Legendary sea monster turns out to be even stranger than we thought

Ancient seas held creatures that were built for speed and survival. The animals had strange bodies and sharp instincts.

Armor plates, blade-like teeth, and wide, grasping jaws turned the waters into a world of relentless motion and fierce competition.

Dunkleosteus terrelli ruled that world with a skull full of bone and cartilage. Museum fossils offer only a silhouette of its strength, but the creature’s real biomechanics stayed concealed until researchers finally looked beneath the surface.

Cleveland’s famous sea creature

Dunkleosteus lived roughly 360 million years ago, growing to about 14 feet long and wielding sharpened bone plates in place of teeth. Those bony blades could shear through sizable prey with startling precision and power.

The creature’s skull plates appear in public displays, yet those plates give only part of the picture.

Much of the head relied on cartilage. That material rarely survives, so early work missed key details about the jaws. The result was a familiar creature with an incomplete story.

New details with better fossils

Older studies made brave guesses. “The last major work examining the jaw anatomy of Dunkleosteus in detail was published in 1932, when arthrodire anatomy was still poorly understood,” said Russell Engelman, a graduate student in biology at Case Western Reserve.

Early fossils came flattened, cracked, or missing deep structures. New material from Australia and Morocco changed everything. Better preserved arthrodires showed muscle scars, cartilage traces, and clear joint outlines.

With that knowledge, the team returned to Cleveland specimens and mapped each surface again. Juvenile and adult skulls helped reveal which traits changed with size and which ones held steady through life.

Jaw muscles with force

The new study shows that the sea creature had a head built for force. The palatoquadrate shaped a deep chamber that held strong jaw muscles.

The lower jaw carried a large Meckelian cartilage that filled the space beneath the bone and acted like a hidden brace.

Together, these structures guided the jaw muscles into a straight, powerful pull rather than a sliding motion. A central raphe separated the muscle blocks, keeping them safely away from the jaw joint during wide openings.

Working in concert, these features helped prevent the jaws from locking or jamming under heavy force.

How the muscles worked

The main adductor muscle filled much of the cheek. Longer cheeks meant more room for muscle growth in older animals. That growth mattered because larger individuals chased larger prey.

The adductor fibers angled slightly forward, gaining leverage near the front of the chamber. This arrangement boosted force and kept the bite quick.

Smaller individuals showed a shorter cheek region, but the same basic plan already existed at early stages.

Hidden features of the sea creature

One part stood out immediately. A huge suborbital groove cut across the outer face of the skull.

The groove widened near the back and opened toward the jaw angle. Its size suggested a major soft tissue feature, not a simple fold of skin.

The closest match was a large m. preorbitalis, likely originating on the ethmoid region and extending toward the jaw corner.

The muscle’s path let it deliver strong torque at large gape angles when the main adductor lost mechanical advantage. This pattern appears in several sharks that use similar closing strategies.

A mouth that opened wide

The m. preorbitalis also helped shape the mouth opening. Its anchor on the ethmoid increased surface area for stronger pull.

Larger adults likely relied on this muscle to start closure before the main adductor took control.

This early push prevented the jaws from drifting or misaligning during the first moments of the bite. By the time the mouth narrowed, the adductor could take over and finish with force.

The joint sat far back on the skull. That position increased gape and let the long cutting edges clear each other.

The joint surfaces included separate contact points that guided smooth movement and steadied the lower jaw during impact. With the joint so far back, the mouth opened wider without straining the rest of the skull.

Evolution of Dunkleosteus terrelli

The study rejects the idea of suction feeding. Nothing in the skull supports strong suction pulses. Instead, the animal used a bite driven strategy with wide opening followed by fast closure.

Strong muscles and a broad chamber let it crush or slice large prey. This approach resembles certain sharks with long jaws and strong closing systems. It gives Dunkleosteus a clear role as a top predator.

The anatomy shows a path toward bigger prey and stronger bites. Long cheeks, deep chambers, and extended joints mark a shift within arthrodires.

“These discoveries highlight that arthrodires cannot be thought of as primitive, homogeneous animals, but instead a highly diverse group of fishes that flourished and occupied many different ecological roles during their history,” said Engelman.

The study expands that view and shows how varied these fishes became over time. Fossils from black shale continue to reveal new ideas about this predator. The research reveals a creature built for precision, not just power.

The study is published in the journal The Anatomical Record.

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