Nanotyrannus lansensis — For years, many believed these specimens were juvenile tyrannosaurus rex — However, a team of American paleontologists analyzed the ceratobranchia (hyoid bone) and concluded that it was actually a distinct, fully grown small tyrannosaurid. This finding shed light on the growth history of Nanotyrannus lansensis.
Encounter between adults of Nanotyrannus lansensis (left) and two juvenile tyrannosaurus rex, with a sub-adult tyrannosaurus rex watching from a distance. Image credit: Jorge Gonzalez.
Nanotyrannus lansensis measures 5.5 meters (18 feet) tall, which is relatively small compared to its length of over 12 meters (40 feet) compared to tyrannosaurus rex.
Initially discovered in 1942, Nanotyrannus lansensis was first classified as gorgosaurus, but through further scientific investigation, it was redefined as a separate species in 1988.
The main cause of the lengthy debate over the classification is that the holotype consists solely of an isolated skull, while traditional assessments of maturity often rely on limb bones.
Until recently, studies on other more complete fossils of Nanotyrannus lansensis yielded conflicting conclusions.
In a recent study, paleontologist Christopher Griffin from Princeton University and his team discovered that the microstructure of small bones in the throat ceratobranchial can accurately indicate the age and skeletal maturity of fossil specimens.
“At the time, there was a general consensus that the holotype skull of Nanotyrannus represented an immature tyrannosaurus rex rather than a distinct species,” Dr. Griffin stated.
“We initially aimed to follow that consensus, but upon examining samples of the hyoid bone and noticing characteristics strongly indicating maturity, we realized we needed to reevaluate that notion.”
The researchers found that in both modern archosaurs (like ostriches and crocodiles) and extinct theropods, the ceratobranch bones retain unique signals of growth and maturation, making them particularly useful for determining the developmental stage of fully mature individuals.
Using this technique, the team studied the holotype of Nanotyrannus lansensis and analyzed three sections of its ceratobranchia, identifying several features that suggest near-complete skeletal maturity.
This indicates that Nanotyrannus lansensis is a taxonomically distinct species that coexisted with tyrannosaurus rex, suggesting a greater diversity of predators in Late Cretaceous ecosystems than previously understood.
“This smaller-bodied tyrannosaurus rex indicates that the carnivore’s hyoid displayed a growth pattern suggesting it had reached or was close to maturity,” remarked paleontologist Ashley Poust of the University of Nebraska State Museum.
“We can now confidently affirm our classification of Nanotyrannus as separate from tyrannosaurus rex.”
“This implies the presence of at least two carnivores of different sizes in the same environment, which has significant implications for dinosaur ecology and extinction,” he added.
“Understanding what existed helps us gauge the size of the fossil record and how species have evolved over time.”
“Furthermore, it’s crucial to comprehend the complexity of the ecosystem.”
Results from this study are published in the journal Science.
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Christopher T. Griffin et al. A small Tyrannosaurus rex lived alongside tyrannosaurus rex. Science published online on December 4, 2025. doi: 10.1126/science.adx8706
Source: www.sci.news
