The first point to be made is that T.rex was a large creature and it has been said that the only evolutionary reason for this is to be top Predator. In modern birds however, T.rex's possible decedents, the scavengers are the biggest. One theory proposed is based on footprint evidence showing a T.rex chasing some Iguanadonts into a river which suggests it was a predator. T.rex was also believed to have had good eyesight. T.rex had forward facing eyes and perhaps stereoscopic vision that would give T.rex an advantage in locating prey. It was also believed that T.rex had a good sense of smell that could have located prey or a rotting carcass.