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,
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.