The oceans of the Cretaceous of North America teemed with life. Gigantic fish and enormous marine reptiles hunted the Western Interior Sea. A unique new fossil reported today demonstrates rare evidence of direct conflict between these apex predators.
Scientists discovered evidence for this clash of Cretaceous titans tucked away in a specimen drawer in the collections of Chicago’s Field Museum of Natural History. The fossil, a four-meter-long Polycotylus from the Mooreville Chalk of Alabama, held a hidden surprise in one of the bones: a huge tooth embedded in one of the animal’s neck vertebrae.
The specimen was noticed by Professor Christopher Brochu, of the University of Iowa Department of Earth and Environmental Sciences, while taking a break from looking at fossil crocodiles.
“I sometimes look at other material to see if there’s anything I can show in my classes, and that’s when I saw the bitten vertebra,” said Brochu.
The violence of the bite, partnered with millions of years of burial, fossilization, and eventual excavation, had left the tooth crushed, broken at base and tip, and embedded inside the bone. The research team turned to technology to help identify the mystery attacker.
The fossil was scanned using computed tomography (CT), a visualization technique that allowed the research team to study the inside of the specimen without damaging it. The fossil was virtually dissected by two University of Tennessee, Knoxville undergraduates, Miles Mayhall and Emma Stalker, who built a three-dimensional model of a tooth from an unexpected culprit: an enormous bony predatory fish called Xiphactinus .
“We sometimes get these fixed ideas in our heads about who the top predator in any given environment is and who might rest a rung or two down on the food chain,” said lead author and paleontologist Stephanie Drumheller, a teaching associate professor from the Department of Earth, Environmental, and Planetary Sciences at UT. “This fossil is a good reminder that nature is rarely that cut and dry.”
It was unlikely that the Xiphactinus , as big as it was, was actually trying to eat the Polycotylus . Several famous “fish-within-a-fish” fossils seem to indicate that the predator likes to gulp down smaller fish whole. The embedded tooth could have been the result of the fight instead of a hunt. No matter the original motivation for the bite, its depth and location would have certainly proved fatal.
“Plesiosaurs are famous for their long necks, but those necks come at a price,” said coauthor Professor Robin O’Keefe, of the Department of Biological Sciences at Marshall University. “The trachea, esophagus, major arteries and veins, important nerves; all these organs lie vulnerable to attack. A bite to the neck by Xiphactinus would have certainly proved fatal to this animal, if the Polycotylus was not already dead.”
The dramatic evidence of this attack joins a whole host of other evidence known from Mooreville Chalk. Bite marks attributable from other bony fish, sharks, and marine reptiles are known from these rocks, painting a picture of a dynamic ecosystem with diverse predators targeting all sorts of prey, from other marine animals, to the occasional hapless dinosaur who was washed out to sea, to one another. Taken all together, these fossils suggest that swimming in these Cretaceous seas would have been a risky proposition for even the largest of these ancient marine predators.
Journal of Vertebrate Paleontology
A bite to the throat: A probable Xiphactinus attack on a Polycotylus from the Cretaceous Mooreville Chalk of Alabama, U.S.A.
12-Mar-2026