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Washington - A tiny tyrannosaur recently discovered in Utah is helping palaeontologists fill a 70-million-year gap in tyrannosauroids' ascent to top predators, scientists said on Thursday.

The "lightweight and exceptionally fast" Moros intrepidus, whose name means "harbinger of doom," lived about 96 million years ago and is the oldest Cretaceous tyrannosaur species yet discovered in North America.

Palaeontologists believe that the diminutive dinosaur, with hind limbs measuring 1.2 metres and weighing around 78 kilograms, is the link connecting medium sized primitive tyrannosaurus dating from the Jurassic period (about 150 million years ago) to the dominant tyrannosaurus rex of the Cretaceous period (about 81 million years ago).

"Tyrant dinosaurs reigned uncontested for 15 million years leading up to the end-Cretaceous extinction - but it wasn't always that way," said Lindsay Zanno from North Carolina State University, lead author of a paper describing the research published in journal Communications Biology on Thursday.

"Early in their evolution, tyrannosaurs hunted in the shadows of archaic lineages such as allosaurs that were already established at the top of the food chain," the scientist said.

"When and how quickly tyrannosaurs went from wallflower to prom king has been vexing palaeontologists for a long time," Zanno added, explaining that the question sparked the decade-long research which led to the discovery.

"Although the earliest Cretaceous tyrannosaurs were small, their predatory specializations meant that they were primed to take advantage of new opportunities when warming temperatures, rising sea-level and shrinking ranges restructured ecosystems at the beginning of the late Cretaceous," Zanno said.

"We now know it took them less than 15 million years to rise to power."

dpa