To date, eco-evolutionary dynamics in the ascent of tyrannosauroids to top predator roles have been obscured by a 70-million-year gap in the North American (NA) record. Here we report discovery of the oldest Cretaceous NA tyrannosauroid, extending the lineage by ~15 million years. The new taxon—Moros intrepidus gen. et sp. nov.—is represented by a hind limb from an individual nearing skeletal maturity at 6–7 years. With a ~1.2-m limb length and 78-kg mass, M. intrepidus ranks among the smallest Cretaceous tyrannosauroids, restricting the window for rapid mass increases preceding the appearance of colossal eutyrannosaurs. Phylogenetic affinity with Asian taxa supports transcontinental interchange as the means by which iconic biotas of the terminal Cretaceous were established in NA. The unexpectedly diminutive and highly cursorial bauplan of NA’s earliest Cretaceous tyrannosauroids reveals an evolutionary strategy reliant on speed and small size during their prolonged stint as marginal predators.
Bibliographical noteFunding Information:
We thank A. Giterman and L. Herzog for exceptional preparation of the holotype specimen; staff, students, and volunteers of the 2012–2015 MDP expeditions for repeated explorations of the site; M. Leschin, S. Foss, G. McDonald, R. Hunt-Foster, staff of the Price Field Office, and the BLM for permitting and expedition support; the Canyonlands Natural History Association for funding fieldwork and analytics in 2014–2015; and Stellenbosch University Department of Earth Sciences and the DRD for travel support. Special thanks to V. Arbour and K. Seymour for assistance checking character states on ROM ornithomimid specimens. We also thank Rich Cifelli, Gregory Funston, and an anonymous reviewer for helpful critiques. The Keyence imaging system was purchased for the NCMNS through a generous gift from the estate of Renaldo Kuhler.
© 2019, The Author(s).