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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.

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A new species of the hadrosaurine hadrosaurid Gryposaurus was discovered in the late Campanian Kaiparowits Formation of southern Utah. Gryposaurus monumentensis, sp. nov. is distinguished from other Gryposaurus species by possessing a more robust skull, enlarged clover-shaped prongs on the predentary oral margin, an anteroposteriorly narrow infratemporal fenestra, and other autapomorphies plausibly associated with feeding adaptations. The derived morphology revealed in G. monumentensis necessitates revision of the generic diagnosis of Gryposaurus, including the addition of synapomorphies that further aid in distinguishing this taxon from Kritosaurus. A revised phylogenetic analysis places Gryposaurus within a monophyletic clade that includes Brachylophosaurus and Maiasaura. Gryposaurus monumentensis represents the most southern example of Gryposaurus, and underlines the remarkable diversification and long duration of this genus. Based on the phylogenetic, geographical, and stratigraphic evidence at hand, Gryposaurus was the most diverse genus within Hadrosaurinae; it also possessed one of the largest geographical and stratigraphic distributions, spanning more than five million years of the Campanian, and ranging from Alberta in the north to Utah (and possibly Texas) in the south.

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Prior studies of Mesozoic biodiversity document a diversity peak for dinosaur species in the Campanian stage of the Late Cretaceous, yet have failed to provide explicit causal mechanisms. We provide evidence that a marked increase in North American dinosaur biodiversity can be attributed to dynamic orogenic episodes within the Western Interior Basin (WIB). Detailed fossil occurrences document an association between the shift from Sevier-style, latitudinally arrayed basins to smaller Laramide-style, longitudinally arrayed basins and a well substantiated decreased geographic range/increased taxonomic diversity of megaherbivorous dinosaur species. Dispersal-vicariance analysis demonstrates that the nearly identical biogeographic histories of the megaherbivorous dinosaur clades Ceratopsidae and Hadrosauridae are attributable to rapid diversification events within restricted basins and that isolation events are contemporaneous with known tectonic activity in the region. SymmeTREE analysis indicates that megaherbivorous dinosaur clades exhibited significant variation in diversification rates throughout the Late Cretaceous. Phylogenetic divergence estimates of fossil clades offer a new lower boundary on Laramide surficial deformation that precedes estimates based on sedimentological data alone.