Eutyrannosauria were the large apex predators that dominated Asian and North American terrestrial faunas in the latest Cretaceous. These predators arose from smaller-bodied tyrannosauroid ancestors during the "middle" Cretaceous, which are poorly known due to the paucity of fossil material. 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. A phylogenetic analysis recovers Khankhuuluu as the species closest to Eutyrannosauria and uniquely recovers the massive, deep-snouted Tyrannosaurini and the smaller, gracile, shallow-snouted Alioramini as highly derived sister clades. Khankhuuluu and Alioramini independently share features related to a shallow skull and gracile build with juvenile eutyrannosaurians, reinforcing the key role heterochrony played in eutyrannosaurian evolution. Within a Eutyrannosauria influenced by peramorphosis or accelerated growth, Alioramini is revealed as a derived lineage that retained immature features via paedomorphosis and is not a more basal lineage as long and widely accepted. Our analyses novelly reveal Eutyrannosauria originated and remained exclusively in North America until their first and only dispersal to Asia in the latest Cretaceous, where the lineage of Alioramini + Tyrannosaurini evolved. Stark morphological differences between Alioramini and Tyrannosaurini likely evolved due to divergent heterochronic trends, paedomorphosis vs. peramorphosis, respectively, allowing these sister clades to coexist in Asia and occupy different ecological niches.

Funding provided by: Killam TrustsROR ID: https://ror.org/021cbtn85Award Number: Funding provided by: Natural Sciences and Engineering Research CouncilROR ID: https://ror.org/01h531d29Award Number: RGPIN 04854