Tyrannosaurus rex is an iconic predator, among the largest land-dwelling meat-eaters of all time, with foot-long teeth and some of the most powerful jaws ever measured. As an adult, that is. Much like us humans, a tyrannosaur’s teenage years were a bit awkward, with lanky legs and long narrow faces. But young T. rex didn’t just look different; they may have been living entirely different lifestyles than their parents.

Ontogeny refers to the changes an animal experiences as it grows into adulthood. To explore tyrannosaur ontogeny, Rich Bykowski, of Georgia Southern University, examined several different species, including T. rex and a number of its close relatives, focusing on the development of three body parts: the hind legs, the upper jaws, and the teeth.

Young tyrannosaurs were built for speed, at least compared to the adults. The

The jawbone of a juvenile Tyrannosaurus at the Burpee Museum of
Natural History, in Rockford, Illinois.
(Courtesy of Rich Bykowski)

proportions of their leg bones were comparable to those of fast-running animals today, while the adults had stockier limbs better suited to support a heftier body (think horse versus elephant). This probably reflects a difference in hunting strategy. “[Younger tyrannosaurs] might have been more apt to run down prey as opposed to ambushing and over-powering it as adults,” says Bykowski, who presented his research last month at the annual meeting of the Society of Vertebrate Paleontology. Whether these predators were doing their hunting alone or in groups is still unknown.

As they grew, tyrannosaurs’ bite changed, too. Their teeth shifted from being narrow and blade-like to being more rounded, like spikes, and their jaws grew proportionately shorter and beefier. According to Bykowski, “that suggests there might have been a change in how these animals used their mouths and teeth to consume and likely catch prey.”

What’s the point of all this dramatic change? Well, imagine being a young tyrannosaur, already an impressive 20 feet long. You may be a fearsome carnivore, but if there’s one creature you don’t want to fight for your food, it’s a 40-foot adult. By changing their lifestyle over the course of development, these dinosaurs could limit how much they competed for resources with other members of their own species.

It also turns out not all tyrannosaur development is equal. This study found that Tyrannosaurus, the latest and largest species in the family, experienced more intense ontogenetic change than its older cousin Daspletosaurus. Bykowski suspects this relates to the dinosaurs’ surroundings. “When Daspletosaurus lived, it shared its ecosystem with another large tyrannosaur: Gorgosaurus; Tyrannosaurus lived alone on top of its ecosystem,” he says. “In the absence of another large competitor, Tyrannosaurus ‘spread out’ ecologically while Daspletosaurus was more restricted.”

With adults and young adults living and hunting in different ways, tyrannosaurs—particularly T. rex—may have essentially acted as two-predators-in-one. “I suspect the adult tyrannosaurs, being roughly equal size or bigger to many of their likely prey species, would have tried to get as close as possible to any prey item and then try to overpower it, like the largest cats: lions and tigers,” Bykowski explains. “The juveniles would have likely just run after their prey and wear it down, more like how modern wolves hunt.”

This kind of niche partitioning between life stages is seen in many animals, past and present, including lots of other dinosaurs. In fact, some dinosaurs changed so much throughout their lifetime that younger individuals can be mistaken for different species, which has even led to some pretty intense scientific argument.

“Dinosaurs had such high metabolisms, so they needed a lot of energy to survive,” says Mike D’Emic, of Adelphi University, who wasn’t involved in this study. With such a great diversity of species, “they needed to have a major division of resources unlike anything we see today.”

A cast of an adult Tyrannosaurus jawbone at the Burpee Museum of
Natural History, in Rockford, Illinois.
(Courtesy of Rich Bykowski)

D’Emic also points out that the study would benefit from more fossils representing more species. “It’s a great study because it incorporates such a large sample, but it’s still low compared to what you would use for [present-day] animals,” he says.

D'Emic admits that this isn't Bykowski's fault. When it comes to dinosaur science, paleontologists are limited by what the fossil record can produce, which isn’t always as much as a thorough researcher might like. Bykowski himself acknowledges that “one of the great problems is that many tyrannosaur genera are only known from one individual and incomplete skeleton.”

“There is also other evidence that has not been collected yet or analyzed in the context of growth and ecology,” he adds. For example, future research might focus on different parts of the skull, particularly the eyes and nose, which are no doubt important to the changing lifestyle of the animals. As is always the case in paleontology, more time and more fossils will be the key to exploring the complicated lives of everyone’s favorite prehistoric tyrants.