Jorge Gonzalez

This goat-size T. rex cousin could reveal how the famed dino got so big

Tyrannosaurus rex and its relatives ruled supreme in North America at the end of the age of dinosaurs—dominating the landscape as gigantic top predators. But exactly how they took over is unclear, because of a gap in the tyrannosaur fossil record: North American specimens that lived between about 150 million and 80 million years ago.

A newly discovered T. rex cousin helps fill that gap. Today in Nature Communications, paleontologists describe a tooth and a hind limb unearthed in Utah, which they say belong to a new species of tyrannosaur that they call Moros intrepidus (Greek for “intrepid harbinger of doom”). It lived about 95 million years ago, and compared with T. rex it was tiny—just over a meter tall at the hip and weighing about 78 kilograms. It was certainly no match for the dominant predators at the time, allosaurs that were up to 12 meters long and weighed 4 tons.

The shape of its leg bone suggests M. intrepidus was a nimble runner, which the researchers say could have helped them both catch prey that the allosaurs couldn’t—and also avoid becoming prey themselves. Together with Asian fossils, the find means tyrannosaurs probably expanded their size relatively quickly—during the past 16 million years of their 100-million-year history.