Teenage Tyrannosaur: Unveiling the Appetite for Baby Dinosaurs through a Rare Fossil
Rare fossil reveals the stomach contents of a teenage tyrannosaur for the first time, shedding light on the changing appetites of top predators This young apex predator, a cousin of T rex, had a taste for baby dinosaurs, filling a unique niche in the prehistoric food chain
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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.
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Rarely do we find preserved dinosaur guts and concrete evidence of their diets in the fossil record. The recent discovery of the stomach contents of a tyrannosaur is particularly exciting, according to co-lead author Darla Zelenitsky, a paleontologist and associate professor at the University of Calgary in Alberta.
The Tyrannosaurs were large predatory species that wandered Alberta and North America during the late Cretaceous period. These iconic top predators, featured in movies, books, and museums, walked on two legs and had notably short arms," Zelenitsky explained.
"It was a relative of the T. rex, which appeared later, around 68 to 66 million years ago. T. rex is the largest of the tyrannosaurs, while Gorgosaurus was slightly smaller, possibly reaching 9 to 10 meters (33 feet) when fully grown."
Beside the juvenile Gorgosaurus specimen, stands Darla Zelenitsky, an associate professor from the University of Calgary, and François Therrien, the curator of dinosaur paleoecology at the Royal Tyrrell Museum in Alberta.
The young Gorgosaurus libratus, a type of tyrannosaur, weighed approximately 772 pounds (350 kilograms), which is less than the weight of a horse. At the time of its death, it measured about 13 feet (4 meters) in length. This information is from the Royal Tyrrell Museum of Palaeontology.
The creature, aged between 5 and 7, seemed to have specific dietary preferences, according to Zelenitsky.
"Its most recent meals were small bird-like dinosaurs, Citipes, with the tyrannosaur only consuming the hind limbs of each prey. There are no other skeletal remains of these predators in the stomach cavity, just the hind legs.
"It must have slain both of these Citipes at separate occasions and then torn off the hind legs, devouring them while leaving the rest of the carcasses," she remarked. "Clearly this teenager had a craving for drumsticks."
An illustration shows a Gorgosaurus libratus eating dinosaurs belonging to the species called Citipes elegans.
Julius Csotonyi/Royal Tyrrell Museum of Palaeontology
Researchers determined that the two baby dinosaurs, belonging to the species Citipes elegans, were less than 1 year old at the time they were hunted down by the tyrannosaur. The almost complete skeleton was discovered in Albertas Dinosaur Provincial Park in 2009.
Following the Chicxulub impact several weeks or months prior, the few remaining non-avian dinosaurs are struggling to survive. The intense heat from the debris that fell initially caused a dramatic rise in temperature, resulting in the rapid death of many animals. As weeks pass, the Earth's atmosphere becomes filled with soot, dust, and other particles, blocking out the Sun and causing a global drop in temperatures. With the lack of sunlight for photosynthesis, plants perish worldwide, leading to the collapse of the entire food chain. In this desolate landscape, a single, wounded and half-starved Dakotaraptor surveys the remnants of her territory in what will eventually become known as Hell Creek, 66 million years later.
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The preservation of the tyrannosaur's stomach contents wasn't initially apparent. However, while preparing the fossil in the lab, staff at the Royal Tyrrell Museum in Drumheller, Alberta, noticed small protruding bones and removed a rock from its rib cage to further investigate. "To our surprise, we found the complete hind legs of two baby dinosaurs, both under a year old, in its stomach," stated co-lead author François Therrien, the museum's curator of dinosaur paleoecology.
By analyzing thin slices sampled from the fossilized bones, the paleontologists were able to determine the ages of both the predator and its prey.
"There are growth marks similar to the rings of a tree, and we can essentially determine the age of a dinosaur by examining the structure of the bone," explained Zelenitsky.
The fossil provides the first concrete evidence of a dietary trend long suspected among large predatory dinosaurs, according to paleoecologist Kat Schroeder, a postdoctoral researcher at Yale University's Earth and Planetary Science department, who was not part of the study.
The adolescent tyrannosaur had a different diet than its parents, and scientists think its eating habits changed as it grew older.
According to a statement by Schroeder via email, "Tyrannosaurs like T. rex had powerful bite forces that could break bone while eating, indicating they fed on large herbivores like Triceratops. Juvenile tyrannosaurs, however, had weaker bites and did not leave similar feeding marks."
The red square highlights the location of the preserved gut contents in the Gorgosaurus libratus.
Fossil find at the Royal Tyrrell Museum of Palaeontology marks the first direct evidence that young tyrannosaurs had different diets from fully developed adults, debunking previous hypotheses.
Fossilized Trisauopodiscus tracks (left) and modern bird tracks (right).
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Schroeder emphasized the importance of the fossil, noting the rarity of juvenile tyrannosaur skeletons. "Teeth can only reveal so much about extinct animals' diets, so discovering stomach contents is like finding a smoking gun," she explained.
Analysis of the tyrannosaur's stomach cavity showed that juveniles hunted fast, small prey at this stage in their lives. This was likely due to the predators not yet being physically adapted to catch larger prey, according to Zelenitsky.
"Throughout their growth, tyrannosaurs undergo significant physical changes, transitioning from slender forms to robust, bone-crushing dinosaurs. These changes are closely linked to their feeding behavior. Upon death, a juvenile dinosaur's mass may only be 10% of that of an adult Gorgosaurus."
How juvenile tyrannosaurs filled a niche
The voracious appetite of teenage tyrannosaurs and other carnivores has been thought to explain a puzzling feature of dinosaur diversity.
The fossil record contains relatively few small and midsize dinosaurs, especially during the Mid- to Late Cretaceous Period, a trend attributed by paleontologists to the hunting activities of young tyrannosaurs.
The Gorgosaurus libratus was the first young tyrannosaur specimen discovered with preserved stomach contents in place inside the skeleton.
Royal Tyrrell Museum of Palaeontology
Zelenitsky stated that the formation in Albertas Dinosaur Provincial Park, where the specimen is from, has been well sampled, providing a good understanding of the ecosystem. With over 50 species of dinosaurs, there is a noticeable absence of mid-sized predators. Therefore, there is a hypothesis that the niche was filled by juvenile tyrannosaurs.
