This 120-Million-Year-Old Dinosaur Fossil Had a Mammal Hiding in Its Stomach

The finding represents only the second recorded instance of a dinosaur consuming a mammal

fossilized foot
The small mammal's foot in the fossil of the microraptor. Alex Dececci

When a team of scientists reanalyzed a cat-sized dinosaur fossil discovered more than two decades ago, they realized the remains of the predator’s lunch had been hiding undetected in its stomach.

Researchers found the foot of a tiny mammal, likely the size of an average mouse, within the dinosaur’s rib cage. This “very rare discovery” is only the second recorded direct evidence of a dinosaur consuming a mammal, they say in a statement. The findings were published Tuesday in the Journal of Vertebrate Paleontology.

Though the small creature discovered in the dinosaur’s stomach was a mammal, it was not a human ancestor, David Hone, a paleontologist at the Queen Mary University of London in England, says in the statement. Still, the finding suggests that ancient relatives of humans may have been a food source for dinosaurs, he adds.

“It’s so rare to find examples of food inside dinosaurs, so every example is really important, as it gives direct evidence of what they were eating,” Hone says in the statement. “This study paints a picture of a fascinating moment in time—one of the first record[s] of a dinosaur eating a mammal—even if it isn’t quite as frightening as anything in Jurassic Park.”

The dinosaur, called Microraptor zhaoianus, was similar in size to a modern-day crow and had long feathers on its arms and legs. It likely hunted by gliding from one tree to another in search of small animals to consume.

The small mammal in its belly, in contrast, likely was a weak climber that spent most of its time on the ground, making it an unexpected meal choice for the tree-dwelling microraptor, the analysis revealed. But when it comes to diet, the microraptor was a generalist: Other studies have found fossils of this dinosaur containing a bird, a lizard and a fish.

The mammal’s identity remains unknown. The researchers noted that its bones had a similar structure to those of Sinodelphys, Yanoconodon and Eomaia, which are all ancient mammals that resemble mice or opossums. Scientists also don’t know whether the microraptor hunted and killed the mammal or discovered and ate it when it was already dead. Despite the mysteries that remain, it’s clear the mammal matched the size of the prey a microraptor might have typically sought out, per the statement.

“The foot seems completely intact, and thus was swallowed whole. How much of the mammal was swallowed is unknown,” Hans Larsson, a paleontologist at McGill University and the study’s lead author, tells Gizmodo’s Isaac Schultz in an email. “However, there were several other unidentified bones around the foot in the rib cage, so I suspect that more of that mammal was consumed.”

Physical, fossilized evidence of a dinosaur’s diet is often difficult to come by, given that by the time the animal eats and digests its prey, there are little to no traces of the meal, reports CNET’s Jackson Ryan. So, discoveries like this one are crucial to advancing scientists’ understanding of prehistoric predation.

“Gut contents are amazing snapshots into the diet of fossil animals, but they are so rare that it can be difficult to figure out whether the preserved ‘last meal’ represents the animal’s normal diet or a weird, one-off event that lucked into getting fossilized,” Stephanie Drumheller-Horton, a paleontologist at the University of Tennessee, Knoxville, who was not affiliated with the study, tells Gizmodo in an email. “Taken together, the authors make a compelling case that this little theropod wasn’t a particularly picky eater, eating all sorts of small-bodied animals in its environment.”