About 70 million years ago in prehistoric South Dakota, a Hesperornis, a 3-foot-tall, flightless diving bird stood on the edge of an ancient sea. All of the sudden, something lunged out of the water and grabbed the bird by the leg. Somehow, it escaped. But the signs of the attack stayed with it for the rest of its life, and were fossilized when the creature passed away.

When researchers came across the fossil, housed in the Princeton University Collection at Yale’s Peabody Museum, they were intrigued by the marks on the tibiotarsus, and decided there were enough teeth indentations to figure out which creature attacked it. The results of their sleuthing were recently published in the journal Cretaceous Research.

Researchers David Burnham of the University of Kansas and Bruce Rothschild from Northeast Ohio Medical University began their search by looking closely at the three indentations on the bone. A rough patch on the condyle, where the foot attaches, told them the bird survived the attack and suffered from an infection from the wound afterwards. The three teeth marks themselves right away ruled out shark attack, because the indentations were rounded, not flattened like a shark’s bite.

So the team began the process of trying to match up the marks with some of many marine predators lurking in the central U.S. during the Cretaceous. “We basically did the Cinderella routine, to see whose teeth fit the ‘slipper,’” Rothschild tells Laura Geggel at Live Science.

The researchers began examining the fossil collection at the University of Kansas, focusing on three main suspects: mosasaurs (a group of giant swimming reptile), Xiphactinus (a genus of 20-foot-long carnivorous fish), and plesiosaurs (a genus of long-necked predator that moved through the water using flippers).

None of the fossils seemed to fit the bite marks until they tried the skull of a juvenile plesiosaur. “The teeth lined up with the indentations in the fossil to within less than a millimeter,” Burnham tells Robin Wylie for the BBC.

The orientation of the bite marks also revealed some of the details of the attack. “Basically, the plesiosaur came in from the side,” Rothschild tells Geggel. “That probably was what allowed the bird to escape, because when [the plesiosaur] got the initial grip, and released to get a better grip, the bird got away.”

It’s an interesting case, but it also adds to our knowledge of the plesiosaur more generally. Tom Stubbs from the University of Bristol tells the BBC that this study shows plesiosaurs may have had a more diverse diet than previously thought. Researchers assumed the animals used their long necks and slender snouts for grabbing fast-moving fish.

“Marine birds were not widely considered as a potential food source,” says Stubbs. “The study presents compelling evidence that plesiosaurs may have been opportunistic predators.”