Scientists Estimate All-time T. Rex Population Was 2.5 Billion The URL has been copied to your clipboard 0:00 0:05:25 0:00 Pop-out player A new study suggests that up to 2.5 billion Tyrannosaurus Rex dinosaurs lived on Earth over a period of a few million years. However, the research shows that not many of these powerful creatures lived in the same place at the same time. Tyrannosaurus Rex, commonly known as T. rex, existed on Earth for millions of years. Scientists involved in the study estimate about 2.5 billion of them lived over a period of 2.4 million years. The dinosaurs mostly lived in the area of the world now known as North America.