Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous.

Abstract

Tyrannosaurus rex ranks among the most comprehensively studied extinct vertebrates¹ and is a model system for dinosaur palaeobiology¹. As one of the last surviving non-avian dinosaurs, Tyrannosaurus is a crucial datum for assessing terrestrial biodiversity, ecosystem structure and biogeographic exchange immediately preceding the end-Cretaceous mass extinction-one of Earth's greatest biological catastrophes. Palaeobiological studies of Tyrannosaurus, including ontogenetic niche partitioning^2-4, feeding, locomotor biomechanics^5,6 and life history^7-9 have drawn upon an expanding skeletal sample comprising multiple hypothesized growth stages-and yet the Tyrannosaurus hypodigm remains controversial^10-13. A key outstanding question relates to specimens considered to exemplify immature Tyrannosaurus^1,14-19, which have been argued to represent the distinct taxon Nanotyrannus^11,13,20,21. Here we describe an exceptionally well-preserved, near somatically mature tyrannosaur skeleton (NCSM 40000) from the Hell Creek Formation that shares autapomorphies with the holotype specimen of Nanotyrannus lancensis. We couple comparative anatomy, longitudinal growth models, observations on ontogenetic character invariance and a novel phylogenetic dataset to test the validity of Nanotyrannus, demonstrating conclusively that this taxon is distinguishable from Tyrannosaurus, sits outside Tyrannosauridae, and unexpectedly contains two species-N. lancensis and Nanotyrannus lethaeus sp. nov. Our results prompt a re-evaluation of dozens of existing hypotheses based on currently indefensible ontogenetic trajectories. Finally, we document at least two co-occurring, ecomorphologically distinct genera in the Maastrichtian of North America, demonstrating that tyrannosauroid alpha diversity was thriving within one million years of the end-Cretaceous extinction.