Two new peer-reviewed papers have converged on the same conclusion: the small tyrannosaur known as Nanotyrannus was a fully grown predator that shared its world with Tyrannosaurus rex, not a juvenile version of the larger animal. The finding overturns a widely held interpretation that had persisted for years and redraws the picture of predator diversity during the final stretch of the Cretaceous period. The key breakthrough came from an unlikely source: a tiny throat bone called the ceratobranchial, part of the hyoid apparatus, which provided the first reliable maturity signal from the Nanotyrannus holotype skull specimen cataloged as CMNH 7541.
Why a throat bone settled a decades-long dinosaur fight
The Nanotyrannus debate has always been stuck on one problem. The holotype is an isolated skull with no limb bones attached. Without long bones, standard growth-ring analysis could not determine whether the animal was still growing or had reached adult size. A team including researchers from Yale and Princeton solved this by developing an osteohistological technique that uses the microstructure of the ceratobranchial to assess maturity in archosaurs, the group that includes crocodilians, birds, and dinosaurs. They validated the method on living archosaurs first, then applied it to CMNH 7541. The hyoid showed clear maturity markers, meaning the animal had stopped growing. It was an adult, not a teenager.
A separate study published in Nature presented anatomical and phylogenetic evidence from a well-preserved Nanotyrannus-like individual, concluding that two tyrannosaur species coexisted at the close of the Cretaceous. That work compared skull proportions, tooth counts, and limb elements, and it found consistent differences that could not be explained by simple growth-related changes in T. rex. The authors argued that these traits, taken together, better fit the pattern of a distinct small-bodied tyrannosaur than of an immature giant.
The Nature paper has since received a minor publisher correction that adjusted its copyright notice, but the scientific content and conclusions remain unchanged. Together, the two papers attack the problem from different angles-bone microstructure in one, comparative anatomy and evolutionary relationships in the other-and arrive at the same answer: Nanotyrannus was not just a growth stage.
The earlier opposing view had its own osteohistological basis. A 2019 paper in Science Advances titled “Growing up Tyrannosaurus rex” argued that Nanotyrannus-like specimens were juvenile T. rex based on growth-stage reasoning from bone microstructure. That study interpreted thin-section data from limb bones of similar small tyrannosaur specimens as evidence of rapid, incomplete growth, consistent with young animals. The new research does not dispute those observations outright but contends that the ceratobranchial offers a more direct maturity signal for a specimen that lacks limbs entirely. In other words, the conflict is less about whether growth rings exist and more about which bones provide the most reliable window into an individual’s life history.
Ceratobranchial maturity tests and what they could reveal next
The hyoid-based method raises an immediate question for the rest of the tyrannosaur fossil record. Dozens of tyrannosaur specimens from the Two Medicine and Hell Creek Formations span a wide range of body sizes. Many lack associated hyoid bones, and their placement on the growth curve of T. rex has relied on limb-bone histology and overall body proportions. If the ceratobranchial criteria were applied to fossils that do preserve hyoids, some of those size classes could resolve as separate adult species rather than growth stages of a single animal.
The Yale team has noted that the holotype’s status as an isolated skull made the hyoid the first reliable maturity signal available for that specimen. Without limb bones, they argued, prior attempts to classify CMNH 7541 as juvenile or adult were necessarily indirect. The new sections from the ceratobranchial show features such as slowed or halted growth at the bone’s outer margin, patterns that in living archosaurs correlate with reproductive maturity and the end of major size increases. That correlation, built from modern animals, underpins the claim that the Nanotyrannus holotype was fully grown.
Princeton researchers have emphasized the practical difficulty of this kind of work. Destructive sampling of a holotype, the single specimen that defines a species, is an exceptional step that required careful justification and institutional permission. Removing even a small piece of bone permanently alters a unique fossil, and curators tend to be cautious. In this case, the team argued that resolving the long-running taxonomic dispute justified the risk, particularly because the sampled element was relatively small and not central to the skull’s outward appearance.
The validation dataset built across living archosaurs and other theropods was summarized in the published papers, though the full primary tables have not been released as standalone files. Instead, the authors provided aggregated statistics, representative micrographs, and methodological descriptions that allow other specialists to evaluate the approach in broad terms. The analytical data, results, and code supporting the Nature paper have been deposited in a Figshare repository, which lets independent researchers inspect the phylogenetic matrices, character scorings, and model settings that led to the conclusion of two coexisting tyrannosaur species.
Those open data help address one of the main criticisms often leveled at dinosaur taxonomy: that species diagnoses can hinge on a handful of poorly documented skeletal traits. By making their comparative framework and computational tools accessible, the authors invite others to rerun the analyses, tweak assumptions, and test alternative groupings. If future work identifies different optimal trees or clusters, the debate will at least be grounded in shared, inspectable inputs rather than opaque, lab-specific datasets.
Open questions about late-Cretaceous predator diversity
The two papers together make a strong case, but several loose ends remain. The full raw osteohistological images and section counts from CMNH 7541 are referenced in the Science supplement rather than deposited in the Figshare record tied to the Nature study. That means independent verification of the hyoid maturity assessment requires access to the supplementary materials of one journal to check claims made in another. For researchers without institutional subscriptions or interlibrary loan support, those logistical hurdles may slow down close scrutiny of the new method.
There are also gaps in the documentation surrounding the new, reportedly complete Nanotyrannus-like specimen at the heart of the Nature analysis. No primary field notes or collection permits have yet been published alongside the main article, leaving some uncertainty about the exact stratigraphic position and taphonomic context of the find. Those details matter because the diversity of predators in late-Cretaceous North America is partly reconstructed from where, and in what rock layers, different fossils appear. Precise horizon data could clarify whether Nanotyrannus overlapped with T. rex throughout its range or only in certain sub-basins or time slices.
Another open question concerns ecological roles. If Nanotyrannus was a small-bodied adult predator, then the Hell Creek ecosystem supported at least two apex or near-apex tyrannosaur species at once, plus smaller carnivores such as dromaeosaurs. That raises the issue of niche partitioning: how did multiple large meat-eaters avoid direct competition for the same prey? Subtle differences in skull shape and limb proportions hint that Nanotyrannus may have been faster and more agile, perhaps specializing in smaller, quicker prey than the bone-crushing T. rex. But those inferences rest on limited material and would benefit from more complete skeletons and biomechanical modeling.
Finally, the new findings feed into a broader methodological shift in paleontology. As more teams combine traditional comparative anatomy with microstructural and statistical approaches, long-standing taxonomic debates are being revisited. The Nanotyrannus case illustrates both the promise and the friction of that process. New tools can extract surprising information from neglected bones, but they also demand careful calibration, transparent data sharing, and a willingness to revise cherished interpretations when fresh evidence appears.
For now, the ceratobranchial of CMNH 7541 has tipped the scales toward recognizing Nanotyrannus as a distinct, fully grown predator that stalked the same Late Cretaceous landscapes as T. rex. Whether further discoveries will reinforce that picture or complicate it again will depend on what future field seasons, CT scans, and thin sections reveal about the last generation of non-avian dinosaurs.
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*This article was researched with the help of AI, with human editors creating the final content.
