The stomach of the teenage tyrannosaur Gorgosaurus libratus is a gift that keeps on giving. A team of paleontologists in Canada found the remains of two meals preserved inside of its stomach cavity, including the partially digested drumsticks of two birdlike dinosaurs. The findings were described in a study published December 8 in the journal Science Advances and is the first known time that well-preserved gut continents have been discovered in a fossilized tyrannosaur.
Gorgosaurus lived about 75 million years ago, or several million years before its more famous relative the Tyrannosaurus rex. In the study, the Gorgosaurus was estimated to be between five and seven years old when it died, or a teenager in dinosaur years. It probably weighed about 738 pounds, or 13 percent of the body mass of a fully grown Gorgosaurus. Adults were about 33 feet long and weighed upwards of 2,200 pounds.
The fossil was first discovered in 2009 by staff from the Royal Tyrrell Museum of Palaeontology in Dinosaur Provincial Park in Alberta. Technicians noticed strange features poking out of the fossil’s rib cage while they were preparing it. These turned out to be small toe bones. Gut contents like these are rarely preserved in dinosaur fossils, but this specimen had the dismembered remains of two herbivorous dinosaurs–Citipes elegans.
The tyrannosaur only ate the hind limbs of each tiny Citipes and they appear to be the last and second-to-last meal that the Gorgosaurus consumed before it died.
“It must have killed… both of these Citipes at different times and then ripped off the hind legs and ate those and left the rest of the carcasses,” study co-author and University of Calgary paleontologist Darla Zelenitsky told CNN. “Obviously this teenager had an appetite for drumsticks.”
Tyrannosaurs digested the bones of their prey in their stomach, instead of regurgitating them the way present-day birds do. Since the elements of the two Citipes individuals are at different stages of digestion, the team concludes that these were two different meals eaten hours or days apart.
This specimen also shows direct evidence that young Gorgosaurus had different diets than adults. Fully grown Gorgosaurus are known to have hunted megaherbivore dinosaurs including ceratopsians (horned dinosaurs) and hadrosaurs (duck-billed dinosaurs), based on the tooth marks left behind on bones. They used their massive skulls and teeth to capture their large prey, chomp through bone, and then scrape and tear the flesh.
Juveniles were not yet built to hunt such large prey. They had more narrow skulls, blade-like teeth, and long and slender hind limbs. This made them better suited for capturing and dismembering small and young prey like the Citipes found in this specimen. The team also believes that this kind of prey was a preferred snack for these teenage dinos.
“This is a great case of showing small tyrannosaurids fed on small dinosaurs, much smaller than themselves,” University of Maryland paleontologist Thomas Holtz told The Washington Post., “Whereas the grown-up versions, we have the evidence of their bite marks on big adults that were about the same size as adults.”
The dietary shift towards eating bigger prey likely began around the age of 11. This is when the tyrannosaurs’ skulls and teeth started to get more robust. Differences in diet provide a competitive advantage by lessening competition for resources in modern ecosystems. This same strategy could have been applied when tyrannosaurs like Gorgosaurus lived. It would have allowed juveniles and adults to coexist in the same ecosystem with less conflict. Occupying different ecological niches during their lifespan was likely one of the keys to tyrannosaurs’ evolutionary success as some of Earth’s carnivores.