While taking a walk on a lunch break from fieldwork in Alberta, Canada, paleontologist Michael Ryan came across a couple of bones. In one hand he gripped an antler from a modern mule deer. In the other he held a piece of an ornithischian dinosaur bone. Ryan couldn't help but notice that both bones bore highly similar bite marks. And that's when it hit him.

"These bite marks were almost identical, but the bones were separated by 75 million years," Ryan says. "I asked myself what kind of animal could have made them, and all of a sudden a light bulb went off."

In that moment Ryan understood how the ancient creature that enjoyed dinosaur bones so long ago might not be that different from living critters that chomp on deer antlers today. And just what was this dinosaur bone diner? Not so much the fearsome beast you might imagine. Think of something like a squirrel. More accurately, an ancient rodent-like mammal called a multituberculate, somewhere between the size of a large squirrel and an opossum, that likely chewed on scavenged bones to obtain nutrients in the manner of modern day rodents.

That's the conclusion Ryan and Yale paleontologist Nicholas Longrich reached after examining bite marks on several late Cretaceous dinosaur, reptile and marsupial bones from Dinosaur Provincial Park in Alberta, as well as from the collections of the University of Alberta Laboratory for Vertebrate Paleontology and the Royal Tyrrell Museum. The researchers think these are the oldest known mammalian tooth marks, and published their analysis online June 16 in the journal Paleontology.

The bones Ryan and Longrich studied include a large dinosaur rib either from a duck-billed hadrosaurid (something like the character Ducky from The Land Before Time) or a horned ceratopsid (think: Triceratops), along with a femur bone from an unidentified ornithischian. The paleontologists also looked at bite marks on a partial jaw of an ancient marsupial called Eodelphis, akin to an opossum, and on the femur of a crocodilian aquatic reptile named Champosaurus.

The researchers found on the ancient bones shallow U-shaped grooves and other similar bite marks that suggested pairs of opposing teeth—just the kind of incisors that the now-extinct multituberculates possessed. It is possible that 75 million years ago other ancient mammals dined on dinosaur bones as well, but Ryan and Longrich think the rodent-like multituberculates are by far the likeliest candidates for these particular bites. Other nibbling mammals with different dentition would have left the bones pockmarked with bites, rather than marked with distinct pairings.

"There's nothing else that size at the time that would produce those marks," Ryan says. "What we found were marks from paired upper and lower incisors, just like those of multituberculates."

Ryan says the ancient mammals likely encountered the bones while foraging for insects and vegetation, seizing on the opportunity to add a little calcium or sodium to their diet—much like many modern rodents, such as squirrels, voles and porcupines. In parallel with Ryan, Longrich made the connection between the bone-munching habits of ancient and extant mammals when he recalled childhood memories of rodent gnaw marks covering the antlers of deer his father would bring home. Longrich says a few researchers have previously noted the possibility that ancient mammals turned to dinosaur bones as dietary supplements, but this is the first thoroughly documented analysis of such ancient evidence.

A few of the tooth marks the paleontologists examined were shallow enough to suggest that whatever was chewing the bones was more interested in meat than calcium. But other bite marks penetrated the bone to an impressive depth, further convincing the researchers that something was after the minerals in the bone itself.

The researchers also compared bite marks on the Cretaceous (late Mesozoic era) bones with tooth marks on bones from the current Cenozoic era. They found far more bites on the more recent bones—scores of overlapping grooves likely created by early rodents. Multituberculates simply didn’t have the gnawing power of the true rodents that succeeded and potentially outcompeted them. Some paleontologists have even speculated that multituberculate incisors were too weak to bite into anything hard, but Ryan and Longrich's new study suggests the ancient squirrelly mammals may have packed a more powerful bite than previously thought.