Paleontologists in Canada have found a 75-million-year-old skeleton of a juvenile of the tyrannosaurid dinosaur Gorgosaurus libratus with the remains of two young individuals of the small dinosaur Citipes elegans in its abdominal cavity.
A juvenile of Gorgosaurus libratus feeding on Citipes elegans. Image credit: Julius Csotonyi / Royal Tyrrell Museum of Palaeontology / University of Calgary.
Tyrannosaurids are a group of carnivorous dinosaurs that dominated the ecosystems of Asia and North America near the end of the Cretaceous period, between 80 and 66 million years ago.
Among the largest land predators to have ever existed, they grew from meter-long hatchlings to multiton sizes (9- to 12-m long, 2,000 to 6,000 kg) over the course of their life span.
Juveniles were gracile with narrow skulls, blade-like teeth, and long slender hind limbs, whereas adults were robust with massive skulls and large incrassate teeth and were capable of generating bone-crushing bites.
These changes suggest that tyrannosaurids underwent a major dietary shift, in which juvenile and adult individuals occupied different ecological niches.
Fossil evidence reveals that dinosaurian megaherbivores (i.e., species with an adult mass of over 1,000 kg, including ceratopsids, giant ornithomimosaurs, hadrosaurids, and sauropods) were common prey items of large tyrannosaurids, a diet for which the necessary adaptations and bite forces only developed when individuals reached late juvenile or early subadult growth stages.
“Unfortunately, fossil evidence for diet in young tyrannosaurids is largely unknown, thus limiting our understanding of ontogenetic dietary shifts in these iconic predators,” said lead author Dr. François Therrien, a paleontologist at the Royal Tyrrell Museum, and colleagues.
Juvenile Gorgosaurus libratus preserving stomach contents: photographs of specimen in (A) right lateral view and (B) left anterolateral view; (C) interpretive illustration of specimen in right lateral view; skeleton consists of a nearly complete skull, the left side of the body and limbs, and a nearly complete pelvis; red rectangle delineates location of stomach contents; (D) histological photomicrograph of tibia showing the presence of five lines of arrested growths and two annuli (marked by asterisks), indicating that the individual was between 5 and 7 years old. Scale bars – 50 cm in (A-C) and 1 mm in (D). Image credit: Therrien et al., doi: 10.1126/sciadv.adi0505.
In their research, the authors examined a well-preserved specimen of Gorgosaurus libratus found in Dinosaur Provincial Park in Alberta, Canada.
“Gorgosaurus libratus was a tyrannosaur that lived 75 million years ago — several million years before Tyrannosaurus rex — in what is now southern Alberta,” they said.
“The age of this individual when it died has been estimated at between five and seven years old.”
“With an estimated body mass of 335 kg based on its thigh bone (femur) length, the juvenile would have been less than 13% of the body mass of an adult.”
The researchers found the partial remains of two small dinosaurs inside the stomach cavity of the Gorgosaurus libratus specimen.
“Before it died, the carnivore dismembered two young, bird-like herbivorous dinosaurs of the species Citipes elegans,” they said.
“Rather than swallowing its prey whole, the young tyrannosaur only ate the hind limbs (the meatiest parts of the body).”
“The prey were caenagnathid dinosaurs, similar to Oviraptor from Asia.”
The further study of the fossilized bones indicated that both Citipes elegans individuals were within their first year of life when they died.
“The rock within the ribcage was removed to expose what was hidden inside. The complete hind legs of two baby dinosaurs, both under a year old, were present in its stomach,” Dr. Therrien said.
Because the elements of the two individuals are at different stages of digestion, the scientists were able to conclude that Gorgosaurus libratus’ stomach contents represent two different meals, ingested hours or days apart.
The presence of two dinosaurs of the same species and age in the stomach contents, ingested at different times, suggests that young caenagnathids may have been among the preferred prey of juvenile gorgosaurs.
This specimen is the first to provide direct evidence that young gorgosaurs had different diets than their adult counterparts.
Based on tooth marks left on bones, adult gorgosaurs are known to have hunted megaherbivore dinosaurs, such as ceratopsians and hadrosaurs.
Adult gorgosaurs used their massive skulls and large teeth to capture large prey, bite through bone, and scrape and tear flesh from carcasses.
However, juvenile gorgosaurs weren’t built to hunt such large prey. Juveniles were lean, with narrow skulls, blade-like teeth, and long, slender hind limbs. They were ideally suited for capturing and dismembering small and young prey.
The evidence suggests that tyrannosaurs occupied different ecological niches over their lifetime.
As young tyrannosaurs grew and matured, they would have transitioned from hunting small and young dinosaurs to preying on large herbivores.
This dietary shift likely began around the age of 11, when the tyrannosaurs’ skulls and teeth started becoming more robust.
“It’s well-known that tyrannosaurs changed a lot during growth, from slender forms to these robust, bone-crushing dinosaurs, and we know that this change was related to feeding behavior,” said University of Calgary’s Dr. Darla Zelenitsky, co-author of the study.
“They appear to have gone from hunting prey like Citipes elegans (small fraction of their size) as teenagers to hunting megaherbivore dinosaurs (as large, or larger, than their size) as adults.”
The team’s paper was published in the journal Science Advances.
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François Therrien et al. 2023. Exceptionally preserved stomach contents of a young tyrannosaurid reveal an ontogenetic dietary shift in an iconic extinct predator. Science Advances 9 (49); doi: 10.1126/sciadv.adi0505