These Tiny Tyrannosaurus Rex Weren't Pygmies—They Were Teenagers Before a Colossal Growth Spurt

For over 50 years a tiny Tyrannosaurus rex has been the subject of a scientific debate. Was it a previously unknown pygmy species—a miniature version of the iconic predator—or was it just a young T. rex that had not reached its full size?

The mystery skull was discovered in 1946 in Montana's Hell Creek Formation and it is housed in the Cleveland Museum of Natural History. In the 1960s, scientists said it was a juvenile T. rex. However, in 1970, another team said it belonged to an Albertosaurus, a genus of tyrannosaurid theropod dinosaur. Eighteen years later, another group said it was an entirely new species, which they called Nanotyrannus, meaning pygmy tyrant in reference to the translation of T. rex—tyrant lizard king.

While most believed the skull to belong to a juvenile T. rex, the idea of pygmy species persisted, Holly Woodward, from the Oklahoma State University Center for Health Sciences, told Newsweek. This changed, however, in the early 2000s when two more mini T. rex skulls were discovered.

Woodward and colleagues have now analyzed these fossils to show they are teenage T. rex that died just before going through a colossal growth spurt. Findings are published in Science Advances.

The two fossils are named Jane and Petey. The Jane fossil consisted of an almost complete skull and skeleton, while Petey had a skeleton but the skull was lacking. "Our research independently tests the 'Nanotyrannus' hypothesis by using bone histology, rather than bone morphology evidence," Woodward explained. "Bone tissue microstructure reveals growth rate at time of death, age, and whether or not an animal was an adult. And our results show that Jane and Petey were juveniles of something. Thus, the most parsimonious conclusion is that they, and the Cleveland skull, are juvenile T. rex."

Jane and Petey were between 13 and 15 years old when they died. They were about half the length of Sue, the biggest known T. rex at 40 feet long, and slightly taller than a draft horse. They are estimated to have weighed between 975-1,000 kilograms—about a tenth of the estimated weight of Sue.

What they died from is not known.

Further analysis of the bones allowed the team to find some unexpected details about how T. rex grew. "I was surprised the juveniles were so small for so long," Woodward said, adding this growth strategy is different from other dinosaurs that get as big as they can as fast as possible. "It was even more surprising to me that in order to stay small for so long, the juvenile T. rex seemed to have been able to grow less during some years—presumably more severe years—and more during other years. This flexibility seemed to allow it to take advantage of the body-size niche it happened to be in as long as it needed, before growing larger."

Researchers believe that by staying smaller, juveniles would have been slender and fast with knife-like teeth, compared with slower-moving, bone-crunching T. rex adults. This would have allowed the juveniles to take advantage of different forms of prey, fulfilling different roles in the ecosystem.

T. rex are believed to have reached their full size by around the age of 20. Sue was in its mid 20s when it died, but because so few adult T. rex have been analyzed, their life expectancy is not known. The current findings are part of a larger project to better understand how T. rex and its closest relatives grew.

artist impression of t. rex
Artist impression of two T. rex. Researchers have now analyzed the growth rates of these extinct apex predators. iStock