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

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

In: Biology

Where are you getting the idea that dinosaurs were big solely because of the oxygen level of the atmosphere?

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.

Dinosaurs were big because of a lot of reasons, Oxygen being only one, and even then only a minority of them.

Most Prehistoric ancestors to Modern day animals were small because they have all been evolving from very small shrew-like ancestors. Many lineages do tend to increase in size and mass as time goes on, just as Dinosaurs in the Triassic were far smaller than the some of the giants of the Cretaceous. Equally, some of the smallest dinosaurs to ever live also existed in the Cretaceous.

The root answer is: Because they were. Evolution is selection pressures acting on a population. That pressure can select for big individuals, small individuals, camouflaged individuals ect ect. And that largely depends on the environment in which those populations live.

Island dwarfism is one of the more common reasons for pygmy species, where a population separated from it’s brothers by the formation or emigration to an island can develop to be far smaller than mainland counterparts in order to subsist on limited resources. Equally some species can experience the inverse such as in the case of Giant Galapagos Tortoises.

It’s all about selection pressures, and the environment the populations live.