A tyrannosaurus fossil is helping scientists learn more about the giant carnivore’s diet — because for the first time its last meals were found embedded in the fossil.

Researchers looked at the fossil of a gorgosaurus libratus — part of the tyrannosaurid group that includes the T. rex — and found two preserved hind limbs that did not belong to the dinosaur. The limbs were those of yearling caenagnathid dinosaurs, found inside the tyrannosaurus’ abdominal cavity.

The dinosaur dismembered and ate the two other creatures at two different times, providing evidence about young tyrannosaurids’ diets, according to the study, published in Science Advances.

Young tyrannosaurids, like the one in the study, may have hunted small dinosaurs, like the caenagnathids, until they grew bigger and began to feed on bigger dinosaurs. 

Only hind limbs were found in the tyrannosaurus, indicating it didn’t eat the caenagnathids whole, but dismembered them, choosing to eat the legs first. Like an older tyrannosaurus, the bones were left in the young dinosaur’s stomach, meaning it digested the bones of its prey rather than regurgitating them.

Both of the caenagnathids were babies within the first year of their lives and weighed an estimated 19 to 26 pounds. Having two inside its stomach suggests this was the tyrannosaurus’ preferred meal. Adult tyrannosauruses, however, wouldn’t choose these dinosaurs to eat because of their small size and low energy value, according to the study.

The evidence suggests tyrannosaurids were both mesopredators — they were smaller carnivores — and apex predators — large carnivores — in their lifetime. Having played both roles may have contributed to their evolutionary success, according to the study. 

That’s because when juveniles and adults are hunting different prey, they may be at an advantage because there is a greater abundance of prey for juveniles. This may have allowed the different age groups to coexist without conflicting.

The fossil is the “first direct evidence of feeding behaviour or diet in a juvenile tyrannosaur,” Darla K. Zelenitsky, who worked on the study, told CBS News via email.

“We now have figured out from this fossil that young tyrannosaurs were probably off chasing small, young and swift prey, while the adults were preying on large herbivorous dinosaurs that travelled in herds,” she said.

Zelenitsky said this makes sense because “young tyrannosaurs were physically very different from their adult counterparts: young tyrannosaurs had blade-like teeth, lightly-built skulls, long legs and were likely much more agile than adult tyrannosaurs, which were massive, had huge skulls, and thicker teeth often described as ‘killer bananas.'”

“From their respective builds, in other words, adult tyrannosaurs were much better equipped for going after very large prey, whereas young tyrannosaurs were able to chase down swift, small prey,” Zelenitsky continued. “Also, whereas adults were more ‘indiscriminate’ feeders, eating carcasses of large plant-eating dinosaurs, often crunching through bones in the process, young individuals were far more picky or precise in how they fed.”

Caitlin O’Kane

Caitlin O’Kane is a digital content producer covering trending stories for CBS News and its good news brand, The Uplift.