Paleontologists have discovered Khankhuuluu, a new species of tyrannosauroid dinosaur from Mongolia that represents a key transitional form in the evolution of Tyrannosaurus rex. This medium-sized predator, weighing about 750 kilograms, lived 86 million years ago and likely migrated from Asia to North America, where larger tyrannosaurs evolved. The find, detailed in a recent Nature study, highlights the dinosaur's agile hunting style and shared traits with later giants.
An international team, led by PhD candidate Jared Voris and Dr. Darla Zelenitsky from the University of Calgary, identified Khankhuuluu after re-examining fossils from Mongolia's Bayanshiree Formation. These specimens, first studied in the 1970s by paleontologist Altangerel Perle and initially linked to the Chinese species Alectrosaurus, revealed distinct differences during Voris's 2023 analysis at Mongolia's Institute of Paleontology. The discovery was published in Nature in 2025.
Khankhuuluu, named the 'dragon prince' in Mongolian, stood as a 'prince' before the 'king' Tyrannosaurus rex in the family tree. At roughly 750 kilograms—comparable to a horse—it was two to three times smaller than its descendants but featured a long, shallow skull, small horn-like structures on its head, and a lean build suited for speed. Voris described it as a mesopredator, akin to modern coyotes, relying on agility rather than the bone-crushing bite of T. rex. The horns, which became more pronounced in species like Albertosaurus and Gorgosaurus, may have served for mating displays or intimidation.
The research suggests Khankhuuluu or a close relative crossed from Asia to North America around 85 million years ago via a land bridge. 'Our study provides solid evidence that large Tyrannosaurs first evolved in North America as a result of this immigration event,' Zelenitsky said. This migration was rarer than previously thought, with Khankhuuluu marking the last known Asian ancestor before North American diversification. There, tyrannosaurs split into massive forms like T. rex and slender 'Pinocchio rexes.'
Voris noted, 'This new species provides us the window into the ascent stage of Tyrannosaur evolution; right when they're transitioning from small predators to their apex predator form.' Future work will explore earlier ancestors to complete the evolutionary picture.