Tyrannosaurus rex and its relatives ruled supreme in North America at the end of the age of dinosaurs-dominating the landscape as big top predators.
"Moros was lightweight and exceptionally fast", Zanno said.
Moros lived about 96 million years ago during the Cretaceous period and is the oldest Cretaceous tyrannosaur species discovered in North America, according to a news release.
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.
"With a lethal combination of bone-crunching bite forces, stereoscopic vision, rapid growth rates, and colossal size, tyrant dinosaurs reigned uncontested for 15 million years leading up to the end-Cretaceous extinction-but it wasn't always that way", says Lindsay Zanno, paleontologist at North Carolina State University, head of paleontology at the North Carolina Museum of Sciences.
"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 added. 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. However, our knowledge has been limited by the lack of North American tyrannosauroid fossils dating from the 70-million-year "dark period", which begins just before the start of the Cretaceous Period. It was likely a juvenile that was almost full-grown - at least 6 to 7 years old - when it died, according to the study.
A life reconstruction of North America's newest tyrannosaur-Moros intrepidus.