"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 N. 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 speciesN. lancensis and N. lethaeus, sp. nov."
"Tyrannosaurus rex ranks among the most comprehensively studied extinct vertebrates1 and a model system for dinosaur paleobiology1. 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. Paleobiological studies of Tyrannosaurus, including ontogenetic niche partitioning2-4, feeding, locomotor biomechanics,5,6and life history7-9 have drawn upon an expanding skeletal sample comprising multiple hypothesized growth stagesand yet the Tyrannosaurus hypodigm remains controversial10-13."
An exceptionally well-preserved, near somatically mature tyrannosaur skeleton (NCSM 40000) from the Hell Creek Formation shares autapomorphies with the holotype of N. lancensis. Comparative anatomy, longitudinal growth models, ontogenetic character invariance observations, and a novel phylogenetic dataset were used to assess Nanotyrannus validity. Nanotyrannus is distinguishable from Tyrannosaurus and is positioned outside Tyrannosauridae. The genus unexpectedly contains two species, N. lancensis and N. lethaeus sp. nov. Dozens of existing hypotheses based on current ontogenetic trajectories require re-evaluation. At least two co-occurring tyrannosaur species are documented.
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