Not every dinosaur was gigantic, but the ones that were were absolute behemoths. So, why did they get so much bigger than animals that walk the land today?
Image: The sizes of some of Mongolia's common dinosaurs in comparison to a person, including the adorably petite Velociraptor. / Conty
Matthew Carrano, the curator for the Museum of Natural History's Dinosauria, explains his thoughts on how dinos attained their size — and why the real question might be not why dinosaurs were so big, but why are we so small?:
It's not clear how oxygen relates to size evolution, but that's one hypothesis out there. I tend to think that evolution takes advantage of opportunity until some sort of limit is reached, and dinosaurs had different limits than modern mammals. In fact, I'd say the real question is why mammals (at least on land) are so small.
One thought is that dinosaurs laid eggs and so did not have any problems reproducing even at huge sizes. So population crises could be overcome simply by laying more eggs. Mammals have a terrible time because at large sizes they can only make one or two young every one or two years. If something goes wrong (diseases, etc.), it takes a long time for the population to recover. So mammals are constrained from being very big in a way that dinosaurs weren't.
You can read more about dinosaurs — including what dinosaur would have made the best pet (it's the parrot, don't let them fool you: birds are dinosaurs) — in Carrano's full Q&A here.