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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Instead the researchers found that the need for a male had a lot to do with temperature. Wynne-Edwards: The worst thing that can happen to a [ P. ] campbelli mum is that she's in a warm environment. Feder: These hamsters, being so well adapted to conserve heat, they're prone to overheat, especially if they're sitting day after day in a nest, nursing pups that are getting better and better at thermoregulating each day. Wynne-Edwards: The pups become more of a problem later because they're too hot. Feder: When solitary females were held at a comfortable 18 degrees Celsius, they were actually pretty successful. More than 90 percent of their pups survived without a dad present. But if it got even a few degrees hotter, suddenly male presence mattered a whole lot for pup survival and for growth. So why would that be? Well, Katherine found that males help the females regulate their own temperature. Females go for walks away from the nest to cool off. The hotter the temperature in the lab, the longer these cool-down walks are. For the mom, this is essential, but it's not great for the pups. They start to lose heat—and water, too— unless there's someone else there to sit on them and keep things nice and warm and humid, aka another parent. So ultimately, it seems like these dads are a product of evolving in a superharsh environment. In a place this cold, it's just hard for a single parent to retain heat and raise their kids without overheating. In fact, it's often the case that harsh environments tip the scales in favor of active fathers. Harsh environments can just mean that offspring need more help to grow up, selecting for more parental care in general. And we can actually see this play out in this hamster's very close relative, Phodopus sungorus, also known as the Siberian hamster. [ P.] sungorus lives right across a mountain range from our hamsters, [ P.] campbelli. Where they live, it's also a harsh environment but not quite as harsh. And in[ P.] sungorus the fathers are often—but not always—involved in the care of offspring. And when they are, they're not quite as attentive as [ P.] campbelli dads. Katherine has conducted experiments in the lab, where she'll remove a hamster pup from a nest and plop it in a far corner of its cage. Wynne-Edwards: The male—if the female is not there—the male will leave the nest, go to the pup, pick it up, bring it back to the nest and just sit down on it again. Feder: And in our star hamsters, [ P.] campbelli, the male will rush over right away, wasting no time. But in their close relative, [ P.] sungorus, the males respond, too, but they take more than twice as long to go over to the pup. And then, more than half the time, they don't even pick it up. So a harsh environment is one explanation for why [ P.] campbelli hamster dads are so devoted. But when it comes to fatherhood, Nick says there are lots of other factors that come into play. Confidence in paternity, for example—so if the female mates with multiple males that can make it harder to determine who the dad is. And that will affect whether the father helps out. In [ P.] campbelli hamsters, that's not much of an issue. In lab experiments, Katherine found that a female won't get pregnant if she mates with more than one male. Another potential factor favoring active dads is females preferentially mating with males who seem like they would be good at fatherhood. Here's Nick again. Royle: There's definitely kind of a selection for good quality parents effectively, and there's some evidence for that, particularly in birds, where biparental care is strongest, so you can get females kind of making choices of males based on their likely parental care. Feder: And so, even though active fatherhood is not the norm in most animals, there are actually lots of species where evolution favors it—[such as] seahorses, famously. Males carry their broods around in special pouches. They even have placentas in there! In some fish species, the male carries the fertilized eggs in his mouth, forgoing food, until they've hatched and grown and are ready to swim around independently. Even some beetles care for their young. Nick studies a species where males and females regurgitate meat for their little larvae kids. And of course, we have humans—fatherhood definitely varies from dad to dad and culturally, too. But there are a lot of devoted, caring human dads. There are dads who feed their kids, change their diapers, teach them how to drive, pay for college tuition—all activities I have no doubt [ P. ] campbelli dads would jump at the chance to do, given access to cars, currency and higher education. So if you happen to be in the arid semi-desert of Inner Mongolia one summer—summer being this hamster's breeding season—just know that you're in proximity to parental greatness. All around you in burrows just below ground are tiny hamster dads, working their fluffy butts off to operate birthing centers—they're delivering babies, keeping them warm, and just generally doing their very best to help their kids survive the harsh, dry land on which you walk. Feltman: That's all for today's Friday Fascination. We'll be back bright and early on Monday with our usual weekly news roundup. Science Quickly is produced by me, Rachel Feltman, along with Fonda Mwangi, Kelso Harper, Naeem Amarsy and Jeff DelViscio. This episode was reported and co-hosted by Elah Feder and edited by Jeff DelViscio. Shayna Posses, Emily Makowski and Aaron Shattuck fact-check our show. Our theme music was composed by Dominic Smith. Subscribe to Scientific American for more up-to-date and in-depth science news.
