In the end of the Cretaceous period, the sounds of Tyrannosaurus rex and Tarbosaurus bataar's roar echoed over today's North America and Asia. With a weight of over five tons and devastating biting power, they stood at the top of the food chain on each continent.
But their common ancestors, which existed at the beginning of the Cretaceous period, tens of millions of years before these giants, were much more humble and weighed perhaps only 50 kilograms.
There must have been something in between T-rex and these mini-tyrannosauroids – but there are hardly any finds of these intermediate species.
A recovered link
Canadian researchers have managed to change that. In a new study, they describe the species Khankhuuluu mongoliensis.
Khankhuuluu is the lost connection between the small, early, and these large top predators, says paleontologist Darla Zelenitsky, lecturer at the University of Calgary and one of the co-authors of the study.
The discovery was made by Jared Voris at the same institution. He studied skeletons found in Mongolia in 1972, which turned out to belong to the new species, which lived 86 million years ago.
It was like today's jackal or prairie dog. It did not hunt the large ankylosaur dinosaurs or the long-necked sauropods. It had a body built for speed, with long legs, and teeth that cut through meat, he says.
In the family tree, it is a sister branch to the branch that eventually ended with, among others, T-rex.
The really cool thing is not just that it's a new species, but how it fits into the evolutionary history of tyrannosaurus, says Voris.
Forward and backward
The analysis gives clues about where the species originated.
About 85 million years ago, Khankhuuluu, or a similar species, migrated over the land bridge between Siberia and Alaska and colonized North America, where it developed into many large tyrannosaurus species such as gorgosaurus and albertosaurus, says Zelenitsky.
Seven to eight million years later, someone returned to Asia, which in turn became the starting point for two large subgroups, one smaller and longer-snouted, the other became the giant tyrannosaurinae, weighing up to three to four tons.
One of them then returned to North America, right at the end of the dinosaurs' reign.
It gave North America T-rex, the largest of the largest. It ruled over North America for one to two million years before the asteroid hit, and then it was over, says Voris.
Gustav Sjöholm/TT
Facts: New dinosaur discovery
TT
The new dinosaur is called Khankhuuluu mongoliensis. The name comes from the Mongolian word for prince and dragon. Mongoliensis is a reference to the country where the fossils were found.
In addition to describing the tyrannosauroid itself, which lived during the Cretaceous period, the researchers present a theory about how relatives of the species were spread in three instances forward and backward between Asia and North America.
The study is published in the journal Nature.
