Data and code from: A new Mongolian tyrannosauroid and the evolution of Eutyrannosauria

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.