Study finds important differences among ‘teenage’ tyrannosaurs

New research led by scientists at the University of Calgary has shed fresh light on the “teenage” years of some of the largest predators to have ever lived on land.

The study, which was recently published in Scientific Reports, looked at juvenile tyrannosaurs, or tyrannosaurids. They were a group of related species of meat-eating dinosaurs whose largest member was Tyrannosaurus rex, or T. rex.

As an adult, T. rex weighed as much as a modern elephant, giving a sobering dimension to other characteristics such as powerful, crushing jaws, teeth like steak knives, and short forelimbs sporting two-fingered hands. What might it have been like for a human being to face tyrannosaur junior?

“They were about the size of a horse, and thus much smaller than the elephant-sized adults of their species,” says the study’s lead author, Jared Voris, a PhD student in the Department of Geoscience in the university’s Faculty of Science. “But at that size, they would likely still have been very dangerous animals, and they probably would have been able to hunt a human very easily.”

Fossils historically rare

Scientists define juvenile dinosaurs as animals that were about half the body length of the largest adult individual of their species. Juvenile tyrannosaurs probably averaged four metres long (13 feet) when they were six years old, the equivalent in terms of their life cycle to the age of a human teenager, says Voris.
 

Alberta’s badlands preserve the greatest variety of tyrannosaurid fossils in the world. Besides T. rex, these include species such as Albertosaurus, Daspletosaurus and Gorgosaurus, which collectively lived during the Late Cretaceous between 76 and 66 million years ago.

But fossil specimens of juveniles have historically been extremely rare, leading scientists to believe there had been little difference between each species until they became adults. Voris was able to correct this impression by examining numerous fossils of young tyrannosaurids in the collections of the Royal Tyrrell Museum, some of which had yet to be studied in detail.

“The number of skeletons of juvenile tyrannosaurs present in the collections has nearly doubled in the last decade,” says study co-author Dr. François Therrien, PhD. As a curator at the museum, he advised Voris, who was supervised by co-author Dr. Darla Zelenitsky, PhD, an assistant professor and dinosaur paleontologist in the University of Calgary’s Department of Geoscience.

Study 'strong foundation'

The greater abundance of fossils allowed Voris to discover juvenile tyrannosaurids were actually far more different from each other than had been previously suspected. The study reveals it is possible to distinguish between the different species of young tyrannosaurs in Alberta, a finding of importance to paleontologists around the world, he says.

A specimen long thought to be the only known juvenile individual of a Daspletosaurus, a close cousin of T. rex, was found by Voris to actually belong to Gorgosaurus, which is a distant cousin, he says. “While Gorgosaurus is known from several specimens representing different growth stages of its life cycle from juveniles to fully grown adults, the new identification of the skeleton means Daspletosaurus is pretty much known only from adult individuals,” he says.

Young Daspletosaurus are “now only represented by a few isolated bones instead of a nearly complete skull,” says Zelenitsky. “Regardless, we still have been able to figure out the earlier growth stages in the life cycle of both tyrannosaurs, Gorgosaurus and Daspletosaurus.”

The study provides “a strong foundation to better understand growth in tyrannosaur species from Alberta and the rest of the world,” says co-author Dr. Philip J. Currie, PhD, who is a professor at the University of Alberta.

Badlands near inland sea

Previous research by Currie suggested tyrannosaurids may have hunted in packs that included both juveniles and adults. But these dinosaurs likely weren’t the ruthless killing machines of fictional films such as Jurassic Park, mindlessly lunging after anything that moved, says Voris.

“They probably only killed when they needed to — when they were hungry, or they needed to establish their territory or something like that, just like predators today,” he says.

The new research into juvenile tyrannosaurs gives scientists a clearer picture of a group of important apex predators that lived during a time when much of the centre of North America was covered by a vast inland sea, dividing the continent into two major landmasses, says Voris. What is now Alberta’s badlands was roughly near the sea’s western shore, he says.

The area likely contained a lushly forested environment somewhat like today’s Mississippi delta, he says. “There were meandering streams along the western shore of that seaway, and that’s why we have so many fossils down at Dinosaur Provincial Park, and in the collections of the Royal Tyrrell Museum,” says Voris. “It’s because of all the sediment — which had been washing off the Rocky Mountains at the time — buried and preserved dinosaur remains in the river channels and floodplains.”