A new Mongolian tyrannosauroid and the evolution of Eutyrannosauria

Eutyrannosaurians were large predatory dinosaurs that dominated Asian and North American terrestrial faunas in latest Cretaceous times. These apex predators arose from smaller-bodied tyrannosauroids during the 'middle' Cretaceous that are poorly known owing to the paucity of fossil material^1-3. Here we report on a new tyrannosauroid, Khankhuuluu mongoliensis gen. et sp. nov., from lower Upper Cretaceous deposits of Mongolia that provides a new perspective on eutyrannosaurian origins and evolution. Phylogenetic analyses recover Khankhuuluu immediately outside Eutyrannosauria and recover the massive, deep-snouted Tyrannosaurini and the smaller, gracile, shallow-snouted Alioramini as highly derived eutyrannosaurian sister clades. Khankhuuluu and the late-diverging Alioramini independently share features related to a shallow skull and gracile build with juvenile eutyrannosaurians, reinforcing the key role heterochrony had in eutyrannosaurian evolution. Although eutyrannosaurians were mainly influenced by peramorphosis or accelerated growth^4-10, Alioramini is revealed as a derived lineage that retained immature features through paedomorphosis and is not a more basal lineage as widely accepted^11-19. Our results reveal that Asian tyrannosauroids (similar to Khankhuuluu) dispersed to North America, giving rise to Eutyrannosauria in the mid-Late Cretaceous. Eutyrannosauria diversified and remained exclusively in North America until a single dispersal to Asia in the latest Cretaceous that established Alioramini and Tyrannosaurini. Stark morphological differences between Alioramini and Tyrannosaurini probably evolved due to divergent heterochronic trends-paedomorphosis versus peramorphosis, respectively-allowing them to coexist in Asia and occupy different ecological niches.