A one-of-a-kind Tyrannosaur fossil helps explain why these predators ruled the top of the food chain

The discovery of the remains of baby bird-like dinosaurs shows that young tyrannosaurs' diets evolved with age.

• Tyrannosaurs were some of the most formidable predators of their time, known to eat large herbivores.
• Researchers found the well-preserved remains of bird-like dinosaurs inside a tyrannosaur's stomach.
• The discovery suggests that tyrannosaurs' diets changed as they grew, which helped them become apex predators.

A 75-million-year-old fossil of a juvenile tyrannosaur helps explain why the apex predators ruled the food chain, new research shows.

According to a study published Friday in the peer-reviewed journal Science Advances, scientists recently examined fossils of a 13-foot-long Gorgosaurus libratus — a cousin of the Tyrannosaurus rex that lived 75 million years ago — in Alberta, Canada.

Tyrannosaurs, which include Gorgosaurus, were some of the most formidable predators of their time. Evidence suggests that older tyrannosaurs typically ate large herbivores.

What makes this specimen unique, however, is the well-preserved fossilized remains of two bird-like dinosaurs in the tyrannosaur's stomach, which offers insight into how juveniles hunted differently from adults of the species.

"This is the first time that such well-preserved stomach contents have been found inside the skeleton of a large species of tyrannosaur," Darla Zelenitsky, the co-author of the study, said.

How the Tyrannosaur became the apex predator of its time

The creature weighed over 700 pounds — only around 10% of the mass of an adult— and was believed to have died young, between the ages of five and seven. The fossil was first found in 2009 in Dinosaur Provincial Park, Canada.

According to the study, adults and juveniles didn't compete for prey — as a juvenile, the tyrannosaurs sought out smaller prey, due to their "narrow skulls, blade-like teeth, and long slender hind limbs," the researchers reported.

Adults, however, had "massive skulls and large incrassate teeth and were capable of generating bone-crushing bites" that could take on larger prey, according to the study.

Because juveniles weren't competing with adults, this, "allowed juvenile and adult tyrannosaurids to coexist in the same ecosystem with limited conflict," and likely why the species dominated the top of the food chain, the researchers reported.

"Tyrannosaurids occupied both mesopredator and apex predator roles during their life span, a factor that may have been key to their evolutionary success," the study said.

Tyrannosaurs were smart, picky eaters

Looking at the fossilized remains of the tyrannosaur's prey, the researchers only found evidence of the prey's hindlimbs, suggesting that the tyrannosaur was focused on the animals' meaty thighs for its meal, according to the study.

"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," according to study coauthor François Therrien.

A picky palate can be associated with a high level of intelligence. Orcas, for example, are considered one of, if not the, smartest marine animals on Earth today, and they are notoriously picky eaters, sometimes targeting sharks just for the liver and leaving the rest of the body to wash ashore and rot.

Thomas Carr, an associate professor of biology at Wisconsin's Carthage College who wasn't involved in the study, told the Wall Street Journal that the juveniles were "smart, picky eaters."

Saying tyrannosaurs were the orcas of their time might be a stretch. But it's probably safe to say that tyrannosaurs were not only ruthless predators but intelligent ones, at that.

In other words, they're not necessarily an animal that humans would want to coexist with today.