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

Small but fearsome dinosaurs once thought to be pygmy kin of tyrannosaur instead may are mere juveniles of the long-lasting 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 quite a century ago. Paleontologists estimate that the most important individuals of the species measured quite 12 meters from snout to tip of the tail. The dinosaurs had teeth about the dimensions and shape of bananas, likely tipped the scales at quite 8,000 kilograms and should have lived to be 30 years or older.

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

But for the last 15 years approximately , debate has raged about whether Nanotyrannus was indeed break away T. rex, says Holly Woodward, a paleohistologist at the Oklahoma State University Center for Health Sciences in Tulsa. as an example , a number of the anatomical features originally thought to be unique to Nanotyrannus have now been found in another tyrannosaurs, including T. rex.
So Woodward and colleagues decided to research the microstructure of leg bones of the 2 last discovered Nanotyrannus specimens, nicknamed Jane and Petey. especially , the team sliced into each fossil’s femur and tibia, the main weight-bearing bones of the upper and lower leg.

Cross sections of the bones revealed features almost like growth rings that suggest that Jane, the smaller of the 2 specimens, was a minimum of 13 years old at death. the marginally larger Petey was apparently a minimum of 15 years old. More importantly, Woodward says, the microscopic structure of the bones — and particularly the amount and orientation of blood vessels therein — hints that the tissues were still growing vigorously, as they might in individuals that weren’t fully mature.

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

Scientists have yet to return to a consensus on whether the primary known example of Nanotyrannus — the 1940s skull — was an adult or a juvenile. Some paleontologists claim that individual bones therein 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 considerable 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 an equivalent dinosaur (SN: 10/27/09).

Even though a young T. rex was an equivalent species as an adult, it still may need 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 in the least (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 special species” — that’s , youngsters probably served a special role within the ecosystem than adults. Nevertheless, he says, the juveniles were likely the dominant predator among dinosaurs of their size.

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