The animals are considered the horrors of the Cretaceous period. The Tyrannosaurus rex could be up to twelve meters long and weigh six tons. Fossils show that the predator even attacked huge prey, its bite shattering bones. Presumably no Late Cretaceous animal could really feel safe from him. But researchers led by the Canadian paleoecologist François Therrien from the Royal Tyrrell Museum of Paleontology in Drumheller near Calgary and the paleontologist Darla Zelenitsky from the University of Calgary are now showing that even predatory dinosaurs like this giant started out very small.
Like the scientists in the magazine Science Advances to report, they examined the fossilized stomach contents of a young Gorgosaurus libratus, whose remains were found in the Dinosaur Provincial Park Formation in Alberta. Like the T. rex, the Gorgosaurus belongs to the tyrannosaurid family: it lived several million years earlier and was smaller, but the findings are transferable, they say. The animal studied died around 75.3 million years ago at the age of around six years. The researchers estimate its live weight at 335 kilograms. A fully grown Gorgosaurus could probably weigh more than two tons.
In the belly of this still relatively small young animal, the researchers found the hind legs of two much smaller dinosaurs of the species Citipes elegans. These were two-legged dinosaurs that probably fed on plants. According to the bones, the two animals each weighed between nine and twelve kilograms when alive. The researchers working with Therrien believe that they were such small animals that an adult Gorgosaurus would hardly have been able to hang out with them due to their lack of nutritional value. And that in turn allows us to draw conclusions about how the predatory dinosaurs once learned to hunt. The idea that young tyrannosaurids hunted for prey in a pack with adults and ate together is apparently wrong, because otherwise the bones of larger prey would have ended up in the young animal’s stomach. Instead, the young predatory dinosaurs probably hunted alone – and initially fed on the animals they were already a match for.
Darla Zelenitsky (left) and François Therrien (right) in size comparison with the Gorgosaurus fossil.
(Photo: Royal Tyrrell Museum of Palaeontology)
If this is true, with this strategy the young predators not only avoided the larger and sometimes dangerous herbivores, but also did not get in the way of their adult counterparts. The researchers write that tyrannosaurids evolved from intermediate to top predators over the course of their lives. Their skeleton changed accordingly: the skull became larger and more robust, the teeth became more massive, and the entire animal grew to many times its size. The Gorgosaurus, for example, would probably have dared to attack large herbivores from around the age of eleven and a body weight of around 600 kilograms, the researchers believe. In this way, the animals would have occupied different ecological niches over time. Perhaps that is a secret of their success.
And the stomach contents of the young Gorgosaurus reveal something else: the animals were apparently picky. At least the specimen examined had not swallowed the entire animal, but rather torn up its prey and only eaten the hind legs, which were presumably particularly meaty. And the two prey animals were less than a year old. The young tyrannosaurid may have specifically targeted young animals. This is a behavior that can also be observed in today’s predators.
But the Gorgosaurus apparently didn’t like its prey well. Because the bones of the eaten animals were hardly corroded by stomach acid, the researchers suspect that it died shortly after its meal.