Enormous predatory dinosaurs, known as tyrannosaurids, underwent significant changes in skull strength and body shape as they grew up, suggesting they probably lived in different ecological niches. Juveniles mainly ate dinosaurian megaherbivores, but we do not have any other information about what young Tyrannosaurids ate. In this study, we present a unique fossil: the preserved articulated hind limbs of two yearling caenagnathid dinosaurs inside the abdominal cavity of a juvenile Gorgosaurus libratus. Two separate feedings can be identified because prey was chosen, torn apart, and eaten at different times. This predator-prey relationship represents direct evidence for ontogenetic dietary shifts within the Tyrannosauridae family when they hunted small and young dinosaurs until individuals grew big enough to require more energy provided by feeding on dinosaurian megaherbivores. During its lifetime, a dinosaur could be both an intermediate-level consumer (mesopredator) and a top-level predator (apex), perhaps reflecting why these animals were so successful over time.
We have all known about the infamous tyrant lizard king Tyrannosaurus Rex since childhood. These titans were the apex predators of the Cretaceous period and hunted a variety of large prey, such as Triceratops and Ankylosaurs. However, a new fossil discovery reveals that juvenile tyrannosaurs, unlike their adult counterparts, had a surprisingly picky diet.
The fossil in question is a young Gorgosaurus libratus, a close relative of T. rex, with a surprising dinner in its belly: the hind legs of two small, bird-like dinosaurs. This finding, published in a recent study, sheds light on the fascinating dietary shift tyrannosaurs underwent as they grew up.
Adult tyrannosaurs were fearsome predators known for taking down massive herbivores. However, this new evidence suggests juvenile tyrannosaurs were much more selective eaters. The Gorgosaurus fossil shows they specifically targeted the hind legs of their small prey, likely because they were the meatiest part. This selective behavior is similar to how modern carnivores, like lions, often eat the hindquarters of their prey first.
The fact that the Gorgosaurus swallowed its prey whole suggests its throat wasn’t quite big enough to handle larger animals, which makes sense, considering its smaller size than an adult. These young predators might have focused on hunting smaller dinosaurs and young animals until they grew big enough to tackle giants.
This discovery tells us about a T. rex’s picky eating habits as a juvenile and hints at why this dietary shift might have been significant. Preying on smaller dinosaurs would have allowed young tyrannosaurs to avoid competition (and potential injury) from fighting with adult herbivores and large carnivores. It was essentially a “survival of the fittest” strategy, allowing them to grow big and strong before taking on the biggest prey.
A young Gorgosaurus, estimated to be only 13% the size of an adult based on leg bone measurements, died at 5-7 years old. The fossils show a very interesting last meal: two juvenile Citipes dinosaurs found in its stomach. This discovery is evidence that those smaller theropods were prey for the Gorgosaurus. The extreme flexion of their limbs supports this idea, too. When they were eaten alive, they must have been squeezed by the powerful stomach muscles of their killer, causing them to bend so much. Therefore, we can see how this predator fed from its prey using such a tight coil position. We now know what the Gorgosaurus ate and how it could hunt when still young.
This study supports the idea that tyrannosaurs, such as the Gorgosaurus, were not only top predators but likely began as “mesopredators”. They preyed on medium-sized animals. As they matured, their physical characteristics changed, particularly their skulls and teeth becoming more robust, enabling them to hunt much larger prey. This transition from mesopredator to apex predator was a crucial factor in the success of tyrannosaurs during the Cretaceous period. By occupying both mesopredator and apex predator roles at different stages of their lives, tyrannosaurs secured a consistent food supply while minimizing competition with other predators. Their impressive growth rates and eventual massive size also contributed to their dominance as some of the largest carnivorous dinosaurs in history.
So when picturing a T. rex next time, remember that it didn’t always reign as a formidable predator; it likely began its journey as a selective eater targeting small dinosaur legs before evolving into the formidable king we recognize today.
References:
- Therrien, F. et al. Science Advances, 2023, 9(49), eadi0505. doi:10.1126/sciadv.adi0505.
