Microscopic analysis of an unconventional throat bone helps resolve a long-standing debate in paleontology, researchers report, revealing evidence that Nanotyrannus lancensis – long thought by many to be a teenage Tyrannosaurus rex – was in fact a fully mature, distinct species of smaller tyrannosaurid. Because the fossil record is often fragmentary, it is difficult to discern the full range of distinct dinosaur species that lived in ancient ecosystems. This is made more challenging when trying to determine true new species from juveniles of known ones. One of the most persistent debates of this kind concerns Nanotyrannus lancensis – a small tyrannosaur from Cretaceous North America that has been variously interpreted for the past 60 years as either its own species or simply a young Tyrannosaurus rex. This decades-long debate has persisted largely because the N. lancensis holotype exists as an isolated skull, and traditional maturity assessments are often based on limb bones. Until recently, previous studies on other, more complete fossils thought to belong to Nanotyrannus have produced conflicting results. Here, Christopher Griffin and colleagues tested whether the microscopic structure of a small throat bone, the ceratobranchial, can be used to reliably indicate age and skeletal maturity in fossil remains. Griffin et al. found that, in both living archosaur species (i.e., ostrich and crocodilians) and extinct theropods, the ceratobranchial bone preserves clear signals of growth and maturity, making them valuable for determining developmental stage, especially in fully mature individuals. Applying this method to the controversial N. lancensis holotype, the authors examined three sections of its ceratobranchial and discovered several hallmarks of near-complete or complete skeletal maturity. This suggests that N. lancensis is a taxonomically distinct species that coexisted with T. rex, implying a greater predatory diversity in late Cretaceous ecosystems than previously recognized.
