We know that the big switch happened some time in the middle part of the Cretaceous, between about 110 and 84 million years ago. Before this time, there were many small to midsize tyrannosaurs living all over the world, with only a few random bigger species like Yutyrannus. After this time, enormous tyrannosaurs reigned throughout North America and Asia, but only those continents, and no species smaller than a minibus remained.
