Canada: T-rex fossil found with its last meal still visible
Recently an incredible first fossil has been discovered in the Dinosaur Park Formation in Alberta, Canada: a young Gorgosaurus skeleton preserved with its last two meals still in its stomach cavity. Each feast included a pair of severed hind legs from small bird-like dinosaurs (Citipes elegans). The stomach contents of a juvenile Gorgosaurus reveal that it feasted on small bird-like species 75 million years ago.
“The young tyrannosaur tore off its legs and swallowed them whole, that's what it looks like,” says Darla Zelenitsky, a paleontologist at the University of Calgary and co-author of the study. Each pair of legs reveals different levels of digestion on the bone surfaces, showing that they were eaten at two different meals hours or days apart. This unique discovery provides some evidence to support a long-held hypothesis: as they grew, tyrannosaurs adapted to hunt and eat different types of prey during different stages of their lives.
The agile young tyrannosaurs were able to run down, kill and survive by feeding on animals such as the smaller Citipes. Once they grew to adult size, they hunted equally substantial prey among the enormous herbivores of the Late Cretaceous, such as duck-billed dinosaurs and horned dinosaurs.
“It's the first evidence we have that tyrannosaurs changed their diet as they transitioned from adolescents to adults, which has long been suspected based on their skeletons,” Zelenitsky says. Tyrannosaurids are best known as huge, fearsome predators like Tyrannosaurus rex, which could tip the scales at nearly eight tons and grow up to 40 feet long. Of course they didn't start out that way.
The little Tyrannosaurs were probably the size of a border collie, and over the course of their lives underwent major changes not only in size, but also in physiology. Younger tyrannosaurs were leaner and more agile, with narrow skulls and blade-like teeth that were critical for catching, dismembering, and devouring smaller prey. The huge, broad skulls and enormous teeth they developed as adults, on the other hand, were better suited for chewing much larger prey and for crushing and biting bones.
The caloric values of smaller prey would probably not have been worth the effort for such large adult predators, if they had still managed to catch them after losing some speed and agility with size and age.
“This is definitely a great discovery,” says Hans-Dieter Sues, a paleontologist at the Smithsonian's National Museum of Natural History who was not involved in the research. “Although the apparent change in the diet of juvenile tyrannosaurs towards adult ones like Gorgosaurus is not a surprise, it is wonderful to have concrete evidence of this now.”
Modern crocodiles and the Komodo dragon undergo similar dietary changes as they grow, Sues adds. The young fossil or “adolescent” Gorgosaurus libratus was 5 to 7 years old. The two young, small bird-like Citipi that he ate weighed as much as a male wild turkeys. But co-author François Therrien, of Alberta's Royal Tyrrell Museum, notes that superficially they were more like emus or cassowaries. They would have been among the fastest runners in the ecosystem, as were young tyrannosaurs.
Gorgosaurus is a large predatory dinosaur species found in the well-known Dinosaur Provincial Park ecosystem, dating back 75-80 million years. Over the years, more than 50 species of dinosaurs have been identified here, as well as numerous mammals, birds and other reptiles. Few of these fossil finds rival the adolescent Gorgosaurus.
“Surprisingly, here we have four legs that represent the most complete Citipes skeleton ever discovered, and it was preserved because it was swallowed by a tyrannosaur and the stomach actually protected the bones of the prey,” Therrien says.
Paleontologist Thomas Holtz of the University of Maryland stated that fossil stomach contents are rare and usually found among small, whole fossilized animals, such as the Oviraptor philoceratops specimen that had a lizard in its stomach. Large dinosaurs are a different story.
“This fossil provides evidence that they fed on small species of dinosaurs, and even young dinosaurs: the individuals it swallowed were one-year-olds who had not yet celebrated their first birthday,” says Therrien. She also notes that the juvenile selectively chose only the fleshy hind legs to consume. “This shows that at least our solitary Gorgosaurus not only fed on different animals than adults, but also fed differently.”
“In the past, when we found something still in the belly of a tyrannosaur or in the coprolite of a tyrannosaur, it was pulverized bones,” explains Holtz, who was not involved in the study.
“We can say it's a dinosaur, but we can't really say more than that.” In the absence of stomach contents, paleontologists had to learn about the diet of tyrannosaurs by other means. Researchers look for fossil bones with telltale bite marks or punctures, made by what could only be Tyrannosaurus teeth. If such wounds show signs of healing, they were probably caused by predators attacking live prey, without searching for carcasses, although the animals did both in abundance.
Discovery of dinosaur droppings reveals further clues. Adult and juvenile tyrannosaurs are so different physically that they are almost two distinct animals. Smaller, lighter juveniles with thin, long legs were probably agile, fast runners without enormous bite force. As they grew, around the age of 11, the animals' size and physiology changed radically. Adults had enormous skulls and exponentially more powerful bite forces. As predators and scavengers, adults fed indiscriminately on all parts of a carcass, crushing bones and swallowing animals whole. Because of these differences, scientists suspected that juveniles would not be successful at hunting giant herbivores, but researchers didn't know exactly what the animals ate.
Could they have relied on scraps and spoils from their elders' kills? Did they hunt in packs or groups? Did they get their meals by turning their attention to more reachable prey, including dinosaurs smaller than themselves?
The article is available at: https://www.tandfonline.com/doi/abs/10.1080/02724634.2022.2144337
