For 40 years, dinosaur experts have been locked in a huge debate over diminutive fossils found in the Western United States. Did they belong to a teenage version of Tyrannosaurus rex, or to another species in the tyrannosaur family entirely?
A new study may have settled the dispute. Paleontologists said in the journal Nature on Thursday that a fossil specimen in a North Carolina museum belonged to Nanotyrannus lancensis, part of a distinct group of tyrannosaurs.
“We have this animal that’s been hiding in plain sight, and it raises all kinds of questions that we as paleontologists weren’t asking until now,” said Lindsay Zanno, a paleontologist at the North Carolina Museum of Natural History and an author of the study.
In the 1980s, fossil hunters found a small tyrannosaur skull from the same rock formation that had produced T. rex, said Thomas Holtz, a tyrannosaur specialist at the University of Maryland who was not involved in the paper. Researchers initially identified the skull as a distinct dinosaur species.
But in 1999, a group of researchers argued that the skull and similar specimens belonged to T. rex as a teenager, suggesting the more famous species underwent an extraordinary growth spurt before its fearsome adulthood.
Scientists used later discoveries, like the relatively complete “Jane” found in 2001, to bolster their competing sides of the argument. However, the “teen rex” hypothesis remained dominant.
What made the question so difficult to resolve — and so contentious and emotionally charged — was that none of the smaller tyrannosaur specimens were finished growing, Dr. Holtz said. If Nanotyrannus was really a separate animal, then where were the adults?
The turning point came in 2006, Dr. Zanno said. Paleontologists dug up fossils in Montana known as the “dueling dinosaurs,” which were eventually acquired by the North Carolina Museum of Natural Sciences in 2020. There, the small tyrannosaur in the pair could be properly studied (the identity of the other animal, a triceratops, was not in doubt).
Like most scientists, Dr. Zanno initially thought the dinosaur was a juvenile T. rex. But soon she and her colleague, James Napoli, realized they might have something different.
To nail down an identity, they cut into the limb bones of the specimen to count the growth rings, and examined its spinal fusions and developmental anatomy. They were able to show that the animal was 20 years old when it died, and was a nearly mature adult.
Dr. Zanno and Dr. Napoli also traveled around the world to study more than 200 tyrannosaur fossils for comparison. That helped them identify anatomical differences from T. rex in the dueling dinosaurs specimen, Dr. Napoli said. It had significantly larger forelimbs and hands than those found on T. rex, and a hint of a vestigial third finger. The skull had more teeth, and CT scans showed distinct patterns of skull nerves. The tail had fewer vertebrae. All these traits are set early in an animal’s development.
“For it to change from that to an adult tyrannosaurus would defy our knowledge of how vertebrates grow,” Dr. Zanno said. “it just doesn’t hold up as a teen rex.”
The team’s analyses show that Nanotyrannus most likely belongs to an early offshoot of the tyrannosaur lineage that split 100 million years ago, around the time that an inland sea broke North America into separate islands. It may be more closely related to the East Coast branch of the family.
“In a very real way, this discovery overturns decades of research on the growth and biology of Tyrannosaurus rex,” Dr. Zanno said, particularly assumptions that the animals changed radically as they grew, and that T. rex was the lone large predator of its environment.
The team also found evidence that Jane — long considered an immature T. rex — was in fact yet another species in the Nanotyrannus genus. The team named this unexpected animal Nanotyrannus lethaeus.
Dr. Holtz — despite taking issue with certain details in the study — called the paper’s identification of the elusive adult Nanotyrannus a decisive blow.
Dave Hone, a paleontologist at Queen Mary University of London who was not involved in the study, added that “this represents a real shift in the current balance of the evidence.” The findings are enough to partially convince some skeptics.
Thomas Carr, a paleontologist at Carthage College in Wisconsin who has long been a proponent of the “teen rex” theory, called the paper “pretty conclusive” in its identification of the dueling dinosaurs specimen. But he disputed the team’s suggestion that Jane represented another such species.
If all of the small tyrannosaur specimens from western North America are Nanotyrannus, Dr. Carr added, “then where are the juvenile T. rex? That part of the picture doesn’t add up.”
There are many more questions to answer going forward, said Steve Brusatte, a paleontologist at the University of Edinburgh who was also a longtime skeptic of the animal’s existence.
But “the overarching mic drop of this paper,” Dr. Brusatte added, “is that Nanotyrannus is real.”
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