Scientists have discovered a new species of tiny tyrannosaur which helps explain how the dinosaurs evolved from small, speedy hunters, into the bone-crushing apex predators that we know.

The species called Moros intrepidus is a small tyrannosaur who lived about 96 million years ago in the lush, deltaic environment of what is now Utah during the Cretaceous period.

The tyrannosaur, described in the journal Communications Biology, is the oldest Cretaceous tyrannosaur species yet discovered in North America, narrowing a 70-million-year gap in the fossil record of tyrant dinosaurs on the continent.

Lindsay Zanno, paleontologist at North Carolina State University in the US said that, 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.

Researchers said that a decade spent hunting for dinosaur remains within rocks deposited at the dawn of the Late Cretaceous finally yielded teeth and a hind limb from the new tyrannosaur.

The lower leg bones of Moros were discovered in the same area where Zanno had previously found Siats meekerorum, a giant meat-eating carcharodontosaur that lived during the same period.

Moros is tiny by comparison — standing only three or four feet tall at the hip, about the size of a modern mule deer.

Zanno estimates that the Moros was over seven years old when it died, and that it was nearly full-grown.

The bones of Moros also revealed the origin of T rex’s lineage on the North American continent. When the scientists placed Moros within the family tree of tyrannosaurs they discovered that its closest relatives were from Asia.