A Tyrannosaurus was found fossilized, and so was its last meal
About 75.3 million years ago, a dinosaur swallowed the Cretaceous equivalent of a turkey leg. It would prove to be the predator’s last feast.
Within days of eating that leg, the dinosaur – a juvenile Gorgosaurus that stood 5 feet tall at the hip – ended up dead in a river. By a stroke of geological luck, sediments quickly covered much of the carcass, protecting the dinosaur and its food from decay.
The resulting fossil was unveiled on Friday in the journal Science Advances, is the first tyrannosaur skeleton ever found and whose stomach contents are still present, providing a wonderful snapshot of its feeding behavior. The fossil also preserved much of the Gorgosaurus’ skull, pelvis and left side of the body.
Gorgosauruses were ancestral relatives of Tyrannosaurus rex, but this fossil contains no trace of the large herbivores that adult tyrannosaurs feasted on. Instead, this Gorgosaurus tore the hind legs 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 based on existing fossil evidence.
“With this specimen, we have physical evidence that young tyrannosaurs not only fed on animals different from their adult counterparts, but also attacked or dissected them differently,” the researchers said. François Therrienthe curator 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 crunching enthusiasm. But before they could take down megaherbivores, tyrannosaurs had to grow bigger, and their skulls and teeth had to become wide and robust enough to generate one of nature’s most powerful bites.
However, young tyrannosaurs had skinny skulls, narrow jaws, blade-like teeth, and long legs. Paleontologists had interpreted these features as signs that young tyrannosaurs must have been agile, an idea supported by the new fossil. “I jokingly call them the ballerinas of doom: fast running, fast turning and able to chase small, fast-running prey,” said Tom Holtza paleontologist at the University of Maryland who was not involved in the study.
Tyrannosaurs’ ability 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 displacing other predatory dinosaurs. The prowess of young tyrannosaurs could even explain an oddity in the fossil record of North America during the late Cretaceous: a “missing middle” predator format between heavyweight adult tyrannosaurs and a menagerie of dinosaurs no bigger than humans.
“What makes sense is that these juveniles filled the niche of medium-sized predators,” he said Darla Zelenitsky, a paleontologist at the University of Calgary and 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 on a hill in Alberta’s Dinosaur Provincial Park. However, the lucky find came during the last 45 minutes of the museum’s 2008 field season, complicating the recovery of the Gorgosaurus. Mr. Tanke did not receive it at the museum until March 2010.
As Mr. Tanke then removed the excess rock from the fossil, he decided to dig deeper into the animal’s ribcage. To his horror, he discovered several toe bones too small to belong to Gorgosaurus, within a distinctive area later found to represent stomach contents.
“This find will be the find of my career,” Mr. Tanke said, looking back 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 legs and a partial tail of 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 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 being stewed in the gastric juices of the Gorgosaurus, 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 more years of hunting small animals before moving on to larger prey. In 2021, a team consisting of Dr. Therrien and Dr. Zelenitsky that Gorgosaurus could not exert higher bite forces – and compete against large herbivores – until the age of 11 years. 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 that there is little doubt that he was well fed. “Everyone loves drum sticks,” he said.