Tyrannosaurus rex was a formidable predator when fully grown. They could take down enormous herbivores that were bigger than them with their strong jaws and sharp teeth. However, palaeontologists report today in Science Advances that juvenile tyrannosaurs consumed different, much smaller prey. It is confirmed by a recent fossil containing remnants of a last meal that these carnivores changed their diet as they matured.
About 77 million years ago, in what is now Alberta, Canada, the tyrannosaur Gorgosaurus libratus ruled over an area as massive as that of a white rhino, measuring nine metre from tip of tail to tooth. Researchers know that adults used their bone-crushing teeth to eat large plant-eating dinosaurs, like Triceratops and duck-billed dinosaurs, the size of an African elephant, based on fossilised excrement and gnawed bones. But because juvenile tyrannosaur’s teeth and skulls appear less robust than those of adults, it has been unclear what exactly they consumed. According to researchers, this altered anatomy suggests that their diet most likely changed as they grew.
A new fossil of a juvenile Gorgosaurus that lived approximately 75 million years ago has been found in Alberta’s Dinosaur Provincial Park, one of the world’s richest dinosaur fossil deposits, according to a new study by Darla Zelenitsky, a dinosaur paleobiologist at the University of Calgary, and her colleagues. At 4 metres long and 350 kilogrammes in weight, the whippersnapper was roughly 1/10 the weight of its parent and as big as a large domestic pig. It was thought to be between 5 and 7 years old. Because the rivers that crossed the region’s floodplains tended to break up the smaller, more fragile juvenile skeletons after death, Zelenitsky explains why fossils of young dinosaurs from the area are rare.
What makes the juvenile skeleton even rarer are the fossilised remnants of two smaller dinosaurs found within. They were recognised by the researchers as the avian-looking raptors Citipes elegans; since neither had any growth marks on their bones, they were most likely younger than a year old and weighed as much as an adult turkey. “This is the first time that we have concrete proof that the juvenile tyrannosaurs were consuming younger, smaller dinosaurs,” claims Zelenitsky. She goes on to say that since these small animals didn’t provide many calories, adult tyrannosaurs probably wouldn’t mess around with them.
Zelenitsky notes that the juvenile Gorgosaurus appears to have been selective in its diet, as it only consumed the meatiest portions of its prey—its two hindlegs—of each Citipes. The state of the bones indicates that the dinosaur ingested both in a matter of hours or days. There might have been a lot of tiny Citipes scuttling around because small raptors like Citipes built nests that held about thirty eggs, she speculates.
It has long been suspected by palaeontologists that tyrannosaurs had a growth spurt around the age of eleven, during which they fed on prey of equal or smaller size. Because they roamed in herds, they were unable to take on large herbivores or their small offspring. However, tyrannosaurs gradually adapted to hunting these massive herbivores after they had gained sufficient mass.
Smith concurs, but she believes that adults might have also given larger prey pieces to their young because of the similarities in tooth wear between very young and adult tyrannosaurs. The new discoveries, she continues, may also contribute to the understanding of why dinosaur diversity was lower than that of other animal groups at the time. They played the roles of two distinct species in the ecosystem at different stages of their lives, according to her. Other than the teenage tyrannosaurs, that left less room for other dinosaurs with intermediate body sizes.