In the early 1960s, palaeontologists exploring rural South Dakota unearthed the fossilised leg bone of a Hesperornis: a large aquatic bird rather like a penguin, which lived during the dinosaur era.

At first glance the fossil looked unremarkable. It took over half a century for scientists to recognise the bone's true significance.

This Hesperornis had survived a run-in with a plesiosaur: a marine reptile larger than a human being. It is only the second fossil found to date that shows signs of being attacked by a plesiosaur.

The story was uncovered by David Burnham of the University of Kansas and his colleagues. They were packing the leg fossil away for storage when they noticed unusual markings on its surface.

The seas were full of things that might have enjoyed dining on a Hesperornis

The bone's condyle – the rounded joint where it would have attached to the bird's foot – had an uneven, lumpy appearance. This was unusual, because in healthy bones condyles are usually smooth (even after fossilisation). Rough condyles suggest that the bone suffered some kind of trauma.

The misshapen joint prompted the team to examine the fossil more closely. They found three small, evenly-spaced indentations positioned discreetly on the underside of the fossil.

It looked as if the Hesperornis had been bitten by something.

To show that the three indentations were made by a predator's jaw, the team needed to identify a marine predator whose teeth fitted the three marks. There was no shortage of potential culprits.

The Hesperornis lived around 80 million years ago, a time called the late Cretaceous. This was not the best time to be an aquatic bird: the seas were full of things that might have enjoyed dining on a Hesperornis, including sharks and large reptiles like mosasaurs and plesiosaurs.

The plesiosaur's teeth lined up with the indentations in the fossil to within less than a millimetre

The fossil collection at the University of Kansas holds specimens of many of these predators, so the researchers were able to compare their jaws to the marks on the Hesperornis fossil, to see if any of them matched.

The shapes of the three indentations ruled out a shark attack right away. The marks were rounded, so they could not have been made by a shark, whose teeth have flattened tips.

Similarly, none of the fossil mosasaur jaws even came close to lining up with the bite marks.

But the team's fortune changed when they tried the skull of a juvenile plesiosaur. The size and spacing of its teeth fitted neatly with the marks on the Hesperornis fossil.

"The plesiosaur's teeth lined up with the indentations in the fossil to within less than a millimetre," says Burnham.

You might think that being bitten by a ravenous reptile would have done for the Hesperornis, but the team believes it escaped.

There was only one uncontested example of a healed plesiosaur bite

The roughness of the bone's condyle suggests that it had experienced an infection. This was probably brought on by the plesiosaur bite, in which case the bird must have survived the attack.

This makes the fossil unusually informative, because it tells us that the Hesperornis was alive when it was bitten.

When palaeontologists find bite marks in fossils, it is usually impossible to say if the animal was bitten before or after death. But in this case, the evidence of infection reveals that the animal must have been alive, and that it survived for some time afterwards.

Before this finding, only a dozen or so examples of healed bite marks had been found in the fossil record. There was only one uncontested example of a healed plesiosaur bite: a fossil crocodile with a plesiosaur tooth embedded in its head. The crocodile's bone had subsequently grown around the tooth.

It's likely that it managed to get the whole of the bird's leg in its mouth

Some extra detective work revealed even more about the moment the Hesperornis was grabbed.

Based on the orientation of the plesiosaur bite, Burnham and co-author Bruce Rothschild were able to reconstruct the direction from which the plesiosaur came at the bird.

"It looks like the plesiosaur came in from the side," says Burnham. "From the orientation of the bite, it's likely that it managed to get the whole of the bird's leg in its mouth."

The findings have been published in the journal Cretaceous Research.

They could change our view of what plesiosaurs ate.

This is the strongest evidence for direct predation available in the fossil record

The jaw that matched the bite marks on the Hesperornis fossil came from a relatively small species of plesiosaur, which had a short neck and a long snout filled with small, pointed teeth: perhaps a Dolichorhynchops or something similar.

Previously, most researchers presumed that such plesiosaurs used their slender snouts for grasping small, fast-moving prey such as fish and ammonites.

However, the new findings suggest that these plesiosaurs had a more diverse diet than was previously thought, according to Tom Stubbs of the University of Bristol in the UK.

"This study presents compelling evidence that plesiosaurs may have been opportunistic predators," says Stubbs. "Marine birds were not widely considered as a potential food source for plesiosaurs. This is the strongest evidence for direct predation available in the fossil record, and suggests that marine birds featured in plesiosaur diets."