Moros intrepidus: North America's Tiny Tyrannosaur

Diminutive, fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record

Published in Communications Biology
21 Feb 2019
DOI: 10.1038/s42003-019-0308-7

Authors: Lindsay Zanno, Terry Gates, Aurore Canoville, Haviv Avrahami, North Carolina State University; Peter Makovicky, Field Musem; Ryan Tucker, Stellenbosch University

Abstract:
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 taxonMoros intrepidus gen. et sp. nov.is represented by a hind limb from an individual nearing skeletal maturity at 67 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 NAs earliest Cretaceous tyrannosauroids reveals an evolutionary strategy reliant on speed and small size during their prolonged stint as marginal predators.

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