Small ‘cousins’ of T. rex may actually have been growing teenagers

Small but fearsome dinosaurs once thought to be pygmy kin of Tyrannosaurus rex instead may have been mere juveniles of the iconic species, new analyses of fossils suggest. The finding bolsters the case that teenage tyrannosaurs had different dining habits than their bone-crushing elders, researchers report January 1 in Science Advances.

T. rex fossils were first discovered more than a century ago. Paleontologists estimate that the largest individuals of the species measured more than 12 meters from snout to tip of the tail. The dinosaurs had teeth about the size and shape of bananas, likely tipped the scales at more than 8,000 kilograms and may have lived to be 30 years or older.

In the 1940s, paleontologists unearthed a fossil skull that, although similar to that of a T. rex, was about half the size and had teeth shaped more like daggers than bananas. After detailed analyses of a similar yet more complete specimen that was dug up in the early 2000s from rocks in the same region and of the same era as T. rex, researchers dubbed the dinosaur Nanotyrannus.

But for the last 15 years or so, debate has raged about whether Nanotyrannus was indeed separate from T. rex, says Holly Woodward, a paleohistologist at the Oklahoma State University Center for Health Sciences in Tulsa. For instance, some of the anatomical features originally thought to be unique to Nanotyrannus have now been found in some other tyrannosaurs, including T. rex.

So Woodward and colleagues decided to investigate the microstructure of leg bones of the two most recently discovered Nanotyrannus specimens, nicknamed Jane and Petey. In particular, the team sliced into each fossil’s femur and tibia, the major weight-bearing bones of the upper and lower leg.

Cross sections of the bones revealed features similar to growth rings that suggest that Jane, the smaller of the two specimens, was at least 13 years old at death. The slightly larger Petey was apparently at least 15 years old. More importantly, Woodward says, the microscopic structure of the bones — and especially the number and orientation of blood vessels therein — hints that the tissues were still growing vigorously, as they would in individuals that weren’t fully mature.

“It’s clear that these creatures were not adults,” says Thomas Holtz Jr., a vertebrate paleontologist at the University of Maryland in College Park who wasn’t involved in the study. “They were still growing and still changing,” he says.

Scientists have yet to come to a consensus on whether the first known example of Nanotyrannus — the 1940s skull — was an adult or a juvenile. Some paleontologists claim that individual bones in that skull are fused together, indicating that the creature was an adult, but other researchers aren’t convinced.

Previous studies have suggested that teenage tyrannosaurs experienced a substantial growth spurt before adulthood (SN: 8/11/04), Woodward notes. And other analyses have found that fossils first thought to be anatomically distinct species were actually different life stages of the same dinosaur (SN: 10/27/09).

Even though a young T. rex was the same species as an adult, it still might have behaved much differently, Woodward says. While juveniles were probably fleet-footed, an adult T. rex was a lumbering behemoth that probably couldn’t run well if at all (SN: 2/27/02). And a juvenile’s daggerlike teeth were strong enough to puncture the bones of prey but couldn’t crush them like adult T. rex teeth could. That difference suggests that youngsters and adults probably chased and consumed different prey, Woodward notes.

Holtz argues that such differences in lifestyle mean that T. rex adults and adolescents “were functionally a different species” — that is, youngsters probably served a different role in the ecosystem than adults. Nevertheless, he says, the juveniles were likely the dominant predator among dinosaurs of their size.