T. rex teens looked wildly different than the adults we’re familiar with

There’s a reason that Tyrannosaurus rex has earned its reputation as the king of dinosaurs. Its massive size and powerful, bone-crunching jaws made T-rex a force to reckon with (or, more likely, flee from). But the species wasn’t born that way. Like humans, T-rexes have a period of teenage growth years. Adolescent T-rexes were built and hunted markedly differently from adults, indicates a study published January 1 in the journal Science Advances.

By analyzing leg bones from two skeletons, paleontologists found evidence that they belonged to half-grown T-rex and not, as some researchers have proposed, a smaller species of tyrannosaurid. The team concluded that T-rex didn’t come into its full strength and bulk until late in adolescence. The two youngsters provide a rare window into this early—but no less ferocious—period in T-rex’s life.

“These specimens were clearly very young, fast-growing animals,” says Lawrence Witmer, a paleontologist at Ohio University in Athens who was not involved in the research. “So the question becomes, are these young Tyrannosaurus rex? I think that the chances of that are really quite high.”

What T-rex looked like during its teen years—whether it was a mini version of its adult self or distinctively shaped—has been an ongoing controversy in the paleontology world. The debate started with a skull discovered in the 1940s and which now resides in the Cleveland Museum of Natural History, according to Holly Woodward, a paleontologist at the Oklahoma State University Center for Health Sciences in Tulsa and lead author of the new study. Its shape was odd enough that in the 1980s a group of researchers pegged it as belonging to a separate genus they called Nanotyrannus.

Then in the early 2000s, the two skeletons that Woodward investigated—which go by the nicknames of “Jane” and “Petey”—were discovered in Montana’s Hell Creek Formation and the debate continued. Paleontologists had noticed that the Cleveland fossil and Jane (whose bones include a nearly complete skull) had more teeth in their jaws than an adult T-rex would, Witmer says. He and his colleagues have reported that young members of a close relative called Tarbosaurus bataar have the same number of teeth as adults.

However, Witmer says, it’s very possible that T-rex lost teeth as it matured. He and most other paleontologists now believe that the mysterious skeletons are probably all adolescent Tyrannosaurus rex.

“Not all pieces of evidence necessarily point in the same direction…but to me the really exciting part of this [new research] is what it tells us about T-rex itself, how it grew and how it lived its life,” he says.

Rather than examining the shape of the bones, Woodward and her team peeked inside them, zooming in on slivers of femur and tibia bone less than a millimeter thick. “Bone fossilizes even on the microscopic scale, so you can see those microstructures and it looks just like modern bone does,” Woodward says.

To preserve as much information from the skeletons as possible, the researchers created 3D laser scans of the bones before cutting into them as well as replicas of the bone slivers they removed.

She and her team saw a number of features that suggested the dinosaurs had been teenagers when they died. The fine fibers within the bone tissue were arranged in all different directions, rather than neatly layered, similar to the fast-growing bones found in present-day juvenile animals. The bone was also full of marks where blood vessels once grew.

“If bone is growing very quickly it needs more nutrients to help the bone cells that are producing the bone live and do their job,” Woodward says. “The rich vascular networks [together] with the presence of disorganized bone tissue suggests that they were juveniles and they were growing quickly, rather than the slow growth that you see when something is getting to be its final size.”

She and her colleagues also examined growth rings, which resemble tree rings and form each year when bone growth briefly pauses for a couple months before firing up again. By counting the rings in Jane and Petey’s bones, Woodward concluded that the dinosaurs were, respectively, 13 and 15 years old when they died. Intriguingly, the amount of space between each ring varied from year to year—meaning the dinosaurs could grow more slowly in lean years and rapidly during times of plenty.

An adult Tyrannosaurus rex like “Sue,” a skeleton at the Field Museum of Natural History in Chicago, would take 20 years to reach its full length of around 40 feet and weigh in at about 21,000 pounds. Jane and Petey, which are housed at the Burpee Museum of Natural History in Rockford, Illinois, were only half that size and probably weighed a mere 2,100 pounds.

Not large, but still in charge

The analysis of Jane and Petey’s bones suggest that T-rex took a long time to approach its adult size. This means that during their lengthy adolescence, Jane and Petey were more svelte and would have followed a very different lifestyle than their mature counterparts.

T-rex was probably similar in many ways to modern alligators, which prey on different animals at different stages of their lives. Young alligators dine on insects and small fish, while adults can eat pretty much whatever they want, Woodward says.

Both teen and adult T-rexes would have scavenged when possible. However, when a free meal wasn’t available, a teenaged T-rex could have chased after small herbivores and other young dinosaurs that hadn’t reached their full size, such as the duck-billed Edmontosaurus. A young T-rex like Jane was more delicate than an adult and had teeth resembling steak knives that were built for tearing through flesh.

“Their hunting style was probably more of an attack and retreat, attack and retreat, whereas the adults would just basically wade right into battle and chomp down on their prey,” Witmer says. “These young T-rexes in some respects are even more terrifying than the adults, because the adults were relatively slow-moving and slower to turn, but these young T-rexes were very quick.”

A bulky adult T-rex wouldn’t have been fast enough to pursue small prey. But then, it wouldn’t need to. “The adults we know became incredibly powerful animals,” Witmer says. “Their jaw muscle chamber just exploded in size.” As T-rex’s teeth thickened and became more banana-shaped, it gained the ability to bite down with enough force to crush bone (it could even have easily ripped through a car roof, were one available). This transformation matches what Witmer has observed in juvenile Tarbosaurus specimens excavated in Mongolia, which also appear more delicate than their adult form.

T-rex’s ability to pause or speed up growth may explain how it thrived at all sizes during its heyday in the late Cretaceous period. “It also maybe shows the reason why we don’t find any other medium to large carnivores in the Hell Creek ecosystem,” Woodward says. “T-rex only had to compete with other T-rexes of similar size; it didn’t have to compete with other species because it was dominating each niche as it was getting bigger.”