Researchers have discovered that a tyrannosaurus' last meal is perfectly preserved in its stomach cavity.
What was on the menu 75 million years ago? The hind legs of two baby dinosaurs, according to new research into the fossil published Friday in the journal Science Advances.
Dinosaur guts and hard evidence of their diet have rarely been preserved in the fossil record, and this is the first time the stomach contents of a tyrannosaur have been uncovered.
The revelation makes this discovery particularly exciting, says co-lead author Darla Zelenitsky, a paleontologist and associate professor at the University of Calgary in Alberta.
"Tyrannosaurs are large predatory species that roamed Alberta and North America during the late Cretaceous. These were the iconic apex or apex predators that we have all seen in movies, books and museums. They walked on two legs (and) had very short arms," Zelenitsky said.
"It was a cousin of T. rex, which appeared later in time, 68 to 66 million years ago. T. rex is the largest of the tyrannosaurs, Gorgosaurus was slightly smaller, perhaps fully grown it would have been 9.10 meters (33 feet)."
The tyrannosaurus in question, a young Gorgosaurus libratus, would have weighed about 350 kilos - less than a horse - and reached a length of 4 meters at the time of death.
The creature was between 5 and 7 years old and seemed picky about what it consumed, Zelenitsky said.
"Its last and penultimate meal was these small bird-like dinosaurs, Citipes, and the tyrannosaurus actually only ate the hind legs of each of these prey animals. There are actually no other skeletal remains of these predators in the stomach cavity. It's just the hind legs.
"It must have killed both Citipes at different times and then ripped off the hind legs and ate them, leaving the rest of the carcasses behind," she added. "Clearly this teenager had an appetite for drumsticks."
The two baby dinosaurs both belonged to the species called Citipes elegans and would have been younger than 1 year old when the tyrannosaur hunted them, the researchers determined.
The nearly complete skeleton was found in Alberta's Dinosaur Provincial Park in 2009.
It was not immediately clear that the tyrannosaur's stomach contents had been preserved, but staff at the Royal Tyrrell Museum in Drumheller, Alberta, while preparing the fossil in the laboratory, noticed small protruding bones and removed a stone from the ribcage to to take a closer look.
"And lo and behold, the complete hind limbs of two baby dinosaurs, both under a year old, were present in the stomach," said co-lead author François Therrien, the museum's curator of dinosaur paleoecology, in a statement.
The paleontologists were able to determine the age of both the predator and its prey by analyzing thin slices of the fossilized bones.
'There are growth marks like the rings of a tree. And we can essentially tell how old a dinosaur is by the structure of the bone," Zelenitsky said.
Changing appetites of apex predators
The fossil is the first hard evidence of a long-suspected diet among large predatory dinosaurs, says paleoecologist Kat Schroeder, a postdoctoral researcher at Yale University's Department of Earth and Planetary Sciences, who was not involved in the study.
The teenage tyrannosaurus didn't eat what his parents ate. Paleontologists believe its diet would have changed over its lifespan.
"Large, robust tyrannosaurs like T. rex have bite forces strong enough to hit bones while feeding, so we know they bit into megaherbivores like Triceratops," Schroeder said via email. "Young tyrannosaurs cannot bite very deeply and therefore do not leave feeding marks."
She said scientists previously hypothesized that young tyrannosaurs ate a different diet than fully developed adults, but the fossil discovery marks the first time researchers have direct evidence.
"Combined with the relative rarity of juvenile tyrannosaurus skeletons, this fossil is of great interest," 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 showed that young people at this stage of life were hunting for fast, small prey. It was likely because the predator's body was not yet well suited for larger prey, Zelenitsky said.
"It is well known that tyrannosaurs changed a lot as they grew, from slender shapes to these robust, bone-crushing dinosaurs, and we know that this change was related to their feeding behavior."
When the dinosaur died, its mass was only 10% of that of an adult Gorgosaurus, she said.
How young tyrannosaurs filled a niche
The voracious appetites of teenage tyrannosaurs and other carnivores are thought to explain a puzzling feature of dinosaur diversity.
There are relatively few small and medium-sized dinosaurs in the fossil record, especially in the Middle to Late Cretaceous. Paleontologists have determined that this is due to the hunting activities of young tyrannosaurs.
"In Alberta's Dinosaur Provincial Park, where this specimen comes from, we have a very well sampled formation. And so we have a pretty good picture of the ecosystem there. More than 50 species of dinosaurs," Zelenitsky said.
"We are missing medium-sized predators from that ecosystem. So yes, there is a hypothesis that the young tyrannosaurs filled that niche."
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