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 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.
Highlights
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
The timing and tempo of trophic restructuring within Late Cretaceous terrestrial ecosystems, including the extinction of allosaurian megapredators and the ascent of tyrannosauroids to top predator roles, are poorly constrained. Teasing out these eco-evolutionary dynamics requires a comprehensive record of tyrannosauroid biodiversity and relatively contiguous spatiotemporal data, during the poorly sampled interval spanning the late Early to early Late Cretaceous, or ‘mid-Cretaceous’, during which time this niche shift and associated anatomical transformations are hypothesized to have occurred[6,7,8]
Whereas the Cretaceous record of Asian tyrannosauroids is rapidly growing, and significant progress has recently been made understanding the timing of major evolutionary transitions within Asian representatives of the clade[5,9], the pre-Campanian record in North America (NA) has remained entirely devoid of diagnostic body fossils
Summary
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. The timing and tempo of trophic restructuring within Late Cretaceous terrestrial ecosystems, including the extinction of allosaurian megapredators and the ascent of tyrannosauroids to top predator roles, are poorly constrained Teasing out these eco-evolutionary dynamics requires a comprehensive record of tyrannosauroid biodiversity (currently upwards of 30 valid species) and relatively contiguous spatiotemporal data, during the poorly sampled interval spanning the late Early to early Late Cretaceous, or ‘mid-Cretaceous’, during which time this niche shift and associated anatomical transformations are hypothesized to have occurred[6,7,8]. The new taxon represents the oldest Cretaceous diagnostic tyrannosauroid on the continent, extending the definitive record of this clade by ~15 million years, and providing a key datum for untangling evolutionary, paleobiogeographic, and ecological dynamics during a turbulent interval in Earth’s history
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