Before the king of the tyrant lizards evolved, there were other, smaller tyrannosaurs, such as this Albertosaurus liberatus. Albertosaurus lived about 5 million years before Tyrannosaurus rex, and was about half of its size. However, it still possessed the atrophied forelimbs, and D-shaped teeth, perfect for punching through tough tendon and bone.
The same Albertosaurus stalks a high-altitude plain of the newly-formed Rocky Mountains. During the Cretaceous, the Sevier orogeny (mountain-building event) was occurring, pushing up mountains from New Mexico up to Canada. This created a lot of sediment, as these quickly rising mountains were eroding, which allowed the massive deposits of the North Horn, Hell Creek, and Two Medicine formations to formed, among others.
to the Cretaceous..