Dinosaur Happy Meal? Young Tyrannosaurs Ate From the Kids' Menu

A new fossil of a tyrannosaurid dinosaur has revealed that the youngster's last meal included another tiny dinosaur.

The fossilized juvenile is of the species Gorgosaurus libratus, thought to have lived around 75.3 million years ago, and was discovered in the Upper Cretaceous Dinosaur Park Formation in Alberta, Canada, according to research published in the journal Science Advances.

Inside the dinosaur's belly, the paleontologists discovered the leg and foot bones of two small caenagnathid dinosaurs from the species Citipes elegans, both of which died aged less than a year old.

"The fossil is unique because it represents the first time that the prey of a tyrannosaur was found preserved inside the stomach," paper co-authors Darla Zelenitsky, an associate professor at the University of Calgary, and Francois Therrien, the curator of dinosaur palaeoecology at the Royal Tyrrell Museum, jointly told Newsweek.

"Importantly, the fossil gives the first direct evidence of feeding behavior or diet in a juvenile tyrannosaur. Paleontologists have long known from tooth-damaged prey bones that large or mature Gorgosaurus individuals fed on duckbilled dinosaurs and horned dinosaurs, large species of herding herbivorous dinosaurs. The diet or hunting strategy of young tyrannosaurs, prior to this, was a bit of a mystery."

Tyrannosaurids roamed the planet towards the end of the Cretaceous period, between around 80 and 66 million years ago. The most famous tyrannosaurid is T. rex, but there have been numerous other species discovered in the fossil record, ranging in size from only a few feet to gargantuan predators.

It has been long theorized that tyrannosaurids changed their diet throughout their lives, hunting larger prey only once they hit adulthood. The fossil evidence of juvenile tyrannosaurids has been relatively sparse, meaning that the shift in diet between youth and adulthood remained unconfirmed until now.

"This specimen tells us that young individuals occupied a different ecological niche than adult individuals. Not only did they feed on different dinosaur species, but the way they processed their food was different," Zelenitsky and Therrien said. "We now have figured out from this fossil that young tyrannosaurs were probably off chasing small, young, and swift prey while the adults were preying on large herbivorous dinosaurs that traveled in herds.

This makes sense because we knew from their skeletons that young tyrannosaurs were physically very different from their adult counterparts: young tyrannosaurs had blade-like teeth, lightly-built skulls, and long legs and were likely much more agile than adult tyrannosaurs, which were massive, had huge skulls and thicker teeth often described as 'killer bananas.'

"From their respective builds, in other words, adult tyrannosaurs were much better equipped for going after very large prey, whereas young tyrannosaurs were able to chase down swift, small prey. Also, whereas adults were more 'indiscriminate' feeders, eating carcasses of large plant-eating dinosaurs, often crunching through bones in the process, young individuals were far more picky or precise in how they fed."

"As shown by this fossil, the legs of the prey Citipes were selectively dismembered and swallowed whole, yet the remainder of the carcasses were not eaten. Young Gorgosaurus may have fed preferentially on small and young dinosaurs, likely because they were very abundant and relatively easy to catch," they said. "Then as Gorgosaurus grew through adolescence, it would have reached a point when feeding on small dinosaurs was no longer enough to provide them with the energy they needed, so they underwent a dietary shift and started feeding on larger dinosaur species."

This new fossil provides further evidence of the dietary habits of young tyrannosaurids being different from their adult counterparts. The dinosaur is thought to have died relatively soon after its final meal, as the bones of its last prey are not much more decayed than the tyrannosaurid itself, so they hadn't had time to be digested away.

"With an estimated body mass of 335 kg [738 lbs] based on its femur length, the Gorgosaurus individual would have been less than 13 percent of the body mass of an adult conspecific," the authors wrote. "Bone histology, specifically bone fabric and the presence of growth marks, reveals that the animal was a juvenile between 5 and 7 years of age at the time of its death."

Therefore, this study supports the theory that juvenile tyrannosaurids hunted smaller prey than adults, and that tyrannosaurids moved between ecological niches throughout their lives. Several modern reptile species do this too, including crocodiles and Komodo dragons, which hunt increasingly large prey as they get older and bigger.

This allows the adults and youngsters to co-exist in the same ecosystem, as they do not compete for resources.

"This study shows us that tyrannosaurs occupied various ecological niches during their lifespan: they dominated the niche of 'mesopredators' (or mid-size predators) when they were young and then transitioned to apex or top predators as they grew," Zelenitsky and Therrien said."That is probably the reason why tyrannosaurs were so successful and dominated their ecosystems at the end of the Cretaceous in North America and Asia.

"Earlier in geologic time, ecosystems contained several different species of predatory dinosaurs, some small, some medium, and some large, and those different predators partitioned the ecosystems, each occupying their separate ecological niche. Around the start of the Late Cretaceous, tyrannosaurs were just one of several small to medium-sized predators and the apex predator role was occupied by other large predatory dinosaurs called allosauroids.

"An extinction event occurred sometime during the Late Cretaceous and the large allosauroids went extinct. After that, tyrannosaurs were able to evolve and invade the apex predator niche left vacant by the extinct large allosauroids, while still keeping their ancestral role of mesopredators as juveniles. So by the end of the Cretaceous, the ecosystems of North America and Asia were completely dominated by tyrannosaurs because they occupied all the predatory ecological niches over the course of their lifespan," they said.

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Update 12/12/23, 12:49 p.m. ET: This article was updated with comment from Darla Zelenitsky and Francois Therrien.