If dinosaurs were giant because of a lot of oxygen then how come horses and rhinoceroses used to be teeny tiny?

Question

If dinosaurs were giant because of a lot of oxygen then how come horses and rhinoceroses used to be teeny tiny?

In: Biology

Answer 1

Dinosaurs were not giant because of lots of oxygen in the air. Your starting premise is flawed. Dinosaurs evolved to be large, since evolution tends to favour gigantism if resources are plentiful. Mammals came on the scene during the time period when dinosaurs were dominant, and thus tended to be very small to avoid predation. Once the dinosaurs went extinct, mammals started to increase in size . Peak mammal size occurred just before the onset of humans. Mass extinctions of large mammals immediately followed once humans reached areas where they lived.