Like, I’m aware of there being exceptions like Penguins, Ostriches, and Bats. But in general, why is there such a distinct land/air split between mammals and birds? Why don’t mammals share the ground with ecosystems of plant- and meat-eating walking birds? Why didn’t we get birds that evolved to slither like snakes, or tunnel like rodents? Why isn’t it (land+sky) all just mammals, where we’d have parrot- and vulture-like bats that don’t lay eggs? If we started the simulation again, might things like this evolve?
I suspect it’s partly just coincidence. Prior to the K–Pg extinction, the sky was ruled by non-birds. Pterosaurs. In fact, the late Cretaceous saw some of the largest creatures to ever fly, like Quetzalcoatlus and Hatzegopteryx.
At the same time, it was also not mammals that ruled the ground, it was, as many an eight year-old can attest, dinosaurs.
The K–Pg extinction event may have killed off all the non-avian dinosaurs and pterosaurs, in part because of their larger size. So the very thing that made them “rule” before is what killed them off, and conversely, the very thing that kept avian dinosaurs (i.e., birds) and mammals from ruling during the Cretaceous is what allowed them to survive that event and evolve to fill the many niches vacated by non-avian dinosaurs after the event.
Arthropods who’ve been ruling the world for 500 million years: Why do dinosaurs get all the hype? 😭 😭 😭
I think it comes down to the fuzzy definition of “ruling”. The physical size and position in the food chain are among the factors that make dinosaurs rule the mesozoic, in my mind.