"Binocular vision requires eyes set slightly apart that have overlapping fields of view, meaning the same eye sees the same thing at the same time from a slightly different perspective," Loewen explained. "This allows the animal to have depth perception."
Although T. rex emerged some 10 million years after Lythronax, both lived at a place called Laramidia, which existed along the western shores of a great seaway that separated North America. The tyrannosaurid dinosaurs likely evolved in isolation on the island continent, with incursions of the seaway separating small areas of land from each other. This further allowed different species of dinosaurs to evolve on different parts of the landmass.
Sertich explained that rising mountains and fluctuating sea levels made for a very dynamic landscape, which set the stage for the evolution of unique dinosaurs and their ecosystems.
"Laramidia over its history was a paradise of dinosaur evolution," Loewen said. "Many groups, including ankylosaurs, ceratopsians, hadrosaurs, ornithomimids and tyrannosaurs underwent radiations on this island continent. In some ways, it was the crucible of evolution during the Late Cretaceous."
The researchers unearthed the remains of a new specimen of Teratophoneus that is the most complete tyrannosaur from southern Laramidia. Together, all of the fossils suggest that the evolution of T. rex and its relatives was much more complex than previously thought.
Co-author Randall Irmis, of the Natural History Museum of Utah and the University of Utah's Department of Geology and Geophysics, told Discovery News that the recent discoveries reveal ancient ecosystems that were nearly unheard of just a decade ago.
"Nearly every dinosaur species we find is new to science, and the same is true for other animals like crocodiles and turtles," he said. "Many folks are under the impression that one has to go to exotic locales, such as China, Mongolia, Argentina or Madagascar, but these finds exemplify the fact that we need not look any further than our own backyard to make important new dinosaur discoveries."
The very toothy skull of Lythronax is now on permanent display at the Natural History Museum of Utah.