Asked by: David Paylor, Oxford
Dinosaurs lived during the Triassic, Jurassic and Cretaceous. During these periods, the climate was much warmer, with CO2 levels over four times higher than today. This produced abundant plant life, and herbivorous dinosaurs may have evolved large bodies partly because there was enough food to support them. But being large also helps to protect against predators.
The giant sauropods had to eat plants as fast as they could, to grow big enough to be safe from carnivores like T. rex and Spinosaurus. Meanwhile, the carnivores were becoming larger just so they could tackle their enormous prey. Another possibility is that the herbivorous dinosaurs were ectothermic (cold blooded), and being huge helped them regulate their temperature. This theory is problematic though because evidence increasingly suggests that the large carnivores were endothermic (warm blooded), which means that dinosaurs would have evolved two different metabolic systems, side by side.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.