A diminutive tyrannosaur lived alongside Tyrannosaurus rex.

Whether Nanotyrannus lancensis represents a distinct taxon or an immature Tyrannosaurus rex is a decades-long controversy. The N. lancensis holotype (CMNH 7541) is an isolated skull and ceratobranchials, but limb osteohistology of Nanotyrannus-like individuals implies that these individuals were immature Tyrannosaurus, suggesting that the N. lancensis holotype is also immature. We demonstrate that ceratobranchial ("hyoid") histology is useful for ontogenetic assessment in extant and extinct archosaurs. The ceratobranchial histology of the N. lancensis holotype indicates that it was nearing or had reached skeletal maturity, suggesting that it is taxonomically distinct from the coeval Tyrannosaurus rex and that Hell Creek (and equivalent) ecosystems supported a diverse assemblage of predatory dinosaurs approaching the Cretaceous-Paleogene extinction. Editor's summary: Interpreting fossils is tricky business. This can be especially true when determining an animal's age. For over 25 years, debate has persisted about whether a small tyrannosaurid fossil is a juvenile Tyrannosaurus rex or if it is a distinct, and much smaller, species. Determining which scenario is correct was especially challenging because interpreting growth patterns is easiest in postcranial bones, which were missing for the type specimen. Griffin et al. examined a previously ignored bone, the hyoid, and found that it could be used to assess maturity, leading them to conclude that the small type specimen is not a young T. rex, but instead is from a smaller species (see the Perspective by Farke). —Sacha Vignieri [ABSTRACT FROM AUTHOR]