Scientists have identified a previously unknown 86 million-year-old dinosaur species that fills an early gap in the fossil record of tyrannosaurs, revealing how they evolved to become massive apex predators.
Researchers analyzing the species’ remains have named it Khankhuuluu mongoliensis, which translates to “dragon prince of Mongolia,” because it was small compared with its much larger relatives such as Tyrannosaurus rex, whose name means “the tyrant lizard king.” The newly identified dinosaur was the closest known ancestor of tyrannosaurs and likely served as a transitional species from earlier tyrannosauroid species, according to the findings published Wednesday in the journal Nature.
Based on a reexamination of two partial skeletons uncovered in Mongolia’s Gobi Desert in 1972 and 1973, the new study suggests that three big migrations between Asia and North America led tyrannosauroids to diversify and eventually reach a gargantuan size in the late Cretaceous Period before going extinct 66 million years ago.
“This discovery of Khankhuuluu forced us to look at the tyrannosaur family tree in a very different light,” said study coauthor Darla Zelenitsky, associate professor within the department of Earth, energy, and environment at the University of Calgary, in an email. “Before this, there was a lot of confusion about who was related to who when it came to tyrannosaur species. What started as the discovery of a new species ended up with us rewriting the family history of tyrannosaurs.”
Multiple migrations over millions of years
Tyrannosaurs, known scientifically as Eutyrannosaurians, bring to mind hulking dinosaurs like Tyrannosaurus rex and Tarbosaurus, which weighed multiple metric tons and could take down equally large prey.
With short arms and massive heads, they walked on two legs and boasted sharp teeth, Zelenitsky said.
But tyrannosaurs didn’t start out that way. They evolved from smaller dinosaurs before dominating the landscapes of North America and Asia between 85 million and 66 million years ago, the researchers said.
While Tarbosaurus, an ancestor of T. rex, clocked in at between 3,000 and 6,000 kilograms (6,613 pounds and 13,227 pounds), the fleet-footed Khankhuuluu mongoliensis likely weighed only around 750 kilograms (1,653 pounds), spanned just 2 metres (6.5 feet) at the hips and 4 metres (13 feet) in length, according to the study authors.
Comparing the two dinosaurs would be like putting a horse next to an elephant —Khankhuuluu would have reached T. rex’s thigh in height, Zelenitsky said.
“Khankhuuluu was almost a tyrannosaur, but not quite,” Zelenitsky said. “The snout bone was hollow rather than solid, and the bones around the eye didn’t have all the horns and bumps seen in T. rex or other tyrannosaurs.”
Khankhuuluu mongoliensis, or a closely related ancestor species, likely migrated from Asia to North America across a land bridge between Alaska and Siberia that connected the continents 85 million years ago, Zelenitsky said.
Because of this migrant species, we now know that tyrannosaurs actually evolved first on the North American continent and remained there exclusively over the next several million years, she said. “As the many tyrannosaur species evolved on the continent, they became larger and larger.”
Due to the poor fossil record, it’s unclear what transpired in Asia between 80 million to 85 million years ago, she added. While some Khankhuuluu may have remained in Asia, they were likely replaced later on by larger tyrannosaurs 79 million years ago.
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