In communities with megatheropods, Ms. Schroeder and colleagues found what they call a “carnivore gap”: a large swath of medium-size niches that were empty. For instance, the Hell Creek Formation, a well-studied fossil area that stretches from modern day Utah to Alberta, was once home to 630-pound grown-up dromaeosaurs, 7-ton adult T. rexes — and no mature meat-eaters in between.
Source: wiktionary