Pivotal new dinosaur research has finally settled a 40-year-old debate: Was the small tyrannosaurus Nanotyrannus a separate species or just a teenage Tyrannosaurus rex? Now, an astonishingly complete fossil reveals that Nanotyrannus was real.
Many paleontologists had long believed that the key fossil was a juvenile Tyrannosaurus rex that lived in western North America 67 to 66 million years ago. However, rather than settling all debates, this “nano” discovery opens a new chapter in the understanding of Tyrannosaurus rex biology and further debate.
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“This is one of the most controversial topics in all of dinosaur paleontology,” study co-author Lindsay Zanno, a paleontologist at North Carolina State University and the North Carolina Museum of Natural Science, told Live Science.
Aside from an isolated skull, the best skeleton of one of these small-bodied tyrannosaurs was collected from the Hell Creek Formation, which also spans parts of North Dakota, South Dakota, and Wyoming. The specimen, known as Jane, was still growing rapidly and was about 11 years old at the time of death, and differed from the only skull in several ways.
Now, Zanno and study co-author James Napoli, an anatomist at Stony Brook University in New York, have described a complete skeleton of a Tyrannosaurus rex that is part of the “Dueling Dinosaur” fossil, a 67-million-year-old tyrannosaurid fossil that is believed to be the most complete on record, but a smaller Tyrannosaurus and a Triceratops that may have been trapped in combat at the time of its death.
In the case of Nanotyrannus
Zanno and Napoli say this “dueling dinosaur” Tyrannosaurus skeleton, also from the Hell Creek Formation, is not a Tyrannosaurus rex, but instead shares features with the skull of N. lancensis. Importantly, analysis of their bone growth rings, spinal fixation data, and developmental anatomy shows that these fearsome dinosaurs were not juveniles, but were approximately 20 years old and nearly fully grown at the time of their deaths.
“We were able to take a thin section of limb bone from this animal and determine that it was actually a nearly full-grown Tyrannosaurus, even though it was only half the length and about one-tenth the mass of a full-grown Tyrannosaurus,” Zanno said.
It weighed only 1,500 pounds (700 kg), while an adult Tyrannosaurus rex weighed around 14,700 to 18,000 pounds (6,700 to 8,200 kg). They also have large forelimbs, more teeth, fewer caudal vertebrae, and unique neural patterns in their skulls. The researchers published their findings Thursday (October 30) in the journal Nature.
Zanno said the two dinosaurs would have had completely different ecologies. Tyrannosaurus was a bulky predator with a huge skull, powerful bite, and banana-shaped serrated teeth. Nanotyrannus was smaller and slender, faster and more agile, with enlarged hands and claws used to capture prey, she said.
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paleontologist answers
With this new evidence, the broader research community appears convinced that this small dinosaur and Tyrannosaurus are different species.
“Basically and on balance, it looks pretty solid,” paleontologist Dave Horne of Queen Mary University of London told Live Science. “I, and many others who have argued that we don’t think Nanotyrannus is legitimate, have always said that the main reason for that is that there just isn’t an obvious adult small skeleton, which is obviously a pretty big problem. And this really, really looks like an adult small skeleton.”
Steve Brusatte, a paleontologist at the University of Edinburgh in Scotland, agrees. “As a long-time researcher of tyrannosaurids, I have believed that the series of smaller skeletons found in the same rocks as the famous skeletons of giant tyrannosaurids were juvenile tyrannosaurids rather than characteristically smaller species,” he told LiveScience in an email. “Evidence from this stunning new specimen shows that I was wrong, at least in part. The case for Nanotyrannus, a type of long-armed tyrannosaurus smaller than Tyrannosaurus, appears strong and I think it has now been proven beyond a reasonable doubt.”
Thomas Carr, a paleontologist at Carthage College in Wisconsin, has previously argued that the fossils are all juvenile Tyrannosaurus rex, but now he has changed his mind. “I think they’ve conclusively shown that Duelist is a small adult tyrannosaurus, so I’m totally fine with that,” he told Live Science.
However, Carr disagrees with the family tree proposed by Zanno and Napoli. This family tree includes Nanotyrannus as a more primitive group outside of the tyrannosaurid family. He said the specimen should be considered a sister species to Tyrannosaurus rex and be renamed Tyrannosaurus lansensis.
Is Jane a new species?
Other parts of Zanno and Napoli’s paper are more controversial. They examined more than 200 other tyrannosaur fossils and said Jane’s skeleton was different from both tyrannosaurs and the dueling N. lancensis. Jane was slightly larger than Duelist, had a distinctive sinus pattern on the roof of her mouth, and had a different shape of the bone behind her eyes.
This suggests that Jane is a new species, Nanotyrannus letheus (named after the River Lethe in Greek mythology), but it has not yet been fully explained.
“Maybe they were separated over time, or maybe they overlapped, but we don’t know that yet,” Zanno said.
Unless there are further discoveries that further clarify Jane’s anatomical features, the distinction here is sufficient to justify two species, Thomas Holtz, a paleontologist at the University of Maryland, told Live Science in an email.
However, many researchers remain unsure that Jane is a new species. “This second described species of Nanotyrannus is based on a small skeleton that has clearly not stopped growing, so I think it’s frankly very difficult to tell whether this is a Nanotyrannus or a juvenile Tyrannosaurus,” Brusatte said.
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“I look at the evidence differently, and that is that Jane is a juvenile Tyrannosaurus rex,” said Kerr, who has studied Jane extensively.
The new study suggests that multiple tyrannosaurus species shared the same western North American ecosystem for the last million years before the asteroid impact about 66 million years ago, Zanno said.
“There is certainly no a priori problem of multiple species of carnivores existing at the same time,” Horn says. “It was so weird not having anyone else around.”
But if, as Zanno and Napoli suggest, most of the small tyrannosaurs at Hell Creek are non-tyrannosaurid species, that means there is a dearth of juvenile skeletons that have been analyzed as clearly tyrannosaurid.
“Tyrannosaurus has been running around for millions of years,” Horn says. “They’re huge, we found a lot of adults, and they don’t suddenly exist at 10 meters. [33 feet] 5 tons long [5.5 tons]. So where are the boys? ”
This lack of juvenile specimens also means we need to reevaluate our ideas about how tyrannosaurs grew. Previous thinking was that this species changes very dramatically as it reaches maturity. “Nanotyrannus has been used for decades as data to understand Tyrannosaurus and its biology, so we need to rethink much of what we know about Tyrannosaurus life history, growth, and paleontology,” Zanno said.
She, Horne, and Carr suggest that a model for the growth of Tyrannosaurus should be based on the development of one of its closest relatives, a Mongolian dinosaur called Tarbosaurus bataar. There are many skeletons of Tarbosaurus bataar, ranging from babies to adults. Young Tarbosaurus specimens look more like scaled-down adults than have major skeletal differences.
“The most important point of this paper is that Nanotyrannus is real and is itself a distinct tyrannosaurid species, which requires a fundamental reassessment of tyrannosaurid taxonomy and evolution,” Brusatte said.
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