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Young tyrannosaurs may have eaten completely different food to their parents
An extraordinary dinosaur fossil has revealed what young tyrannosaurs were eating over 70 million years ago.
The bones of a juvenile Gorgosaurus suggest that it was a much fussier eater than its parents, snacking on just the juicy legs of small dinosaurs rather than swallowing them whole.
Tyrannosaurs may have cornered the market as predators by changing their diet as they aged.
While adult tyrannosaurs had some of the most powerful bits of any animal, allowing them to eat large dinosaurs, their young wouldn’t have been able to keep up this pressure. Instead, juvenile tyrannosaurs were thought to be eating smaller prey, but evidence for this was thin on the ground.
However, new research published in Science Advances confirms this theory after all, following the discovery of a well-preserved juvenile Gorgosaurus from Alberta, Canada. The young carnivore, believed to be between five and seven years old, had been snacking on the legs of a small, bird-like dinosaur known as Citipes shortly before its death.
Jared Voris, one of the study’s co-authors, says that this separation in diets would have helped younger and older tyrannosaurs to avoid competing with each other for food.
‘What we see in the rocks of the Dinosaur Park Formation is different from what’s been observed in other dinosaur-dominated ecosystems,’ Jared explains. ‘Normally, entirely different species occupy the roles of middle and apex predator in an environment, but that’s not the case here.’
‘By dominating these two niches, tyrannosaurs would have prevented other dinosaur species from evolving into them. This is likely one of the reasons why tyrannosaurs were so successful in the last couple million years of the Cretaceous.’
The Citipes legs (at the bottom of the fossil) were found where the stomach is expected to have been and showed signs of digestion. Image © Darla Zelenitsky, University of Calgary (specimen courtesy of Royal Tyrrell Museum).
Young vs old tyrannosaurs
While Tyrannosaurus rex is the first known (and most famous) member of the group, there are many other species of tyrannosaurs, including Daspletosaurus, Gorgosaurus and Albertosaurus.
They were all among the largest carnivores to ever walk the Earth, with large skulls and sharp teeth that allowed them to shear and crush bone. This would have allowed fully grown tyrannosaurs to eat large herbivores like the duck-billed hadrosaurs which many other predators couldn’t.
While the bite of adult T. rex is estimated to be the most powerful ever recorded, the bites of its young were more than 10 times weaker. This is due to differences in the shape and size of the dinosaurs as they grew up.
Dr François Therrien, the lead author of the paper and Curator of Dinosaur Palaeoecology at the Royal Tyrell Museum, explains, ‘Adult and juvenile tyrannosaurs looked very different. Adults were very heavily-built animals with massive skulls, and robust teeth that we often compare to killer bananas.’
‘In comparison, juveniles were far more gracile animals with lightly built skulls, blade-like teeth, long legs, and comparatively weak bites. As a result, it had been suggested that they fed on different things.’
Finding out exactly what young tyrannosaurs were eating, however, is easier said than done. Tooth marks from these juveniles are relatively rare, as are the preserved guts and faeces, making it hard to know what they were preying on.
Fortunately, in 2009 the researchers were given exactly what they were looking for. While the well-preserved juvenile Gorgosaurus was already an important find, it had toe bones emerging from its rib cage, suggesting the remnants of its last meal were still inside.
‘We were all in disbelief when we realised what we had,’ François says. ‘We don’t often find the skeleton of young tyrannosaurs because their bones are so fragile that they tend to either not be fossilised or are destroyed before they're discovered.’
‘Finding that the skeleton also preserves stomach contents makes this specimen truly unique. These are really rare in large meat-eating dinosaurs, as the organs often decompose or are eaten by scavengers before burial.
‘In fact, this specimen represents the first occurrence of stomach contents preserved in place in a tyrannosaur.’
The team then began the painstaking process of preparing the specimen, slowly revealing the rock that had built up around the Gorgosaurus to see what was inside.
The legs found in the Gorgosaurus stomach probably belonged to Citipes less than one year old. Image © Royal Tyrrell Museum of Palaeontology, art by Julius Csotonyi.
Two dinosaur drumsticks, please
The researchers found the toe bones were still attached to legs that belonged to small bird-like dinosaurs known as Citipes.
The Citipes legs showed signs of being digested, having been worn away by the stomach acid of Gorgosaurus. One set of the bones was more worn than the other, suggesting that they were eaten perhaps hours or days apart.
While eating just the legs might have been chance, eating two in the same way one after the other suggests this might be a specific behaviour of young tyrannosaurs.
‘Whereas adult tyrannosaurs fed on all parts of a carcass, often pulverizing and swallowing bones in the process, younger animals seem to have been far more surgical,’ François says. ‘They selectively dissected away the legs to target their muscle-rich flesh and left the rest of the carcass behind.’
‘This is similar to living crocodiles, which usually consume the internal organs and legs of prey first.’
As Gorgosaurus grew, its shape and size changed, allowing it to eat different prey. Image © Jared Voris, University of Calgary.
Crocodiles also change their diets as they grow, switching from insects to fish, birds and mammals as they get larger. In fact, this process, known as an ontogenetic dietary shift, is seen in many different animals today.
While studies of the size of different dinosaur fossils have previously suggested this dietary shift also took place in dinosaurs, the Gorgosaurus fossil provides direct evidence that it might be the case.
After eating small and young dinosaurs during their youth, Gorgosaurus would have started to undergo a massive growth spurt at around the age of 11. As it grew, its teeth and skull would have become more robust to allow it to tackle large prey to fuel its increasingly large body size.
While the evidence is certainly compelling, more fossils will be needed to confirm that the dietary difference between young and old tyrannosaurs is widespread, and not just the result of one individual’s longing for legs.
