An amazing feature of many dinosaurs was their size, and the group included some of the largest terrestrial animals to inhabit Earth. But how did the largest dinosaurs get so big? Did they grow slowly, like living reptiles, reaching adulthood over periods perhaps exceeding a century? Or did they grow very rapidly, like living birds and mammals, attaining adult size in just a few decades? How did birds, now known to be direct descendants of carnivorous dinosaurs, acquire their exceptionally fast growth rates? Was it through their dinosaurian ancestry?