The smallest known non-bird dinosaurs were far larger than the tiniest mammals, birds, and reptiles alive today. Energy-based models could not explain this size gap, suggesting competition and other ecological pressures constrained dinosaur evolution. Early birds may have escaped those limits through flight, opening new habitats where much smaller bodies became possible. A dinosaur could tower over a house, yet even the smallest known non-bird species never shrank to mouse size. That missing lower limit may reveal a forgotten evolutionary rivalry. An analysis in the journal Evolution suggests physiology alone cannot explain why non-avian dinosaurs stayed unusually large. Competition with small mammals may have blocked them from occupying the tiniest body-size ranges. “Everyone loves a giant dinosaur,” said Roger Benson, the Macaulay Curator of Dinosaur Paleobiology at the Museum. “But we decided to look at the other end of the scale. The absence of tiny dinosaurs may be just as interesting as the existence of giant ones. We already knew that dinosaurs prevented mammals from evolving to large sizes before the end-Cretaceous mass extinction. Here …