For more than three decades, a small tyrannosaur skull sat at the center of one of paleontology’s most stubborn arguments. Was Nanotyrannus lancensis a real species, or just a young Tyrannosaurus rex that hadn’t finished growing? Two peer-reviewed studies, one published in 2025 in Science and the other in 2025 in Nature, now argue forcefully for the first option: Nanotyrannus was its own animal, a smaller predatory tyrannosaur that lived alongside T. rex in the final stretch of the Cretaceous period, roughly 66 million years ago.
The findings overturn a widely accepted interpretation that had held sway since 2020 and reopen questions about how many apex predators stalked western North America before the asteroid impact wiped them all out.
The bone evidence
The case rests on two independent lines of physical evidence drawn from exceptionally preserved fossils.
The Science study examined the ceratobranchial, a small hyoid bone from the throat region, of the original Nanotyrannus holotype specimen first described by Robert Bakker in 1988. Using histology, the microscopic study of bone tissue, the team identified an external fundamental system (EFS): a dense band of closely spaced growth lines that forms when a vertebrate animal approaches or reaches its maximum body size. “The EFS is unambiguous,” lead author Holly Woodward Ballard of Oklahoma State University told reporters. “This animal was not growing toward T. rex size. It had essentially stopped growing.” The EFS is a well-established biological marker seen across many vertebrate groups, from birds to crocodilians. Its presence in the Nanotyrannus holotype indicates the animal was nearly or fully grown, not a fast-growing juvenile on a trajectory toward multi-ton T. rex proportions.
The Nature study focused on a different specimen entirely: NCSM 40000, the tyrannosaur skeleton recovered alongside a ceratopsian in the famous “Dueling Dinosaurs” fossil pair, now housed at the North Carolina Museum of Natural Sciences. That skeleton is among the most complete tyrannosaur fossils ever found. The research team used comparative anatomy and longitudinal growth modeling, which estimates adult size based on the spacing and number of annual growth marks in bone, to argue the animal had reached near somatic maturity at a body size far smaller than any known adult T. rex.
Together, the two papers attack the same question from different angles, using different specimens and different skeletal elements, and arrive at the same conclusion: the small tyrannosaur fossils previously dismissed as juvenile T. rex belong to a separate lineage.
What they are overturning
The scientific consensus these papers challenge was established by a widely cited 2020 study in Science Advances. That research used limb-bone osteohistology and growth-curve arguments to conclude that specimens resembling Nanotyrannus were juvenile Tyrannosaurus passing through a distinct growth phase. Under that interpretation, young T. rex occupied a different ecological niche than adults, which explained why the smaller specimens looked so anatomically different.
The 2025 papers directly counter that body of evidence. Where the 2020 study extrapolated growth curves from limb bones and projected them onto the known T. rex growth trajectory, the newer work argues those projections assumed the answer: if the specimens don’t actually belong to T. rex, fitting them onto a T. rex growth curve is circular. The EFS in the throat bone and the growth modeling of the “Dueling Dinosaurs” skeleton both point to animals that had stopped growing well short of T. rex size.
A publisher correction was issued for the Nature paper, but it addressed only copyright and licensing details and did not alter the scientific conclusions.
What remains uncertain
Strong as the maturity evidence is, the debate is not over. Limb-bone osteohistology and hyoid histology can yield different signals depending on how growth rates vary across the skeleton, and paleontologists have not yet agreed on which tissue provides the most reliable maturity indicator for tyrannosaurs. No public response from the authors of the 2020 study has appeared as of June 2026. Their reaction, and any reanalysis they undertake, will likely shape whether the field fully accepts the revival of Nanotyrannus or continues to contest it.
Sample size is another problem. Only a handful of fossils preserve the right combination of skull, limb, and throat bones to be confidently assigned to Nanotyrannus. Many smaller tyrannosaur bones in museum collections have historically been cataloged as juvenile T. rex without detailed histological study. Systematically re-examining those specimens could either expand the known Nanotyrannus sample or reveal that some bones do match a juvenile T. rex growth stage after all.
There is also the perennial challenge of defining species boundaries in the fossil record. Paleontologists typically rely on consistent anatomical differences that cannot be explained by age, sex, or individual variation. The 2025 studies argue that features of the skull, limbs, and throat bones, combined with maturity signals, cross that threshold. Skeptics may counter that tyrannosaurs were more variable than the current sample suggests, or that environmental stress could have produced stunted individuals mimicking a smaller species. Resolving that question will require more fossils and more statistical work on variation within known tyrannosaur populations.
Ecological stakes
If Nanotyrannus holds up as a genuine species, the ecological picture of late Cretaceous North America gets more crowded and more interesting. Instead of a single dominant tyrannosaur at the top of the food chain, with juveniles filling a mid-sized predator role, there were at least two distinct tyrannosaur species competing for prey during the Maastrichtian stage, the final interval before the mass extinction.
That matters for models of predator-prey dynamics and food-web structure. A separate mid-sized predator species would have had its own population dynamics, hunting strategies, and prey preferences, rather than simply being a developmental stage of T. rex. But reconstructing those dynamics requires more than a taxonomic name. Population estimates, geographic range data, and dietary analysis for Nanotyrannus remain thin, and the fossil record may not yet support firm ecological conclusions.
Where the burden of proof now sits
For readers trying to evaluate the competing claims, the core question is straightforward: were these small tyrannosaurs still growing rapidly, or had they nearly stopped? The 2020 study said still growing. The 2025 studies say nearly stopped. Both sides present peer-reviewed data, but the recent work benefits from more diverse skeletal samples, refined histological techniques, and access to exceptional fossils like the “Dueling Dinosaurs” pair that were not available to earlier researchers.
That does not automatically make the new interpretation correct, but it does mean the burden of proof has shifted. Where Nanotyrannus was once widely dismissed as a misidentified juvenile, it now stands as a serious, evidence-backed contender for recognition as a genuine smaller tyrannosaur.
Future research will likely focus on three fronts: expanding histological sampling across tyrannosaur skeletons, including museum specimens that have never been sectioned; running more comprehensive statistical comparisons of skull and limb proportions across a wide range of tyrannosaur sizes; and hoping for new fossil discoveries from late Cretaceous deposits that could fill gaps in the record.
Until that work is done, anyone declaring this debate “settled” is getting ahead of the science. But the direction of the evidence has clearly changed. Nanotyrannus, the little tyrannosaur that paleontology tried to erase, is back.
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*This article was researched with the help of AI, with human editors creating the final content.
