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.