Data for: A diminutive Tyrannosaur lived alongside <em>Tyrannosaurus rex</em>

Whether Nanotyrannus lancensis, a putative species of small tyrannosaurid, represents a distinct taxon or an immature Tyrannosaurus rex is a decades-long controversy. The N. lancesis holotype is an isolated skull and ceratobranchials, but limb osteohistology of Nanotyrannus-like individuals implies that these individuals were immature Tyrannosaurus, suggesting that the Nanotyrannus holotype is also immature. We demonstrate for the first time 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 K-Pg extinction event.