A relative of the Tyrannosaurus rex has been discovered in Uzbekistan, bridging a 20-million-year gap in the carnivore's evolutionary tree. The discovery shows T-rex only grew to its giant size toward the end of the dinosaurs' existence.

A 20-million-year gap in fossil records has meant palaeontologists have been unable to learn how and when the Tyrannosaurs grew so large. Research published in the Proceedings of the National Academy of Sciences describes a newly-discovered fossil dating back 90 million years.

"The middle Cretaceous is a mysterious time in evolution because fossils of land-living animals from this time are known from very few places," said Alexander Averianov, researcher working on the study. "Uzbekistan is one of these places. The early evolution of many groups like tyrannosaurs took place in the coastal plains of central Asia in the mid-Cretaceous."

Timurlengia euotica

The scientists uncovered a new species of horse-sized dinosaur in the Kyzylkum Desert, north Uzbekistan, that is an ancestor to the T.rex. The species – Timurlengia euotica – lived 90 million years ago; 24 million years before the T.rex dominated the land.

The distant cousin of the T.rex weighed around 250kg, and had long legs and sharp teeth. The T.euotica existed some 80 million years after the first Tyrannosaurs are known to have existed.

"Timurlengia was a nimble pursuit hunter with slender, blade-like teeth suitable for slicing through meat," explains researcher Hans Sues of the Smithsonian Institution. "It probably preyed on the various large plant-eaters, especially early duck-billed dinosaurs, which shared its world."

Evolution of Tyrannosaurus rex

The discovery gives researchers an insight into how the T.rex evolved to its large size. Seeing as T.euotica was still horse-sized 90 million years ago, scientists say the T.rex must have evolved to its 5m height right at the end of the Tyrannosaur's evolutionary history. In the space of 24-million-years, the Tyrannosaur evolved to three times the height, and 28 times the weight.

Steve Brusatte, the lead researcher said: "The ancestors of T. rex would have looked a whole lot like Timurlengia, a horse-sized hunter with a big brain and keen hearing that would put us to shame." He added: "Only after these ancestral tyrannosaurs evolved their clever brains and sharp senses did they grow into the colossal sizes of T. rex. Tyrannosaurs had to get smart before they got big."