A diminutive tyrannosaur lived alongside <i>Tyrannosaurus rex</i>.

Abstract

Whether <i>Nanotyrannus lancensis</i> represents a distinct taxon or an immature <i>Tyrannosaurus rex</i> is a decades-long controversy. The <i>N. lancensis</i> holotype (CMNH 7541) is an isolated skull and ceratobranchials, but limb osteohistology of <i>Nanotyrannus</i>-like individuals implies that these individuals were immature <i>Tyrannosaurus</i>, suggesting that the <i>N. lancensis</i> 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 <i>N. lancensis</i> holotype indicates that it was nearing or had reached skeletal maturity, suggesting that it is taxonomically distinct from the coeval <i>Tyrannosaurus rex</i> and that Hell Creek (and equivalent) ecosystems supported a diverse assemblage of predatory dinosaurs approaching the Cretaceous-Paleogene extinction.