Misidentified bones that languished in the drawers of a Mongolian institute for 50 years belong to a new species of tyrannosaur that rewrites the family history of the mighty T-Rex, scientists said Wednesday.
This slender ancestor of the massive Tyrannosaurus Rex was around four meters (13 feet) long and weighed three quarters of a ton, according to a new study in the journal Nature.
"It would have been the size of a very large horse," study co-author Darla Zelenitsky of Canada's University of Calgary told AFP.
The fossils were first dug up in southeastern Mongolia in the early 1970s, but at the time were identified as belonging to a different tyrannosaur, Alectrosaurus.
For half a century, the fossils sat in the drawers at the Institute of Paleontology of the Mongolian Academy of Sciences in the capital Ulaanbaatar.
Then PhD student Jared Voris, who was on a trip to Mongolia, started looking through the drawers and noticed something was wrong, Zelenitsky said.
It turned out the fossils were well-preserved, partial skeletons of two different individuals of a completely new species.
"It is quite possible that discoveries like this are sitting in other museums that just have not been recognized," Zelenitsky added.
They named the new species Khankhuuluu mongoliensis, which roughly means the dragon prince of Mongolia because it is smaller than the "king" T-Rex.
Zelenitsky said the discovery "helped us clarify a lot about the family history of the tyrannosaur group because it was really messy previously".
The T-Rex represented the end of the family line.
It was the apex predator in North America until 66 million years ago, when an asteroid bigger than Mount Everest slammed into the Gulf of Mexico.
Three quarters of life on Earth was wiped out, including all the dinosaurs that did not evolve into birds.
Around 20 million years earlier, Khankhuuluu -- or another closely related family member -- is now believed to have migrated from Asia to North America using the land bridge that once connected Siberia and Alaska.
This led to tyrannosaurs evolving across North America.
Then one of these species is thought to have crossed back over to Asia, where two tyrannosaur subgroups emerged.
One was much smaller, weighing under a ton, and was nicknamed Pinocchio rex for its long snout.
The other subgroup was huge and included behemoths like the Tarbosaurus, which was only a little smaller than the T-rex.
One of the gigantic dinosaurs then left Asia again for North America, eventually giving rise to the T-Rex, which dominated for just two million years until the asteroid struck.
