Scientists discover ‘dragon prince' dinosaur, the T. Rex's missing ancestor
Khankhuuluu mongoliensis was slender with features like no other member of the tyrannosaur family tree. This illustration depicts how the slender Khankhuuluu mongoliensis may have appeared as it roamed Mongolia during the Cretaceous period. The newest addition to the tyrannosaur family, the discovery of this "dragon prince from Mongolia" sheds light on the origins of Tyrannosaurus rex. Illustration by Julius Csotonyi
Tyrannosaurus rex is a carnivorous icon. Exceeding 40 feet in length and nine tons, the bone-crushing giant stands out as the largest and last of its meat-eating family. Now a new and far smaller tyrannosaur is filling in the famous dinosaur's evolutionary backstory.
The newest addition to the tyrannosaur family tree is named Khankhuuluu mongoliensis, which translates to 'dragon prince from Mongolia.' Described this week in the journal Nature, the dinosaur has been identified for the first time from two partial skeletons that include skull bones, vertebrae, part of the hips, and limb bones.
Altogether, the pieces reveal a slender tyrannosaur that roamed Cretaceous Mongolia about 86 million years ago and was about 13 feet long—or about the size of juvenile T. rex that would stalk North America 20 million years later. In fact, Khankhuuluu even looked like a juvenile of later, larger tyrannosaurs, with round eye sockets, blade-like teeth, and long, shallow jaws better suited to biting fast rather than hard.
(T. rex had lips, upending its enduring pop culture image.)
Khankhuuluu does more than simply add another dinosaur to the ever-growing roster of dinosaurs. 'Khankhuuluu gives us the origin story of tyrannosaurs,' says University of Calgary paleontologist and study co-author Darla Zelenitsky. Comparing the fossils of mature Khankhuuluu (a, d, g) with fossils of mature Gorgosaurus (c, f, i) and juvenile Gorgosaurus (b, e, h) provides new insights into the evolutionary lineage between the smaller-bodied tyrannosauroids, such as Khankhuuluu, and the larger eutyrannosaurians like Gorgosaurus and Tyrannosaurus. Silhouettes compare the sizes of Khankhuuluu (left) with a juvenile (right) and adult (middle) Gorgosaurus. Scale bars, 5 cm (individual elements) and 1 m (silhouette). Illustration by Voris et al. (2025), Nature
In the early 1970s, Mongolian paleontologist Altangerel Perle found a pair of partial tyrannosaur skeletons in the eastern part of the country. The bones seemed similar to a small tyrannosaur that had been named before, Alectrosaurus. But when University of Calgary paleontologist and study co-author Jared Voris studied the bones during a research trip to Mongolia in 2023, he soon realized that the bones did not belong to Alectrosaurus at all.
The bones from the two skeletons belonged to a new form of tyrannosaur that had been waiting to be discovered in collections for half a century. 'It had features like a hollow air chamber in side its nasal bone, which no other tyrannosaur species has,' Voris says. The fossils deserved a new name and have been recategorized as Khankhuuluu.
Voris has found tyrannosaurs hiding in plain sight before. In 2020, Voris and colleagues named the 80 million-year-old tyrannosaur Thanatotheristes from bones assigned to another species found in Alberta.
("Reaper of Death" tyrannosaur discovered in Canada.)
The finds are part of a burgeoning array of tyrannosaur discoveries. Instead of a simple line of evolution from early tyrannosaurs to T. rex, paleontologists have uncovered a wildly branching evolutionary bush of different tyrannosaur subgroups that came and went through the Cretaceous. The glut of new tyrannosaur species is allowing experts to piece together how big tyrannosaurs, including the gigantic T. rex, evolved and spread across vast stretches of the planet. What the 'dragon prince' tells us about the evolution of T. rex
When compared to other tyrannosaurs, the researchers found that Khankhuuluu is a close relative of the broader group of tyrannosaurs that include Gorgosaurus from Alberta, the bumpy-snouted Alioramus from Mongolia, and the iconic T. rex. The new family tree, as well as where the fossils were uncovered, create an updated picture of how tyrannosaurs evolved over 20 million years.
'It is a pivotal species in understanding the evolutionary success of T. rex and its relatives,' says University College London paleontologist Cassius Morrison, who was not involved in the new study. In particular, the new analysis reveals how tyrannosaurs evolved into many different species as the carnivores wandered into new landscapes.
Sometime around the time of Khankhuuluu, Voris and colleagues propose, such small, slender tyrannosaurs were dispersing from prehistoric Asia into North America over a land bridge.
'Tyrannosaurs evolved into those giant apex predators and diversified very rapidly across North America,' Voris says, the first of what Zelenitsky calls 'two explosions of tyrannosaurs.' Some of the predators remained slender and chased smaller prey while others became bulkier and hunted larger dinosaurs, and they roamed habitats from southern California to New Jersey.
(See how these fierce dinos evolved in our pages over 100 years.)
The new study suggests, however, that the direct ancestors of T. rex, did not evolve in North America. Voris and colleagues propose that around 79 and 78 million years ago at least one lineage of tyrannosaurs ventured back into Asia. The researchers know this because of the close relationship of two tyrannosaur groups that at a glance might seem very different.
When tyrannosaurs returned to Asia during this period and underwent their second explosion, one group was relatively slender and had long snouts decorated with small horns, like the 'Pinocchio' dinosaur Qianzhousaurus. The other group began to grow larger, with deep skulls adept at crushing bones, like Tarbosaurus.
T. rex evolved from ancestors in the second group, a lineage of bone-crushers that once again crossed the land bridge back into North America between 73 and 67 million years ago—making T. rex a new form of predator that arrived from another continent. 'The new analysis provides strong support that the ancestors of T. rex evolved from a group of tyrannosaurs that ventured back to Asia after they had undergone an evolutionary radiation in North America,' Morrison says.
Ultimately, the study suggests that the rise of one of Earth's largest carnivores was due to a back-and-forth between North America and Asia that took place over a period of 20 million years. Had a devastating asteroid impact not abruptly ended the Cretaceous 66 million years ago, tyrannosaurs would have undoubtedly kept changing.
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