“It’s a phenomenal paper, and the discovery is just over-the-top,” said David Burnham, a paleontologist at the University of Kansas who was not involved in the research.

The tyrannosaur Gorgosaurus is a slightly smaller cousin of the most famous Tyrannosaurus rex. This juvenile was probably between 5 and 7 years old and weighed about 740 pounds — 13 percent the size of a fully grown adult. It would have measured about 15 feet from nose to tail and stood about as tall as an average human adult.

Darren Tanke, a technician at the Royal Tyrrell Museum of Paleontology, collected the fossil in 2009 in the badlands of Dinosaur Provincial Park in Alberta, Canada. It had been buried lying on its left side. As he worked to prepare the fossil, he noticed something odd poking through the rib cage — a few small toe bones.

Careful investigation would reveal those to be parts of the hindlimbs of two yearling birdlike dinosaurs called Citipes that would have each been about the size of a turkey.

Outside paleontologists said the find was exceptional for a long list of reasons. The bones were articulated and found in place, rather than jumbled up for scientists to piece together. The animal is a juvenile, providing a critical window into the time before Gorgosaurus bulked up into a bone-crushing apex predator. But most dazzling, its stomach contents were intact, allowing scientists to see that before its death, it had recently dined on two separate occasions.

“This is a once-in-a-career fossil,” said Darla Zelenitsky, a paleontologist at the University of Calgary and one of the authors of the study, published Friday in Science Advances. The adolescent dinosaur “was probably very much a precision eater. It had a very narrow skull, bladelike teeth [and] it could probably just easily rip the hindlimbs off these animals.”

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Dino drumsticks and ‘killer bananas’

For years, scientists have pondered the developmental arc of the tyrannosaurs. The adults are the celebrities of the dinosaur world: Most of the known species were burly, robust animals that are thought to have hunted massive duck-billed and horned dinosaurs.

But until they reached maturity — 11 years old for a Gorgosaurus — they were almost like a different species, more lightly built than their elders, faster and lacking the bone-crushing chomp. Their teeth were more like sharp blades, not rounded like the “killer bananas” of adult tyrannosaurs, said Francois Therrien, the curator of dinosaur paleoecology at the Royal Tyrrell Museum.

The changes in physiology led to theories that, like modern-day Komodo dragons, tyrannosaurs underwent a dietary shift over the course of a lifetime, eating small prey when they were young and possibly occupying a separate ecological niche from that of adults.

“That had been suggested before, but we didn’t really have any proof that was the case. It was all based on modeling and assumptions,” Therrien said.

Now, at least for Gorgosaurus, they don’t have to guess.

“It looks like it was Thanksgiving, because it was mostly eating the legs,” said Thomas Holtz, a paleontologist at the University of Maryland who is one of the scientists who has previously theorized that tyrannosaurs underwent a big dietary shift with maturation.

“This is a great case of showing small tyrannosaurids fed on small dinosaurs, much smaller than themselves,” he said, “whereas the grown-up versions, we have the evidence of their bite marks on big adults that were about the same size as adults.”

A glimpse into the life of Gorgosaurus

Evidence about what tyrannosaurs ate comes mostly from connecting dots in the fossil record. Bite marks on the bones of their prey or analysis of fossilized tyrannosaur dung have helped paleontologists reconstruct their dining habits.

But the new fossil is the first example of a tyrannosaur with stomach contents preserved in place, according to the authors. Any dinosaur with stomach contents in place is a rarity, because the corpses of animals are rarely buried intact right after death. Scavengers might come along and eat them, including their gut contents, and environmental conditions might jumble the bones before they are preserved.

In this case, the feathered dinosaurs whose rear limbs were preserved also happen to be the most complete fossils yet discovered of Citipes — ironically, because they were protected by the stomach of the dinosaur that ate them. One set of bones looks a little more digested than the other, suggesting the two meals were separated by hours or days, Zelenitsky said.

With only a single specimen, it’s hard to tell how generalizable the find is. Did young Gorgosaurus prefer to eat Citipes? Did it selectively dismember the Citipes to eat only the drumsticks? Did it hunt these two animals? Was another juvenile Gorgosaurus gulping down the other half?

It’s hard to say why only the legs were in its stomach, said Joseph Peterson, a paleontologist at the University of Wisconsin at Oshkosh who was not involved in the work. “Maybe they came across a couple carcasses and this was all that was left, and it ate those,” he said. “That’s where we have to be a little careful. It’s so tempting to take it to that next stage.”

Despite the limitations, several scientists said they’d guess the dietary shift wasn’t an idiosyncratic quirk of this one individual or even of this species, but may be common to other tyrannosaurs.

“Tyrannosaurs were not just bone-crunching brutes that ate whatever they wanted, whenever they wanted. They were actually quite sophisticated feeders, whose diets changed as they got older,” Stephen Brusatte, a paleontologist at the University of Edinburgh, wrote in an email. “They had to grow into their bone-crushing personas.”