Some 75.3 million years ago, a dinosaur swallowed the Cretaceous equivalent of a turkey drumstick. It would turn out to be the predator’s final feast.
Within days of eating that haunch, the dinosaur — a juvenile Gorgosaurus that stood 5 and a half feet tall at the hip — ended up dead in a river. By a stroke of geological luck, sediments rapidly covered much of the carcass and protected the dinosaur, and its dinner, from decay.
The resulting fossil, unveiled Friday in the journal Science Advances, is the first tyrannosaur skeleton ever found with stomach contents still preserved inside, yielding an exquisite snapshot of its feeding behavior. The fossil also preserved much of the skull, pelvis and left side of the Gorgosaurus’s body.
Gorgosauruses were ancestral relatives of Tyrannosaurus rex, but this fossil doesn’t contain a speck of the large herbivores on which adult tyrannosaurs feasted. Instead, this Gorgosaurus ripped the hind limbs off two small feathered dinosaurs. Researchers say the fossil provides the first direct evidence that tyrannosaurs changed what they ate as they aged, which paleontologists had predicted from existing fossil evidence.
“With this specimen, we have physical proof that young tyrannosaurs not only fed on different animals than their adult counterparts, but they also attacked or dissected them differently,” said François Therrien, the curator of dinosaur paleoecology at the Royal Tyrrell Museum in Drumheller, Alberta, and an author of the study.
Previously discovered coprolites — fossilized poop — and bones damaged by teeth or stomach acid show that adult tyrannosaurs feasted on large plant-eating dinosaurs such as Triceratops with bone-crunching gusto. But before they could take down megaherbivores, tyrannosaurs had to grow larger, and their skulls and teeth had to grow wide and robust enough to generate one of nature’s most powerful bites.
Juvenile tyrannosaurs, however, had skinny skulls, narrow jaws, blade-like teeth and long legs. Paleontologists had interpreted these traits as signs that young tyrannosaurs must have been nimble, an idea supported by the new fossil. “I jokingly refer to them as the ballerinas of doom: fast-running, fast-turning and able to go after small, fast-running prey,” said Tom Holtz, a paleontologist at the University of Maryland who was not involved with the study.
Tyrannosaurs’ ability to behave as speedy midsize predators in their youth before maturing into adult apex predators may have given the group an evolutionary edge by crowding out other predatory dinosaurs. Young tyrannosaurs’ prowess may even explain a quirk of North America’s fossil record during the late Cretaceous period: a “missing middle” predator size between heavyweight adult tyrannosaurs and a menagerie of dinosaurs no larger than humans.
“What makes sense is that these juveniles were filling that midsized predator niche,” said Darla Zelenitsky, a paleontologist at the University of Calgary and an author of the study. “They were the coyotes of the Cretaceous.”
The Gorgosaurus specimen was discovered in August 2008 by Darren Tanke, a technician at the Royal Tyrrell Museum. Weathering had exposed its ribs in a hillside in Dinosaur Provincial Park in Alberta. However, the lucky find came during the final 45 minutes of the museum’s 2008 field season, complicating the Gorgosaurus’s recovery. Mr. Tanke did not get it into the museum until March 2010.
As Mr. Tanke then removed excess rock from the fossil back, he decided to dig deeper into the animal’s rib cage. To his shock, he uncovered several toe bones too small to belong to the Gorgosaurus, within a distinctive area later found to represent its stomach contents.
“This find will be the find of my career,” Mr. Tanke said, while reflecting on the more than 11,000 fossils he has collected for the museum. “I don’t think I could ever beat this.”
The stomach contents consist of hind limbs and a partial tail from beaked dinosaurs known as Citipes, which resembled shrunken cassowaries. Each of the two Citipes was less than a year old when eaten, and, based on the bones’ degree of acid wear, the Gorgosaurus ate them during the final week of its life, one a few days before the other. Despite stewing in the Gorgosaurus’s gastric juices, the Citipes bones are so well preserved that they are the most complete fossils of the animal ever found.
In all likelihood, this Gorgosaurus had several years left of hunting small animals before moving to bigger prey. In 2021, a team including Dr. Therrien and Dr. Zelenitsky found that Gorgosaurus could not exert higher bite forces — and take on large herbivores — until the age of 11. This dinosaur’s bones indicate that it died between the ages of 5 and 7.
Though this Gorgosaurus never graduated to the adult table, Dr. Therrien thinks there is little doubt that it fed well. “Everybody loves some drumsticks,” he said.