Scientifically named Khankhuuluu mongoliensis, the newly-identified tyrannosauroid species is the closest-known ancestor to Tyrannosaurus rex.
Khankhuuluu mongoliensis. Image credit: Julius Csotonyi.
Khankhuuluu mongoliensis roamed our planet during the Cretaceous period, some 86 million years ago.
This dinosaur was a medium-sized, fleet-footed predator that evolved after the extinction of other large predatory dinosaurs.
As the closest-known ancestor, it shared many characteristics with its Tyrannosaurus descendants.
Khankhuuluu mongoliensis was about the size of a horse (750 kg), making it two to three times smaller than its massive descendants.
It had tiny rudimentary horns that would evolve to be more noticeable in species like Albertosaurus or Gorgosaurus used for mating display or intimidation.
It had a long, shallow skull that shows Khankhuuluu mongoliensis didn’t have the ability to crunch through bone like Tyrannosaurus rex.
The new species can be defined as a mesopredator, similar to coyotes, meaning it used speed and agility to take down its prey.
“Khankhuuluu mongoliensis provides us the window into the ascent stage of tyrannosaur evolution; right when they’re transitioning from small predators to their apex predator form,” said Dr. Jared Voris, a paleontologist at the University of Calgary.
Partial skeletons of two Khankhuuluu mongoliensis individuals were found in the Bayanshiree Formation in southeastern Mongolia in 1972 and 1973.
The team’s phylogenetic analysis of the specimens suggests that the new species is an immediate relative of the dinosaur group Eutyrannosauria and a close ancestor to the massive, deep-snouted Tyrannosaurini and smaller, shallow-snouted Alioramini clades.
“Khankhuuluu mongoliensis, or a closely related species, would have immigrated to North America from Asia around 85 million years ago,” said Dr. Darla Zelenitsky, also from the University of Calgary.
“Our study provides solid evidence that large tyrannosaurs first evolved in North America as a result of this immigration event.”
“The results show the movement of tyrannosaurs back and forth between Asia and North America was less frequent and less sporadic than previously known.”
“Khankhuuluu mongoliensis, or one of its kin, traveled across a land bridge into North America, where it evolved into the famous apex predator Tyrannosaurus rex.”
“The fossil record indicates tyrannosaurs were exclusive to North America for few million years before immigrating to Asia, where the lineage split into two groups.”
“One group branched off to become even bigger apex predators, ultimately evolving into Tyrannosaurus rex, and the other group evolved into a medium-sized long-snouted species, dubbed ‘Pinocchio rexes’.”
The discovery of Khankhuuluu mongoliensis is reported in a paper published today in the journal Nature.
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J.T. Voris et al. A new Mongolian tyrannosauroid and the evolution of Eutyrannosauria. Nature, published online June 11, 2025; doi: 10.1038/s41586-025-08964-6
