Scientists have found the first evidence of prey inside the stomach of a tyrannosaur skeleton, showing that the fearsome dinosaurs had a taste for small, bird-like creatures when they were young. The discovery also sheds light on how these dinosaurs evolved from slender juveniles to massive, bone-crushing adults.
The skeleton of the Gorgosaurus, a member of the tyrannosaurid family that also includes the T-Rex, was around six years old when it died more than 75 million years ago, according to a new study in the journal Science Advances. The fossil was discovered in 2009 at the Dinosaur Provincial Park, east of the Canadian city of Calgary.
But when the scientists examined the skeleton in the lab, they noticed something unusual. The study’s lead author, Francois Therrien of the Royal Tyrrell Museum, told AFP they were astonished to “discover the remains of the last meal of this young tyrannosaur still preserved in place”.
What was even more surprising, he added, was that the small leg bones protruding from the tyrannosaur’s ribcage belonged to two young, bird-like dinosaurs called Citipes. Citipes are thought to have had feathers, wings and a beak and walk on two feet, somewhat resembling modern-day cassowaries, Therrien said. They are far smaller than the huge plant-eating dinosaurs that adult tyrannosaurs had been known to eat.
Study co-author Darla Zelenitsky, a paleontologist at the University of Calgary, told AFP that this particular “fussy eater” used its sharp teeth to carve itself only the legs of the two baby Citipes. “This teenage Gorgosaurus seems to have had an appetite for drumsticks,” she said.
A drastic diet change as they grew older
The discovery also offers a rare clue into how tyrannosaurs grew from one-meter-long at birth to some of the biggest predators to have ever walked the Earth. “This fossil is the first solid evidence that tyrannosaurids drastically changed their diet as they grew from teenagers to adults,” Zelenitsky said.
Young tyrannosaurs had slender heads and legs, sharp knife-like teeth for dissecting carcasses, and could probably run quite fast to catch their turkey-like prey. These youths probably looked more similar to the velociraptors depicted in the movie “Jurassic Park” than the giant T-Rex, Zelenitsky said.
But at roughly 11 years old, as the tyrannosaurs hit their middle-age, their bodies grew almost ten times in size, ending up weighing more than 3,000 kilograms (6,600 pounds). Their heads broadened and their teeth thickened into what Therrien called “killer bananas” capable of crunching through huge bones.
This transformation was driven by a change in diet, as the dinosaurs ditched the drumsticks of their youth and started preying on giant plant-eating dinosaurs.
Implications for understanding tyrannosaur evolution
The study also has implications for understanding how tyrannosaurs evolved and diversified over time. Therrien said that the Gorgosaurus, which means “dreadful lizard”, was one of the most common tyrannosaurs in North America during the Late Cretaceous period, about 80 to 66 million years ago.
He said that the Gorgosaurus was closely related to the Albertosaurus, another tyrannosaur that lived in the same region but at different times. Both species had similar body shapes and sizes, but differed in their skull and tooth morphology.
Therrien said that the study suggests that these differences may have been related to their dietary preferences, as the Albertosaurus may have fed on larger prey than the Gorgosaurus. He said that this could explain why the two species did not compete with each other for food, and coexisted peacefully.
He added that the study also shows that tyrannosaurs were not always the apex predators that they are famous for, and that they had to adapt to different ecological niches as they grew up.
