What did the teenage tyrannosaur have for dinner? Whatever it wanted! That groaner of a dad joke isn’t very funny—and it’s also not accurate. Unlike their enormous adult relatives at the top of the food chain, it seems juvenile tyrannosaurs had to rely on prey more suitable for their smaller, nimbler physiques.
Sometimes that meant a smorgasbord of small, birdlike dinosaurs, which were apparently so plentiful that the young predators selected and devoured the meaty hind legs, leaving the rest for scavengers. Researchers announced that entrée was on the Late Cretaceous menu in a study published Friday in Science Advances.
How did scientists know the items in the 75-million-year-old diet? An incredible fossilized first was discovered in the Dinosaur Park Formation in Alberta, Canada: a young Gorgosaurus skeleton, fortunately preserved with its two last meals still in its stomach cavity. Each feast included a pair of hind legs severed from small, birdlike dinosaurs (Citipes elegans). “The juvenile tyrannosaur just ripped the legs off and swallowed them whole, that’s what it looks like,” says Darla Zelenitsky, a paleontologist at the University of Calgary and a co-author of the study.
Each pair of legs reveals different levels of digestion on the bone surfaces, showing they were eaten during two different meals some hours or days apart.
The unique find delivers some hard evidence for a long-held hypothesis: As they grew, tyrannosaurs adapted to hunt and eat different types of prey during different stages of their lives. Agile juvenile tyrannosaurs were able to run down, kill and subsist on animals like the smaller Citipes. Once they’d grown to massive adult size, they hunted equally substantial prey among the Late Cretaceous’s huge herbivores like the duck-billed dinosaurs and the horned dinosaurs. “It’s the first evidence that we have that tyrannosaurs drastically changed their diet as they grew from teens to adults, which has long been suspected based on their skeletons,” says Zelenitsky.
Tyrannosaurids are best known as massive, fearsome predators like Tyrannosaurus rex, which could tip the scales at close to eight tons and grow to 40 feet in length. Of course, they didn’t start out that way. Baby T. rex were likely the size of a border collie, and throughout their lives they went through major changes not only in size, but also in physiology. Younger tyrannosaurs were more slender and agile, with narrow skulls and blade-like teeth, key for catching, dismembering and devouring smaller prey. The massive wide skulls and huge “killer banana” teeth they developed as adults, on the other hand, were better suited for chomping much larger prey—and for crushing and biting through bone. The caloric values of smaller prey likely wouldn’t have been worth the effort for such large adult predators—if they could even still catch them after losing some speed and agility with size and age.
“This is definitely a great find,” says Hans-Dieter Sues, a paleontologist at the Smithsonian’s National Museum of Natural History not involved with the research. “Although the apparent dietary shift from young to adult tyrannosaurs like Gorgosaurus comes as no surprise, it is wonderful to have actual evidence for this now.”
Current crocodilians and the Komodo dragon undergo similar dietary shifts during their growth, Sues adds.
The fossilized juvenile or “teenage” Gorgosaurus libratus was 5 to 7 years old, stood about human height at the hips, and stretched some 13 feet long. It probably weighed about 740 pounds—but even at that weight it was less than 15 percent the size of its adult relatives and had a lot of growing to do.
The two young, small, birdlike Citipes it ate weighed perhaps 20 or 26 pounds, roughly the size of male wild turkeys. But co-author François Therrien, of Alberta’s Royal Tyrrell Museum, notes they were superficially more like emus or cassowaries. They would have been among the ecosystem’s fastest runners—as would the juvenile tyrannosaurs. “Citipes may have given this juvenile a run for its prey,” Therrien says.
Gorgosaurus is a large species of predatory dinosaur found in the well-known, 75- to 80-million-year-old ecosystem of Dinosaur Provincial Park. Over the years, more than 50 dinosaur species have been identified here, as well as many mammals, birds and other reptiles. Few of those fossil finds rival the teen Gorgosaurus.
In 2009, Darren Tanke, a technician at the Royal Tyrrell Museum, found the spectacular fossil in the badlands of Dinosaur Provincial Park. The well-preserved find was exciting; the fossilized skeletons of young tyrannosaurs are far rarer than those of their full-grown relatives. Bigger dinosaurs with robust skulls and bones survived the fossilization process in the region more often than more fragile juveniles.
But while preparing the fossil, it soon became clear that it was more than initially met the eye. Tanke noted small toe bones that weren’t near the animal’s feet. “The bones were way too small to belong to this tyrannosaur, and they were coming out of the ribcage from inside the animal,” Therrien says. From that point, the team slowly examined the fossil from the inside out to learn what was behind the ribcage.
“That was really exciting, because it was the first time that stomach contents preserved in place were ever found for a tyrannosaur,” says Therrien.
Detective work led to more fortuitous finds. Most species are identified by skulls, but these prey animals survived only as dismembered legs and feet. Luckily, Citipes was known only from previously collected foot bones, which on comparison matched the fossils recovered from the ancient stomach cavity. “Amazingly, here we have four legs that represent the most complete skeleton of Citipes 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 at the University of Maryland said that fossilized stomach contents are rare and usually are found among small animals fossilized whole—like the Oviraptor philoceratops specimen that had a lizard in its stomach. Big dinosaurs are a different story. “In the past, when we found something still in the belly of a tyrannosaur or in a tyrannosaur coprolite, it’s pulverized bone,” explains Holtz, who wasn’t involved with the study. “We can tell it’s dinosaur, but more than that we really can’t say.”
Absent stomach contents, paleontologists have had to learn about tyrannosaur diet by other means. Researchers look for fossil bones with telltale bite or puncture marks on them, made by what could only be tyrannosaur teeth. If such wounds show signs of having healed, they were likely made by predators attacking live prey, not scavenging on carcasses, although the animals did plenty of both. Uncovering dinosaur droppings reveals more clues.
Adult and juvenile tyrannosaurs are so different physically that they are almost two distinct animals. Smaller, lighter juveniles with slender, long legs were probably agile and swift runners without a huge bite force. As they grew, by about age 11, the animals’ size and physiology changed dramatically. Adults had massive skulls and a bite force exponentially more powerful. As predators and as scavengers, adults fed indiscriminately on all parts of a carcass, crushing through bones and swallowing animals whole.
Because of these differences, scientists had suspected that juveniles would not have been successful hunting giant herbivores, but researchers didn’t know exactly what the animals did eat. Might they have relied on scraps and spoils from their elders’ kills? Did they hunt in packs or groups? Did they source their own meals by turning their attention to more attainable quarry—including dinosaurs smaller than themselves?
“This fossil provides the evidence that they feed on small species of dinosaurs, and also young dinosaurs—these individuals that it swallowed were yearlings who hadn’t celebrated their first birthday yet,” Therrien says. He also notes that the juvenile selectively chose only the meaty hind legs to consume. “It shows that at least our lone Gorgosaurus not only fed on different animals than adults, but also fed differently.”