Did someone say "jaws"? Forget the great white shark: a 400-million-year-old, multiton fish may have had a bite powerful enough to chop a shark--or just about anything else--clean in two. To determine its strength, researchers reconstructed the ancient creature's jaw muscles from the grooves of a well-preserved fossil.

A well-known denizen of museum displays, Dunkleosteus terrelli could have exerted up to 1,200 pounds of force with its bite, the investigators estimate. When applied along its jagged snapping-turtle-like jaws, such a force would translate to about 8,000 pounds of pressure per square inch, the researchers find. "It was probably the first vertebrate that was able to fragment its prey before swallowing," says zoologist Mark Westneat of the Field Museum in Chicago.

Dunkleosteus grew up to 33 feet long and was the largest of a group of armor-plated predatory fish, the placoderms. The top ocean predator of the time, its prey could have included early sharks, large nautiluslike mollusks, arthropods and other placoderms, Westneat says.

A strong bite would have helped the "Dunk" contend with such armored fare, says paleontologist Gregory Erickson of Florida State University, who was not involved in the research. "You had to crack fairly thick pieces of shell or thick pieces of bony armor," he says. The estimated bite force of Dunkleosteus is comparable to that of a hyena or lion, and is probably stronger than that of a shark, he notes. "It'll go right through bone," he says of such a bite. "It's like a hot knife through butter."

To reconstruct the size of the Dunk's jaw muscles, Westneat and his colleague Philip Anderson, a graduate student at the University of Chicago, made foam rubber casts of the muscle cavities in a Dunkleosteus skull replica. The length and cross section of each muscle allowed them to calculate maximum contraction forces based on the typical strength of a modern vertebrate muscle.

The researchers combined these values with a two-dimensional simulation of the fish's jaws, which fossils indicate could pivot at several points. Muscles on the top rear of the head and under the chin might have pulled its jaws open or held them in place so crisscrossing muscles in the cheeks could slam them shut. Such a multipivot system, in which some hinges stay still and others rotate, is highly efficient at transmitting force, the researchers observe in a paper published online November 28 in Biology Letters.

The muscle placement and pivot points of the jaw suggest Dunkleosteus may have sucked prey into its mouth before biting, like an extremely large-mouthed bass, the researchers note. Suction might have helped it contend with agile prey such as sharks, they observe.

Despite its bone-crushing bite and dominance of the seas, Dunkleosteus died out after 100 million years for unknown reasons. It would not be the last word in chompers, however. Today's large alligators can generate 3,000 pounds of bite force, and the Tyrannosaurus rex probably outdid that, Erickson says.