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