Evidence that may reveal the evolutionary process of the carnivorous dinosaur 'Tyrannosaurus rex' was found by reanalyzing fossils excavated in Mongolia in the 1970s. The name of this dinosaur, famous as the protagonist in the movie 'Jurassic Park,' means 'tyrant lizard' in Greek, and 'rex' means 'king' in Latin. There has been ongoing debate in the academic community about how the king of dinosaurs came to be, but this research is expected to provide clues to the solution.
An international joint research team involving scientists from Canada, Mongolia, the United States, and Japan announced on the 12th that two partial skeleton fossils discovered in Mongolia between 1972 and 1973 belonged to a new dinosaur that lived 86 million years ago in the Tyrannosaurus lineage. The research results were published in the international journal 'Nature.'
The new dinosaur species had some characteristics of Tyrannosaurus, such as a strong jaw structure, but other characteristics had not yet appeared. With a body length of 4 meters and a weight of around 750 kilograms, it is a small carnivorous dinosaur compared to Tyrannosaurus rex. The researchers named the newly discovered dinosaur 'Khankhuuluu mongoliensis,' which means 'prince of the Mongolian dragon.'
Tyrannosaurus rex was a carnivorous dinosaur that lived during the Late Cretaceous period, approximately 68 to 66 million years ago. It is estimated to have a body length of 12 to 15 meters, a height of 4 meters, and a weight of 7 to 9 tons. Scientists believe that the ancestors of Tyrannosaurus began with small-bodied dinosaurs that lived during the Jurassic period, about 150 million years ago. It later evolved into a gigantic dinosaur during the Late Cretaceous period, but the processes of that evolution remain largely unknown.
Darla Zelenitsky, a professor at the University of Calgary and the corresponding author of the paper, said, 'Khankhuuluu is essentially the missing link between smaller early forms and the large apex predator we know as Tyrannosaurus rex.'
This study began with the reanalysis of fossils discovered in Mongolia over 50 years ago. At the time of discovery, these fossils were believed to belong to a dinosaur called 'Alectrosaurus,' which lived during the Late Cretaceous period, like Tyrannosaurus rex, but the research team found that they belonged to a new species that lived 86 million years ago, much earlier.
The Tyrannosauridae, which includes Tyrannosaurus rex, was a group of large carnivorous dinosaurs that dominated Asia and North America until 66 million years ago. The researchers hypothesized that Khankhuuluu was one of the early ancestors of the Tyrannosauridae. They then used computer models to track subsequent migration processes.
The researchers predicted that as Khankhuuluu migrated from Asia to North America, the Tyrannosauridae were born, and that Tyrannosaurus later migrated back to Asia, splitting into Alioramus and Tyrannosaurus.
Alioramus, known by the name Pinocchio Rex, is a medium-sized predator with a shallow, long snout, while Tyrannosaurus is a giant apex predator with a deep, massive snout. These two groups occupied different ecological roles and could coexist. Ultimately, it is believed that one of the large Asian Tyrannosaurs spread back into North America about 70 million years ago, giving rise to Tyrannosaurus rex.
Steve Brusatte, a professor at the University of Edinburgh, noted, 'This research will be an important key to understanding the evolutionary history of Tyrannosaurus, and its value is heightened by the fact that there are very few fossils from this period.' He added, 'Just as humans formed households through migration, the Tyrannosaurus family did the same.'
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-08964-6