By YUSUKE OGAWA/ Staff Writer
July 9, 2025 at 07:00 JST
Dinosaur fossils unearthed in Mongolia half a century ago belong to a newly identified species that evolved into the massive flesh-eating tyrannosaurs that dominated the planet for millions of years, an international research team said.
“The origin and evolutionary process of large tyrannosaur varieties have been ascertained,” said Yoshitsugu Kobayashi, a paleontological professor at the Hokkaido University Museum, who is part of the team.
According to the researchers’ accounts, tyrannosaur variations weighed 3.5 tons or more. They started as a relatively small species and evolved into huge carnivores with thick skulls and powerful bites.
However, few fossils had previously been found to corroborate this hypothesis.
The scientists studied a pair of fossilized dinosaurs discovered in a 90-million-year-old geological layer in the southeastern area of Mongolia between 1972 and 1973.
The team analyzed various characteristics of the fossils, including skeletal structures of the head and shoulders, inclusive of a cavity in the nasal bone.
Their findings revealed that the fossils are likely from a formerly unidentified variety.
The researchers said the dinosaurs likely had slim bodies and shallow skulls, resembling those of a young tyrannosaur. They were estimated to have weighed less than 500 kilograms, a fraction of the weight of their descendants.
The newly confirmed tyrannosaur variant has been named Khankhuuluu mongoliensis, a label combining the Mongolian words for “prince” and “dragon,” or “Mongolian prince dragon.”
To determine how dinosaur varieties branched off, the team compared more than 300 distinctive traits in the skulls and skeletal frames of a range of tyrannosaurs, including the recently identified species.
Khankhuuluu mongoliensis was highly likely the common ancestor of the giant tyrannosaurs that once thrived in both North America and Asia, the team said.
The scientists retraced details of the branching process, studied geographical data and pored over other elements to reconstruct the evolutionary history of the tyrannosaur.
According to the researchers, the tyrannosaur ancestors originated in Asia and migrated to the North American continent 90 million years ago, where they spread significantly.
Over the next 20 million years, they traveled between North America and Asia.
The scientists are planning further research.
“We will be considering and deciding the exact method and timing of the dinosaurs’ migration between Asia and North America,” Kobayashi said.
The team’s findings were published in the British scientific journal Nature at (https://www.nature.com/articles/s41586-025-08964-6) on June 12.
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A reconstruction of a dinosaur called Khankhuuluu mongoliensis that is believed to be an ancestor of the tyrannosaur (Provided by Masato Hattori)
