About 75.3 million years ago, a dinosaur swallowed the Cretaceous equivalent of a turkey leg. It would be the predator’s final feast.
Within a few days of eating that leg, the dinosaur — a young Gorgosaurus standing 5 1/2 feet tall at the hip — ended up dead in a river. By a stroke of geological luck, sediment quickly 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 Tyrannosaurus skeleton ever found with its stomach contents still preserved inside, providing an exquisite snapshot of its feeding behavior. The fossil also preserved much of Gorgosaurus’ skull, pelvis, and left side of the body.
Gorgosaurus were ancestral relatives of Tyrannosaurus rex, but this fossil contains not a speck of the large herbivores that adult tyrannosaurs feasted on. Instead, this Gorgosaurus tore off the hind limbs of two small feathered dinosaurs. The researchers say the fossil provides the first direct evidence that tyrannosaurs changed what they ate as they aged, something paleontologists had predicted from existing fossil evidence.
“With this specimen, we have physical evidence that young tyrannosaurs not only fed on different animals than their adult counterparts, but also attacked or dissected them differently,” he said François Therriencurator of dinosaur paleoecology at the Royal Tyrrell Museum in Drumheller, Alberta, and author of the study.
Previously discovered coprolites (fossilized poop) and bones damaged by teeth or stomach acid show that adult tyrannosaurs feasted on large herbivorous dinosaurs like Triceratops with munching relish. But before they could take down the megaherbivores, tyrannosaurs had to get bigger, and their skulls and teeth had to become large and strong enough to generate one of nature’s most powerful bites.
Juvenile tyrannosaurs, however, had thin skulls, narrow jaws, blade-like teeth, and long legs. Paleontologists had interpreted these traits as signs that young tyrannosaurs must have been agile, an idea supported by the new fossil. “I jokingly call them the ballerinas of destiny: they run fast, turn quickly and are able to chase small, fast prey,” he said Tom Holtza paleontologist at the University of Maryland who was not involved in the study.
The ability of tyrannosaurs to behave as fast, medium-sized predators in their youth before maturing into adult apex predators may have given the group an evolutionary advantage by crowding out other predatory dinosaurs. The prowess of young tyrannosaurs could also explain a quirk of the North American fossil record during the late Cretaceous period: a “missing average” predator size. between lumbering adult tyrannosaurs and a menagerie of dinosaurs no bigger than humans.
“What makes sense is that these juveniles filled that niche of medium-sized predators,” he said Darla Zelenitsky, paleontologist at the University of Calgary and author of the study. “They were coyotes of the Cretaceous.”
The Gorgosaurus specimen was discovered in August 2008 by Darren Tanke, a technician at the Royal Tyrrell Museum. The elements had exposed his ribs on a hill in Alberta’s Dinosaur Provincial Park. However, the lucky find occurred during the last 45 minutes of the museum’s 2008 field season, complicating the recovery of Gorgosaurus. Mr. Tanke didn’t bring it into the museum until March 2010.
After removing excess rock from the fossil, Mr Tanke decided to dig deeper into the animal’s rib cage. Much to his shock, he discovered several foot bones too small to belong to Gorgosaurus, within a distinctive area later found to represent stomach contents.
“This discovery will be the discovery of my career,” Mr. Tanke said, reflecting on the more than 11,000 fossils he has collected for the museum. “I don’t think I could ever beat him.”
The stomach contents consist of the hindlimbs and partial tail of beaked dinosaurs known as Citipes, which resembled shrunken cassowaries. Each of the two Citipes was less than a year old when it was eaten, and based on the degree of acid wear on the bones, Gorgosaurus ate them during the last week of its life, one a few days before the other. Despite coming from the gastric juices of Gorgosaurus, the bones of Citipes 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 to hunt small animals before moving on to larger 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 – up to the age of 11. The bones of this dinosaur indicate that it died between the ages of 5 and 7.
Although this Gorgosaurus never made it to the adult table, Dr Therrien believes there is no doubt that it fed well. “Everyone loves chopsticks,” he said.