What’s for dinner? UCalgary paleontologist finds out through remarkable specimen

The difference between a juvenile and adult tyrannosaur is massive — both figuratively and literally.

While adults weighed around 3,000 kilograms, the weight of a pick-up truck, juveniles were much leaner. Adults had massive skulls, robust teeth and were capable of generating bone-crushing bites, while juveniles were long-legged with slender skulls and blade-like teeth.

Now, paleontologists from UCalgary and the Royal Tyrrell Museum have discovered that as the body of a tyrannosaur changed throughout its lifespan, so did its diet.

The new study, led by Dr. Darla Zelenitsky, PhD, an associate professor in the Department of Earth, Energy and Environment at the University of Calgary, and Dr. François Therrien from the Royal Tyrrell Museum, is published today in the journal Science Advances.

The research was the result of a groundbreaking discovery: a juvenile Gorgosaurus with prey found in its stomach. This remarkable specimen broadens our understanding of the feeding habits of these iconic meat-eating dinosaurs.

“This is the first time that such well-preserved stomach contents have been found inside the skeleton of a large species of tyrannosaur,” says Zelenitsky.

The prey found inside the gut region was the hind legs of two young, bird-like theropod dinosaurs of the species Citipes elegans.

The well-preserved juvenile Gorgosaurus was found in Dinosaur Provincial Park by Darren Tanke, a technician at the Royal Tyrrell Museum of Paleontology. Gorgosaurus, a species of tyrannosaur, lived about 75 million years ago in what is now Alberta. When the dinosaur died it was between the ages of five and seven, weighed only 335 kilograms — just over 10 per cent the mass of an adult individual — and was four metres long.

While it wasn’t immediately obvious that there was prey inside of the specimen, during preparation work in the museum's lab, staff noticed small toe bones protruding from the ribcage.

“The rock within the ribcage was removed to expose what was hidden inside. Lo and behold, the complete hind legs of two baby dinosaurs, both under a year old, were present in its stomach,” says Therrien.

Zelenitsky explains that the juvenile Gorgosaurus likely only ate the hind limbs from the small prey because it was the meatiest part of the animal.

“The two individuals were probably eaten at different times because the bones of one were more digested than the other. Regardless, it appears that this teenage dinosaur definitely had an appetite for drumsticks.”

It’s unclear whether Gorgosaurus had a preference for Citipes, or if this particular species was simply an abundant prey at a certain time of year. What this discovery does mean is that juvenile tyrannosaurs’ narrow skulls and blade-like teeth meant they were suited for capturing and dismembering small, swift prey while adult tyrannosaurs’ massive skulls and robust teeth meant they could eat much larger prey like megaherbivores, including duck-billed and horned dinosaurs.

“It’s well-known that tyrannosaurs changed a lot during growth, from slender forms to these robust, bone-crushing dinosaurs, and we know that this change was related to feeding behaviour. They appear to have gone from hunting prey like Citipes — a small fraction of their size — as teenagers to hunting megaherbivore dinosaurs — as large, or larger, than their size — as adults,” says Zelenitsky.

While this is only one unique fossil, Zelenitsky and Therrien are hopeful that more specimens will be found to give clues as to what else was on the menu for teenage tyrannosaurs.

Overall, this Gorgosaurus specimen gives direct evidence that tyrannosaurs were changing their diet as they grew from juveniles to adults. This study is a leap forward for the world of paleontology research and fascinating new information for anyone intrigued by these creatures who roamed our earth 75 million years ago.