The animals of the Cretaceous period were considered terrifying. The Tyrannosaurus rex reached a length of up to twelve metres and weighed six tonnes. Fossils prove that this predator hunted even large prey and crushed bones with its bite. However, the fossilised stomach contents of a young predatory dinosaur show that even these prehistoric horrors were once small and were picky eaters.
Died shortly after the meal
It seems that no animal of the Late Cretaceous was safe from it. Nevertheless, researchers such as Canadian palaeo-ecologist François Therrien from the Royal Tyrrell Museum of Paleontology in Drumheller near Calgary and palaeontologist Darla Zelenitsky from the University of Calgary are now revealing something new: gigantic predatory dinosaurs such as the Tyrannosaurus rex began their existence tiny.
In a report in the journal Science Advances, the scientists describe the examination of the fossilised stomach contents of a young Gorgosaurus libratus. The remains of this animal were discovered in the “Dinosaur Provincial Park” formation in Alberta. Although the Gorgosaurus lived several million years earlier than the T. rex and was smaller, the findings can be transferred to it. The Gorgosaurus under investigation died around 75.3 million years ago at the age of about six years. The researchers estimate its weight at 335 kilograms. A full-grown Gorgosaurus could have weighed over two tonnes.
The researchers found the hind legs of two much smaller dinosaurs of the species Citipes elegans in the stomach of this still relatively small young animal. These bipedal dinosaurs presumably fed on plants. Based on the bones, the researchers estimated that each of these animals weighed between nine and twelve kilograms. These animals were so small that a full-grown Gorgosaurus would probably not have spent much time with them, as they offered little nutritional value, Therrien and his team concluded. This suggests how predatory dinosaurs once learnt to hunt. The idea that young tyrannosaurs hunted in packs with adult animals and ate together seems to be wrong, as otherwise bones of larger prey would have been found in the young animal’s stomach. Instead, the young predatory dinosaurs probably hunted alone and initially fed on animals that they could defeat.
The stomach contents of the young Gorgosaurus also reveal that these animals were picky eaters. The examined specimen had not devoured the entire animals, but only their presumably meatiest hind legs. The two prey animals were less than a year old. It seems as if the young Tyrannosaurus was specifically looking for young animals. This behaviour is similar to that of modern predators.
If this is true, the young predatory dinosaurs not only avoided larger and more dangerous herbivores with this strategy, but also did not disturb their adult conspecifics. In the course of their lives, the tyrannosaurs evolved from medium-sized to top predators, as the researchers discovered. Their skeleton changed accordingly: the skull became larger and more robust, the teeth more massive, and the entire animal grew to many times its size. The researchers assume that the Gorgosaurus would probably have tackled large herbivores from an age of around eleven years and a weight of around 600 kilograms. In this way, the animals occupied different ecological niches over time, which may have been one reason for their success. But apparently the Gorgosaurus did not get its prey well. As the bones of the eaten animals were hardly decomposed by stomach acid, the researchers suspect that it died shortly after its meal.
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