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Researchers have found a perfectly preserved last meal in the stomach cavity of a tyrannosaur.
What was on the menu 75 million years ago? The hind legs of two baby dinosaurs, according to new research on the fossil published Friday in the journal Science Advances.
Concrete evidence of dinosaur guts and diets is rarely preserved in the fossil record, and this is the first discovery of the stomach contents of a tyrannosaur.
This makes this discovery particularly exciting, said co-lead author Darla Zelenitsky, a paleontologist and associate professor at the University of Calgary in Alberta.
“Tyrannosaurs are these large predatory species that roamed Alberta and North America during the Late Cretaceous period. They were the iconic apex or apex predators we’ve all seen in movies, books and museums. “They walked on two legs (and) had very short arms,” Zelenitsky said .
“This was a cousin of T. rex that came later, around 68 to 66 million years ago. “T. rex is the largest of the tyrannosaurs, while Gorgosaurus was slightly smaller, perhaps 9, 10 meters (33 feet) if fully grown.”
The tyrannosaur in question, a juvenile Gorgosaurus libratus, must have weighed about 772 pounds (350 kilograms) (less than a horse) and reached a length of 13 feet (4 meters) at the time of death.
Zelenitsky said the creature was between 5 and 7 years old and appeared to be picky about what it consumed.
“Its last and second-to-last meal was Citipes, these small bird-like dinosaurs, and the tyrannosaur actually ate only the hind legs of each of these prey items. There are really no other skeletal remains of these predators in the stomach cavity. Only the hind legs.”
“He must have killed both of these Citips at different times and then ripped off their hind legs, eaten them and left the remaining carcasses,” he added. “Clearly this young man had an appetite for baguettes.”
Researchers determined that both baby dinosaurs belonged to a species called Citipes elegans and were less than 1 year old when the tyrannosaur hunted them.
The nearly complete skeleton was found in Alberta’s Dinosaur Provincial Park in 2009.
It wasn’t immediately obvious that the tyrannosaur’s stomach contents were preserved, but staff at the Royal Tyrrell Museum in Drumheller, Alberta, noticed small protruding bones while preparing the fossil in the laboratory and removed a rock from its ribcage for a closer look.
“Lo and behold, two baby dinosaurs, both less than a year old, had their entire hind legs present in their stomachs,” co-lead author François Therrien, the museum’s curator of dinosaur paleoecology, said in a statement.
By analyzing thin slices from fossilized bones, paleontologists were able to determine the age of both the predator and its prey.
“There are traces of growth, like the rings of a tree. “And essentially by looking at these, the structure of the bone, we can tell how old a dinosaur is,” Zelenitsky said.
Changing appetites of top predators
The fossil is the first concrete evidence of a long-suspected diet among large predatory dinosaurs, said paleoecologist Kat Schroeder, a postdoctoral researcher in Yale University’s Department of Earth and Planetary Science who was not involved in the research.
The young tyrannosaur did not eat what its parents ate. Paleontologists believe that its diet will change throughout its lifespan.
“Large, robust tyrannosaurs like T. rex had bite forces strong enough to hit bone while eating, and that’s why we know they bit megaherbivores like Triceratops,” Schroeder said via email. “Young tyrannosaurs cannot bite that deeply and therefore do not leave such feeding marks.”
He said scientists had previously hypothesized that young tyrannosaurs had different diets than fully developed adults, but the fossil find marks the first time researchers have direct evidence.
“Coupled with the relative rarity of young tyrannosaur skeletons, this fossil is very important,” Schroeder added. “Teeth can only tell us so much about the diet of extinct animals, so finding stomach contents is like picking up the proverbial ‘smoking gun.'”
The contents of the tyrannosaur’s stomach cavity revealed that at this stage of life the juveniles hunted fast, small prey. This is most likely because the predator’s body is not yet suitable for larger prey, Zelenitsky said.
“It’s well known that tyrannosaurs changed a lot during growth, from slender forms to these sturdy, bone-crunching dinosaurs, and we know that this change was related to feeding behavior.”
When the dinosaur died, he said, its mass was only 10% of the mass of an adult Gorgosaurus.
How did young tyrannosaurs fill a gap?
The voracious appetites of juvenile tyrannosaurs and other carnivores are thought to explain a surprising feature of dinosaur diversity.
There are relatively few small and medium-sized dinosaurs in the fossil record, particularly from the Middle-Late Cretaceous Period; Paleontologists have determined that this was due to the hunting activities of young tyrannosaurs.
“We have a very well-sampled formation in Alberta’s Dinosaur Provincial Park, where this specimen came from. And so we have a pretty good idea of the ecosystem there. There are over 50 species of dinosaurs,” Zelenitsky said.
“We are missing medium-sized predators in this ecosystem. “So yes, there is a hypothesis that young tyrannosaurs filled this gap.”
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