After the dinosaurs went extinct, why did they not return over time through evolution?

Question

I never learned much about evolution, so please do explain it like I’m 5

In: Biology

Answer 1

So the asteroid that killed most of the dinosaurs (birds are dinosaurs so some survived!) made almost every animal larger than 25kg extinct.

So what you had left was a bunch of small mammals and a bunch of small flying dinosaurs (birds) left.

So dinosaurs might have evolved back from birds, but it’s thought that mammals took the big animal spots instead because they give birth to live young, which could be much bigger than the young of egg laying animals. 

To grow big an animal from an egg has to compete with a load of small animals, then a load of medium sized animals, then a load of big animals. For mammals they can start reasonably big already (think like a baby elephant, who are born bigger than many adult humans)