This is sort of getting close to trying to construct an evolutionary history of the Pokémon world, which is something I said I would never try to do because trying to work that $#!t out in the real world where we have thousands of prehistoric species to compare is complicated enough (did you know that the closest living relative of the elephant is a small hamster-like creature called a hyrax?), and I am not an evolutionary biologist. But hey, what the hell?
It’s entirely possible that the whole thing is just convergent evolution – that is to say, Meganium and Tropius, despite their common appearance, are actually no more closely related to Aurorus than they are to Pikachu; they just happen to have adapted in similar ways to similar stimuli. If you believe that Pokémon type is a reliable indicator of evolutionary lineage (which I don’t, but just for the sake of argument) then this makes a great deal of sense, because the modern sauropod Pokémon are Grass-types, while Aurorus is an Ice-type. I’m not totally sold on this because I’m not sure I can think of any real-world examples of convergent evolution that are so striking, but it’s possible that the internal biology of these species is radically different and they just happen to look similar…
Anyway. There’s no reason, ultimately, why large reptiles necessarily have to precede large mammals and die out before mammals can become dominant. There’s nothing that makes one inherently superior to the other, which is the thing people tend to forget about evolution; it’s not about general superiority, it’s about suitability to particular conditions. Things just happened to turn out that way in our world, because there was a meteor, and lots of pain, and I think the phrase “global firestorms” has been thrown about, which is never a fun time, and as a result all the largest animals died out very suddenly and all their niches became open all at once, which is a recipe for major adaptive radiation on the part of the smaller animals. People weren’t totally sure for a while what caused the KT extinction but I think most paeleontologists tend to agree now that the ten-kilometre-wide hunk of flaming rock certainly didn’t help. It’s possible that the Pokémon world just didn’t experience KT, or didn’t react to it in the same way. If we believe the events of the first Mystery Dungeon games, Rayquaza (who may well be immortal) has both the power and the inclination to prevent apocalyptic meteor strikes, and while I tend to take Mystery Dungeon with a grain of salt it is worth remembering that there are, and probably always have been, godlike powers at work in the Pokémon world. I would hypothesise that in some parts of the world the ‘saurian’ Pokémon lineages died out in a much more peaceful and gradual fashion (which let’s face it, many dinosaur lineages were beginning to do in the real world anyway, long before KT) and were replaced by large mammalian Pokémon, while in other areas they continued to flourish, adapt and evolve, creating two or more radically different ecological spheres, which were only recently brought together again by humans in a sort of Columbian Exchange-type fashion. Think Australia, but with surviving dinosaurs instead of marsupials. I’d say that’s my best guess.
Pokémaniacal 