Their precise evolutionary origins, however, have long been murky. A critical swathe of the family tree is blank.
Khankhuuluu, known from two partial skeletons in fossil collections at the Institute of Paleontology in Mongolia, helps fill in this gap - a transitional 86 million-year-old species that represents the closest known ancestor to the famed late tyrannosaurs.
Previously, it had been described as an alectrosaurus, another early tyrannosaur.
Asked for a modern-day comparison, University of Calgary graduate student Jared Voris, who led the work, said to imagine a large, predatory horse.
“What makes them so important is their age,” said Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study.
“They are about 86 million years old, a good 20 million years older than T-Rex. It has been a frustrating gap in the record.”
Voris was on a research trip in Mongolia in 2023 when he sent a text halfway across the world to his adviser, Darla Zelenitsky, an associate professor of paleontology.
He told her that he thought some of the fossils he had examined in a museum collection were actually a new species.
Those specimens had been classified for decades as alectrosaurus, “an enigmatic and poorly represented tyrannosauroid species”, Zelenitsky and Voris wrote in their study.
What leaped out to Voris initially was that the snout bone was hollow, a clear sign that this was an early ancestor of the tyrannosaur family.
It was the first of what would come to be dozens of features that suggested this creature was something new.
Zelenitsky, a paleontologist at the University of Calgary, said she was excited but cautioned Voris to take his time.
Some bones were sent to Japan, where the research team was able to conduct CT scans to study them in greater detail.
The scientists also travelled to the American Museum of Natural History in New York to study alectrosaurus fossils to make sure the animals they were researching were distinct.
“There’s no doubt this is a new species,” Zelenitsky said. “There’s also no doubt that it’s the ancestor to these large apex predator tyrannosaurs.”
The new study reveals an intermediate form - a slender, relatively flat-snouted creature.
Other tyrannosaur experts said it was good to see these specimens re-examined with modern techniques and understanding.
Thomas Holtz, a vertebrate paleontologist at the University of Maryland, recalled that in the 1980s, the find in Mongolia was relatively famous within the small community of tyrannosaur specialists.
He was the envy of colleagues back then, because he had been able to hunt down a blurry microfiche of the paper, published in an obscure Mongolian geologic journal in the 1970s.
“It helps fill in a gap in time and in the evolutionary tree,” Holtz said.
Khankhuuluu is part of a gold rush in discoveries that are filling in more chapters of tyrannosaur evolution.
Over the past 15 years, there has been an explosion in discoveries of tyrannosaur species, said Joseph Sertich, a vertebrate paleontologist with the Smithsonian Tropical Research Institute.
Sometimes, new species come to light by dusting off old fossils and finding ones that were misclassified in museum collections, as in the new study. At other times, scientists dig up new bones.
Voris and Zelenitsky paint a picture of species evolution and migration as tyrannosaurs dispersed between Asia and North America over millions of years.
Khankhuuluu, or another closely related species, left Asia and moved into North America, giving rise to tyrannosaurs there around 85 million years ago.
Several million years later, a migration back to Asia resulted in two new branches of the tyrannosaur family tree: the giants on one branch and the “Pinocchio rexes” - smaller dinosaurs with long, slender snouts - on the other.
And at the very end of tyrannosaur evolution, one of the giants migrated back to North America, giving rise to T-Rex.
Sertich drew the analogy to big cats, where many species can live alongside one another - leopards, lions, or tigers coexisting as predators in the ecosystem.
“The patterns of evolution we are uncovering are revealing a complex story of evolution that goes far beyond tyrant dinosaurs and has bearing on the origins of the modern ecosystems of the Northern Hemisphere,” Sertich said.
“Ultimately, this paper is exploring the ancient connections between North America and Asia, two continents that have shared species, including humans, for the past 90 million years.”
