A new study published this week in the journal Science Advances has reignited the longstanding paleontological debate over whether the dinosaur Nanotyrannus is actually a separate genus or merely juveniles of the mighty Tyrannosaurus rex.
Key finding – skulls thought to be Nanotyrannus are more gracile
The research team, led by Denver Fowler from the Museums of Western Colorado, conducted a detailed analysis of multiple small tyrannosaurid skulls held in museum fossil collections that had previously been classified as either juvenile T. rex or the distinct genus Nanotyrannus.
Surprisingly, their examination uncovered key differences in the skulls that set them apart as more gracile or slender compared to confirmed T. rex specimens. This indicates they likely represent a separate species or even genus rather than juvenile T. rex as long presumed.
“The morphological differences we see between the specimens indicate that the group of smaller tyrannosaurids are not T. rex juveniles, but a distinct species with a slender build more adapted for speed and agility,” said Fowler.
| Specimen | Previous Classification | New proposed classification |
|---|---|---|
| LACM 28471 | Juvenile T. rex | New species of small tyrannosaurid |
| CMNH 7541 | Nanotyrannus | New species of small tyrannosaurid |
Implications for tyrannosaur taxonomy and biology
If the conclusions of the study stand up to further scrutiny, it will require extensive revisions to the taxonomic classification of Late Cretaceous North American tyrannosaurids.
“This changes our whole understanding of the size ranges and ecological niches occupied by tyrannosaurs. We previously envisioned T. rex as occupying the large bodied apex predator niche, with smaller tyrannosaurids like Gorgosaurus and Albertosaurus in the mid-size predator niche. This newly recognized small tyrant throws that neat scenario into disarray,” explained Dr Thomas Carr from Carthage College, an expert on tyrannosaur taxonomy.
The identification of a new small agile tyrannosaurid species may also impact our understanding of tyrannosaur biology and development.
“We used to think that T. rex went through a dramatic morphological shift during adolescent growth, transitioning from a sleek and nimble juvenile into the gigantic bone-crushing adult form we all recognize. But these fossils indicate that the juvenile characteristics persist into small adult tyrannosaurs that maintain a gracile form. This opens up a new field of inquiry into tyrannosaur development and variation,” said Carr.
Controversy and doubts remain among some experts
However, the study has proved controversial, with some experts in the field of tyrannosaur paleontology remaining unconvinced that the fossils represent a distinct genus.
“I am cautious about overstating the diversity of Late Cretaceous North American tyrannosaurs. There is a huge amount of variation during growth in tyrannosaur skulls, so we cannot rule out that these specimens fall within that range for T. rex. Extraordinary claims require extraordinary evidence, and I’m not yet convinced,” commented Dr Jack Horner of the Burke Museum of Natural History.
His skepticism was echoed by Prof. Thomas Holtz Jr from the University of Maryland. “While an intriguing idea, I feel more data is needed before concluding that Nanotyrannus, previously considered a juvenile T. rex, is indeed valid. The study did not sufficiently rule out individual, sexual, and ontogenetic variation within T. rex as a possible explanation,” he wrote on Twitter.
This lack of consensus among top paleontologists suggests the debate over Nanotyrannus will continue to rage on until more definitive evidence surfaces.
Reaction from museums – plans to update exhibits
The new findings have major implications for fossil specimens held in natural history museums that have been displayed as juvenile T. rex fossils for decades. These institutions will now need to re-label or re-configure these displays to represent the newly described taxon.
At the Smithsonian National Museum of Natural History, curator Dr. Hans Sues responded enthusiastically to the development. “The skull specimens we have featured in our Tyrannosaurs: Meet the Family exhibit since 2014 will clearly require re-examination and potentially new labels to reflect this study. It’s always thrilling when new scientific insights lead to changes in how we educate the public about paleontological discoveries,” he said.
However, at the Carnegie Museum of Natural History, the source of the original Nanotyrannus holotype specimen described in 1988, curator Dr. Matthew Lamanna took a more cautious attitude. “We cannot make hasty changes to a display that has existed for over 30 years and has been viewed by over 10 million people based solely on one contentious study. We await feedback from more peer reviewers of the research before determining if significant modifications to the exhibit explaining these fossils is truly warranted,” he noted.
Where did the debate originate?
The debate has its roots in a 1988 study describing the dinosaur Nanotyrannus based on a skull collected from Montana in 1942. Paleontologist Robert Bakker and colleagues noted its similarities but also key differences compared to juvenile T. rex cranial material, and assigned it to a new genus.
In the 1990s, further apparent Nanotyrannus skulls were described from New Mexico and Wyoming. But throughout this period many paleontologists argued these all likely represented specimens of juvenile T. rex.
Most infamously, a 2001 study by Thomas Carr conclusively declared Nanotyrannus a juvenile T. rex. The debate then quietened for over a decade, with Nanotyrannus often excluded from listings of valid tyrannosaurid genera.
This latest study revisits the original 1980s debate with fresh fossil evidence and analysis. Time will tell whether the new claims shift consensus back towards recognition of Nanotyrannus as a distinct tyrannosaur.
What could settle the debate?
Further clarity could come from several lines of new evidence that may emerge:
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More complete skeletons – the current debate centers around fragmentary skull and tooth material. Recovery of more complete specimens would provide added context.
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Histological bone analysis – microscopic study of bone cell structure and growth lines could reveal growth stage and maturity.
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Discovery of adult specimens – definitive confirmation of mature specimens of Nanotyrannus would prove it is not merely juvenile T. rex.
But finding such fossils is challenging. “Complete tyrannosaur skeletons are exceptionally rare,” explains Fowler. “Many lines of evidence point towards these [studied skulls] representing adults. But an unambiguous large individual would seal the deal.”
Until such evidence arises, lively debate amongst paleontologists is set to continue as they pick apart the study and argue the true identity of Nanotyrannus. This scientific discourse will ultimately enrich our understanding of the rulers of the Late Cretaceous.
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