relatively weak muscles compared to the adult, so the animal isn’t going to be feeding and hunting the way adult tyrannosaurs do.”

Combat Injuries and Cannibalism?

And what was adult Dunkleosteus hunting? Apparently other Dunkleosteus, judging by the evidence that Dr. Ryan’s former Case Western Reserve University student, Evan Scott, and Museum Preparator Lee Hall have identified. 

Examining three fossil specimens in the Museum’s collection, Scott and Hall documented major damage to the thick bony plates — deep gouges, puncture marks and fractures — that could only have been inflicted by a fish with Dunkleosteus’ massive biting and crushing power.

Some of the wounds show signs of healing, but others were clearly unrepaired and lethal. Consider, for example, specimen CMNH 9882, the bony cap as wide as an airliner seat cushion that sat atop Dunkleosteus’ skull, protecting its brain. The lower right corner of the head shield, which curves down toward the fish’s jawline and forms a triangular point, has been broken and pushed backward beneath the cap by something that left big scrapes in the bone.

“The head shield is the thickest part of Dunkleosteus’ skull and this bite was strong enough to fold it like a taco,” says Hall. “That is a hugely traumatic injury and there’s no getting better from it. The only animal that had the jaw power to do that in that (Devonian) environment was an adult Dunkleosteus.”

Hall saw what seems to be a pattern in the wounds in the three specimens. They’re concentrated in the area immediately behind Dunkleosteus’ jaw hinge, where its gills were. That would be a natural attack point to avoid getting bitten while getting a grip and inflicting damage.

What could explain such violent conflict among members of the same fish family?

So-called intraspecies aggression is well-known in the natural world; it can involve combat over feeding territory or mating rights (the head-butting between male big-horn sheep, for instance), or even courtship behavior between males and females. “Modern female sharks look like they’ve been through a blender with bite marks around their gills,” Hall says.

Cannibalism is another possibility. “The reason we can make that assertion is because we’re seeing what looks like intraspecific aggression scars and bones that have been bitten up and have fragments removed,” Hall says. Other parts of the head shield, include the delicate bones around the eye sockets, remain intact.

“That means there were no environmental forces damaging the bone postmortem,” Hall says. It didn’t get tumbled around and broken by an ocean current. “Clearly, something was biting on this and doing a good job of taking out chunks. When you try to explain what could cause that kind of damage, you put two and two together.

“I wasn’t the first person to notice the damage in these specimens,” Hall says. “My work was putting the three specimens together to tell a story that was a little bit bigger than an individual specimen could tell. We’re really curious to see if we’re onto a trend. Our goal is to expand our search for bite marks into other specimens. I don’t think anyone has ever gone methodically through our collection looking for this information. Some things are invisible until you start looking for them. Suddenly, after you know what to look for, it’s like wow — they’re everywhere.

Science writer John Mangels is The Cleveland Museum of Natural History’s science communications officer. Contact him at jmangels@cmnh.org.

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