In the badlands of southeastern Mongolia, two dusty skeletons sat largely forgotten for fifty years. Dug up in the early 1970s, the bones were long assumed to belong to a known species — an enigmatic predator named Alectrosaurus. But when a team of paleontologists led by Jared Voris and Darla Zelenitsky at the University of Calgary took a second look, they realized the fossils told a far more interesting story.
Those bones belong to a new species, now named Khankhuuluu mongoliensis, or “Prince of Dragons.” This slim, fast-moving predator didn’t rule the landscape like its infamous descendant, Tyrannosaurus rex. But it may have set the evolutionary stage for the tyrant lizard kings that would come millions of years later.
According to the researchers, Khankhuuluu mongoliensis is now the most recent ancestor of tyrannosaurs.
“This new species provides us the window into the ascent stage of Tyrannosaur evolution; right when they’re transitioning from small predators to their apex predator form,” said Voris.
A Missing Link with Bite
Before T. rex became a pop-culture juggernaut and a literal juggernaut in its own time, its ancestors were small, lithe, and, in many ways, ordinary. The early lineage is littered with relatively obscure and fragmentary species. This left a frustratingly blurry picture of how tyrannosaurs grew into the giants of the Late Cretaceous.
Khankhuuluu mongoliensis, described this week in the journal Nature, fills that void. Dating back 86 million years, it’s the most advanced tyrannosauroid yet found outside the true “eutyrannosaurs” — a clade that includes T. rex, Tarbosaurus, and Gorgosaurus. Khankhuuluu represents what scientists call a “mid-grade” tyrannosauroid, and it stands at a key pivot point in the lineage.
Weighing in at about 750 kilograms (≈ 1650 lbs) and stretching four meters long (≈ 13 ft), Khankhuuluu was about the size of a grizzly bear. Its skull — elongated and low — measured around 70 centimeters (≈ 2.3 ft), with a femur nearly the same length. Unlike its bulkier descendants, it had longer lower legs than thighs and longer arms relative to its body, likely making it a swift, nimble hunter.
“You would have been the thing that it would hunt down, and it would have been faster than you,” Voris told The Guardian. “I would much rather run into an adult T. rex than run into Khankhuuluu.”
The Tyrant’s Asian Roots
The fossils come from the Bayanshiree Formation, a stretch of windswept rock in Mongolia that also gave rise to other dinosaurs like Protoceratops. For decades, paleontologists believed the two partial skeletons were just another specimen of Alectrosaurus, first identified in China. But a detailed reanalysis of their bones told a different tale.
Khankhuuluu’s anatomy is distinct: a nasal ridge that’s shallower but longer than its relatives; vertebrae with deep air sacs; a femur with a tall lesser trochanter; and a unique arrangement of skull and jaw bones that sets it apart from other known species.
The researchers didn’t just stop at anatomy. They ran an extensive evolutionary analysis, reconstructing the family tree of tyrannosaurs. Their conclusion: Khankhuuluu sits just outside Eutyrannosauria, the group that includes the true giants.
And this is where things get even more interesting.
According to the study, tyrannosaurs didn’t evolve into giants in Asia. Instead, Khankhuuluu or a close relative likely migrated across a land bridge into North America. There, in a new environment, its descendants exploded in size and power.
“Our study provides solid evidence that large Tyrannosaurs first evolved in North America as a result of this immigration event,” said Zelenitsky.
The Dragon Prince and His Kin
The researchers also used computer models to examine how these predators moved across continents. They learned that the migration wasn’t a constant shuffle. It happened in waves.
The first wave, roughly 86 million years ago, took mid-sized tyrannosauroids like Khankhuuluu into North America. There, they evolved into Eutyrannosaurs.
The second wave — later in the Late Cretaceous — brought those giants back into Asia, where they split into two lineages: the deep-skulled, bone-crushing Tyrannosaurini (like Tarbosaurus) and the slender-snouted, oddly juvenile-looking Alioramini, sometimes nicknamed “Pinocchio rexes.”
A final wave brought members of Tyrannosaurini — giants among giants — back to North America. One of them became T. rex.
“The tyrannosaur family tree was shaped by migration, just like so many of our human families,” said Steve Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study.
How to Grow a Tyrannosaur
One of the most revealing aspects of the study is what Khankhuuluu tells us about dinosaur growth.
Its body, though adult, retained traits usually seen in juveniles of later tyrannosaurs: a slender build, long limbs, a shallow skull, and reduced cranial ornamentation. Meanwhile, its descendants — especially the likes of T. rex — developed deep skulls, short arms, thick bones, and massive bite forces.
This suggests a fundamental change in how these dinosaurs grew, especially what paleontologists call heterochrony: changes in the timing of development.
In the case of giant tyrannosaurs, evolution favored peramorphosis — where growth is extended or accelerated, resulting in oversized features. That’s how we get T. rex, a dinosaur with a 6,000-kilogram body and a skull built to crush bone (and oddly undersized arms).
In contrast, Alioraminids — those “Pinocchio rexes” — appear to have gone the other way. They retained juvenile features into adulthood, a process known as paedomorphosis.
“Alioramini is revealed as a derived lineage that retained immature features through paedomorphosis and is not a more basal lineage as widely accepted,” the study authors wrote.
That means these long-snouted oddballs weren’t early experiments in tyrannosaur evolution. They were a later offshoot — quirky survivors in a family tree of giants.
The Prince Who Became King
Ultimately, Khankhuuluu mongoliensis isn’t just another name on the dinosaur roster. It’s a missing piece — perhaps the missing piece — in the rise of one of prehistory’s most terrifying predators.
It shows that tyrannosaurs didn’t simply grow big overnight. They didn’t just roar into dominance. They came from nimble, fast hunters. From the survivors, but also the travelers.
From princes.
And in time, their descendants would become kings.
