A research group led by Professor Yoshitsugu Kobayashi of Hokkaido University Museum and Professor Darla K. Zelenitsky and Graduate Student Jared T. Voris of the University of Calgary (Canada) announced that fossils from Late Cretaceous strata in Mongolia represent a new species of tyrannosaur called "Khankhuuluu mongoliensis," and that analysis, including analysis of these fossils, has revealed the evolutionary process of large tyrannosaur species. The results are expected to contribute to understanding tyrannosaur evolution. These findings were published in the June 11 issue of Nature.
During the final period of the Cretaceous, approximately 86-66 million years ago, large carnivorous tyrannosaurs with body weights exceeding 1 ton and thick, robust skulls with powerful bite forces reigned as apex predators in terrestrial ecosystems— Tyrannosaurus rex in North America and Tarbosaurus bataar in Asia. Large tyrannosaurs are thought to have evolved from early forms that existed during the Jurassic to Early Cretaceous. But until now, the intermediate forms between the early and large types had a poor fossil record and were considered an "evolutionary gap."
The research group re-examined fossils discovered from approximately 93-83-million-year-old strata at Baishin Tsav in the Gobi Desert of southeastern Mongolia, which were previously described as Alectrosaurus, comparing their morphology with other tyrannosaur fossils. They discovered multiple features unique to these fossils (autapomorphies) in the nasal region, bones at the back of the head, shoulders, and other areas, naming them as a new genus and species: Khankhuuluu mongoliensis. This is said to mean "Mongolian dragon prince."
Next, they reconstructed evolutionary phylogenetic trees by quantifying and statistically analyzing (using maximum parsimony and Bayesian methods) over 300 characteristics of skull and body bones from a wide range of tyrannosaurs including Khankhuuluu, while also modeling migration and evolution. The results showed that it was an intermediate tyrannosaur species that branched out just before large tyrannosaurs such as North American Tyrannosaurus rex and Asian Tarbosaurus.
Furthermore, it was revealed that the small, gracile Alioramini subfamily, whose evolutionary position had been unclear, was actually an advanced group on par with Tyrannosaurus rex.
Based on these analyses, when evolution and dispersal were organized, it was estimated that ancestors of large tyrannosaurs first originated in Asia, gained diversity, then crossed to North America and diversified by 86 million years ago. Lineages distributed in North America that returned to Asia around 78 million years ago diverged and dispersed into Alioramini and Tyrannosaurini subfamilies. During the end of the Cretaceous, 73-67 million years ago, some species, including Tyrannosaurus, moved again from Asia to North America and came to reign as apex predators.
Khankhuuluu is estimated to have weighed less than 500 kilograms and possessed features very similar to juvenile tyrannosaurs not seen in adults, likely enabling coexistence with large tyrannosaurs on the Asian continent as an intermediate predator.
Journal Information
Publication: Nature
Title: A new Mongolian tyrannosauroid and the evolution of Eutyrannosauria
DOI: 10.1038/s41586-025-08964-6
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