More than 85 million years ago, a smaller-bodied ancestor of the Tyrannosaurus rex roamed the plains of Mongolia. The dinosaur’s identity was unknown — until now.
In a study published Wednesday in Nature, scientists describe the identification of a new tyrannosauroid named Khankhuuluu mongoliensis. Apex predators, including the T. rex, eventually arose from these smaller-bodied tyrannosauroids, the study’s researchers said.
“This species, Khankhuuluu, is the missing link between smaller and earlier forms and the larger apex predatory tyrannosaurs,” paleontologist Darla Zelenitsky, a professor at the University of Calgary and study co-author, told CBS News.
Riley Brandt, University of Calgary 
The Khankhuuluu’s skeleton was discovered in Mongolia in the Gobi Desert in 1972 or 1973, but wasn’t studied scientifically until a few years later, Zelenitsky said. The remains were identified by Mongolian paleontologist Altangerel Perle as Alectrosaurus olsoni, another obscure species. After it was misidentified, the bones became part of a museum collection and were relatively untouched for almost 50 years.
Referred to as mid-grade tyrannosauroids, these smaller-bodied predators were evolutionary intermediates, the study found. During the period when Khankhuuluu lived in Mongolia, the tyrannosaurs began growing larger and taking on the features of apex predators. Not much else is known about these elusive tyrannosauroids due to fragmentary fossil remains, but researchers said the dinosaurs weighed around 1,700 pounds. They were one of the larger predators in the ecosystem at the time.
“It’s essentially the immediate ancestor of the tyrannosaur family,” Zelenitsky said.
A surprise discovery
University of Calgary postdoctoral scholar and study co-author Jared Voris traveled to Mongolia a few years earlier to research the bones of various tyrannosaurs and, in the process, gained access to see the bones of what they would later determine are the Khankhuuluu.
Voris thought the remains could be something important, as he often saw references to the specimens pop up, but after he started examining the bones, he knew it could be something special. While studying the individual characteristics of the bones, one of the things he noticed was that the Khankhuuluu’s teeth couldn’t crush bone, Voris said, a defining characteristic of the larger apex predatory Tyrannosaurus.
“This animal had features we had never seen before in more of our iconic tyrannosaurs,” Voris said.
He texted Zelenitsky from Mongolia that he “thought it could be a new species.” Zelenitsky said she responded, “I said, great going, keep looking at this so we can be sure this is a new species.”
Image: Julius Csotonyi 
The two paleontologists aren’t new to groundbreaking scientific discoveries — they’ve had several. In 2023, Zelenitsky was part of a team that identified a Tyrannosaurus fossil with a meal in its stomach, and Voris discovered a new species of tyrannosaurid dinosaur, Thanatotheristes degrootorum — whose genus name translates to “Reaper of Death.”
“Rewriting a family history of tyrannosaurs”
These Asian tyrannosauroids eventually crossed over into North America using a northern land bridge between Alaska and Siberia, Zelenitsky said. Dinosaurs used these land bridges, which appeared periodically over the past 100 million years, to travel between Asia and North America.
And from it, these tyrannosauroids evolved and morphed into apex predators. T. rex is the deadliest land predator ever to live, and there has long been a fascination with the dinosaur that has been extinct for about 66 million years.
Image credit: Jared Voris 
The study co-authors said the new identification shows there are still big discoveries to be made in science. “What really started just as a discovery of a new species ended up with us kind of rewriting a family history of tyrannosaurs,” Zelenitsky said.
