First tyrannosaur fossil discovered with its last meal perfectly preserved in its stomach

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Researchers have found a tyrannosaur’s last meal perfectly preserved inside its stomach cavity.

What was on the menu 75 million years ago? The hind legs of two baby dinosaurs, according to new research on the fossil published Friday in the journal Science Advances.

Dinosaur guts and hard evidence of their diets are rarely preserved in the fossil record, and it is the first time the stomach contents of a tyrannosaur have been uncovered.

The revelation makes this discovery particularly exciting, said co-lead author Darla Zelenitsky, a paleontologist and associate professor at the University of Calgary in Alberta.

“Tyrannosaurs are these large predatory species that roamed Alberta, and North America, during the late Cretaceous. These were the iconic apex or top predators that we’ve all seen in movies, books and museums. They walked on two legs (and) had very short arms,” Zelenitsky said.

“It was a cousin of T. rex, which came later in time, 68 to 66 million years ago. T. rex is the biggest of the tyrannosaurs, Gorgosaurus was a little bit smaller, maybe full grown would have been 9, 10 meters (33 feet).”

The tyrannosaur in question, a young Gorgosaurus libratus, would have weighed about 772 pounds (350 kilograms) — less than a horse — and reached 13 feet (4 meters) in length at the time of death.

The creature was between the ages of 5 and 7 and appeared to be picky in what it consumed, Zelenitsky said.

“Its last and second-to-last meal were these little birdlike dinosaurs, Citipes, and the tyrannosaur actually only ate the hind limbs of each of these prey items. There’s really no other skeletal remains of these predators within the stomach cavity. It’s just the hind legs.

“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,” she added. “Obviously this teenager had an appetite for drumsticks.”

The two baby dinosaurs both belonged to the species called Citipes elegans and would have been younger than 1 year old when the tyrannosaur hunted them down, the researchers determined.

The almost complete skeleton was found in Alberta’s Dinosaur Provincial Park in 2009.

That the tyrannosaur’s stomach contents were preserved wasn’t immediately obvious, but staff at the Royal Tyrrell Museum in Drumheller, Alberta, noticed small protruding bones when preparing the fossil in the lab and removed a rock within its rib cage to take a closer look.

“Lo and behold, the complete hind legs of two baby dinosaurs, both under a year old, were present in its stomach,” said co-lead author François Therrien, the museum’s curator of dinosaur paleoecology, in a statement.

The paleontologists were able to determine the ages of both the predator and its prey by analyzing thin slices sampled from the fossilized bones.

“There’s growth marks like the rings of a tree. And we can essentially tell how old a dinosaur is from looking at those, the structure of the bone,” Zelenitsky said.

The fossil is the first hard evidence of a long-suspected dietary pattern among large predatory dinosaurs, said paleoecologist Kat Schroeder, a postdoctoral researcher at Yale University’s department of Earth and planetary science, who wasn’t involved in the research.

The teen tyrannosaur didn’t eat what its parents did. Paleontologists believe its diet would have changed over its life span.

“Large, robust tyrannosaurs like T. rex have bite forces strong enough to hit bone when eating, and so we know they bit into megaherbivores like Triceratops,” Schroeder said via email. “Juvenile tyrannosaurs can’t bite as deep, and therefore don’t leave such feeding traces.”

She said that scientists have previously hypothesized that young tyrannosaurs had different diets from fully developed adults, but the fossil find marks the first time researchers have direct evidence.

“Combined with the relative rarity of juvenile tyrannosaur skeletons, this fossil is very significant,” Schroeder added. “Teeth can only tell us so much about the diet of extinct animals, so finding stomach contents is like picking up the proverbial ‘smoking gun.’”

The contents of the tyrannosaur’s stomach cavity revealed that at this stage in life, juveniles were hunting swift, small prey. It was likely because the predator’s body wasn’t yet well-suited for bigger prey, Zelenitsky said.

“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 behavior.”

When the dinosaur died, its mass was only 10% of that of an adult Gorgosaurus, she said.

The voracious appetite of teenage tyrannosaurs and other carnivores has been thought to explain a puzzling feature of dinosaur diversity.

There are relatively few small and midsize dinosaurs in the fossil record, particularly in the Mid- to Late Cretaceous Period — something paleontologists have determined is due to the hunting activities of young tyrannosaurs.

“In Alberta’s Dinosaur Provincial Park, where this specimen is from, we have a very well sampled formation. And so we have a pretty good idea of the ecosystem there. Over 50 species of dinosaurs,” Zelenitsky said.

“We are missing mid-sized … predators from that ecosystem. So yeah, there’s been the hypothesis that, the juvenile tyrannosaurs filled that niche.”