According to a recent research, fossils that were previously thought to be
juvenile Tyrannosaurus rex were really adults of a smaller tyrannosaur with
larger arms, longer legs, and narrower jaws. After being identified decades
ago, the species Nanotyrannus lancensis was subsequently reinterpreted as a
juvenile Tyrannosaurus rex.
Paleontologists have debated for decades whether the 1942 discovery of the
first Nanotyrannus skull in Montana was a new species or merely a juvenile
of the much bigger T. rex.
Reexamining the fossils, Drs. Nick Longrich of the Milner Centre for
Evolution at the University of Bath and Evan Saitta of the University of
Chicago examined growth rings, Nanotyrannus's morphology, and an
as-yet-undiscovered fossil of a juvenile Tyrannosaurus rex.
Researchers measured the growth rings of Nanotyrannus bones and found that
as development slowed, the rings were closer together near the exterior of
the bone. It implies that these animals were not rapidly developing
juveniles but rather were almost at full size.
Based on growth models, the animals' greatest size was estimated to be
between 900 and 1,500 kg and five meters, or around 15% of the massive T.
rex's size, which reached up to 8,000 kg and nine meters or more.
The results of the study have been reported in
Fossil Studies.
Longrich remarked, "I was pretty blown away when I saw these results." "It
surprised me because it was so definitive. We're not seeing them develop
like rapidly, gaining hundreds of kilos a year, if they were juvenile
Tyrannosaurus rex. Despite our best efforts, we were unable to achieve
satisfactory growth rates no matter how we modelled the data. The theory
that these creatures are juvenile Tyrannosaurus rex appears to be coming to
an end."
The lack of fossils that combined traits from Nanotyrannus and T. rex—which
would exist if one species evolved into the other—supported the presence of
separate species. They could positively identify each fossil they looked at
as belonging to a particular species.
The idea that they were juvenile Tyrannosaurus rex was further refuted by
the development trends in other tyrannosaurs.
"Jungles of other tyrannosaurs exhibit many of the distinguishing
characteristics of the adults," Dr. Longrich said. A very young Tarbosaurus,
a close cousin of Tyrannosaurus rex, has characteristics unique to the
adults. Just as kittens resemble cats and puppies resemble dogs, so too are
the juveniles of certain tyrannosaurs unique. Furthermore, Nanotyrannus
doesn't resemble a Tyrannosaurus rex at all. It's possible that it is
developing in a manner that sets it apart from all other dinosaurs,
including tyrannosaurs, but it's more likely that it is simply not a T.
rex."
However, this begs the question, why hasn't a young Tyrannosaurus rex been
discovered if Nanotyrannus isn't a juvenile?
That has always been a major concern. As it happens, we had discovered
one," Longrich remarked. "But the fossil was collected years ago, stuck in a
box of unidentified bones in a museum drawer, and then forgotten."
Through their investigation, Longrich and co-author Evan Saitta were able
to identify a juvenile Tyrannosaurus from a prior specimen found and kept in
a San Francisco museum.
The frontal bone of the baby Tyrannosaurus rex's skull is a representation
of it; it has unique characteristics that identify it as a Tyrannosaurus but
are absent from Nanotyrannus. It originates from a small mammal, around five
meters in length and with a 45 centimeter long skull.
"Yes, it's just one specimen and one bone, but it only takes one," Dr.
Longrich stated. None of the other skull bones resembles the unique
appearance of T. rex skull bones. Young Tyrannosaurus rex are real, they're
simply extremely rare, much as other dinosaur juveniles."
These results, according to the researchers, provide compelling evidence
that Nanotyrannus is a distinct species that is not closely linked to
Tyrannosaurus. Compared to its densely packed sibling, it had longer legs
and a lighter build. Also, in contrast to the well-known T. rex with short
arms, it possessed longer arms.
"In actuality, the arms are longer than T. rex's. The limbs and claws of
even the largest Tyrannosaurus rex are shorter than those of these tiny
Nanotyrannus. The arms of this animal were really rather strong weapons.
It's basically simply a tiny, swift, and nimble mammal all together. Whereas
T. rex depended on power and bulk, this animal depended on speed."
It may have existed outside of the Tyrannosauridae family, of which T. rex
is a member, in its own family of predatory dinosaurs, as shown by its long
arms and other characteristics, which indicate that it was only distantly
related to T. rex.
The new research is the most recent in a decades-long sequence of papers on
the issue.
"Nanotyrannus is highly controversial in paleontology," Longrich stated. It
felt like we'd figured out this problem with the baby T. rex not too long
ago.
"I was very skeptical about Nanotyrannus myself until about six years ago
when I took a close look at the fossils and was surprised to realize we'd
gotten it wrong all these years."
The authors speculate that we may be underestimating the variety of
dinosaurs and other ancient animals since it is difficult to distinguish
between them based just on their sometimes fragmentary bones.
"It's astounding to consider how much we still don't know about the most
well-known of all the dinosaurs," commented Longrich. It begs the question,
"What else have we done wrong?"
Provided by
University of Bath
