That's where we disagree I suppose. You're saying the Ornimegalonyx/prehistoric route is highly likely, I disagree.
The Bulbasaur like many other pokemon in the generation looked dinosaur-esque (Charmander line, Lapras, Kangaskhan ect.) But I don't believe they were ever intended to actually be based off of pre-dinosaur reptilians.
I believe it's the same with the Chikorita line, the dinosaur/monster style is just popular.
Sceptile is a large lizard, not a dinosaur.
Torterra is a giant tortoise with snapping turtle influences, while turtles/tortoises have been around a while, they are also popular animals to be used for inspiration. Look at the Squirtle line, even Shuckle has turtle influences.
Serperior is the same as Torterra in the sense of, while snakes have also been around for quite some time. It's just a popular animal to base designs off of like the Arbok line, Dunsparce somewhat, Seviper and Rayquaza.
The Chespin line is where I feel like the prehistoric theory really falls apart. The line is inspired by hedgehogs and chestnuts mostly. Chesnaught's name is even a combinatio of chesnut and juggernaut. The shell on it's back does not resemble that of the glyptodon which was a relative of the armadillo. Chesnaught does not resemble this, nor does it resemble armadillos. The shell on it's back is inspired by a chestnut shell.
I guess so.
Venusaur is obviously well... a dinosaur.
Meganium is obviously a Sauropod.
The Sceptile line takes some inspiration from raptors and dilophosaurs though, such as the arm leaves, which velociraptors have arm feathers similar to that and the head shape resembles that of a dilophosaur.
Torterra's design has some inspiration from prehistoric tortoises(Carbonemys for example.) They are absurdly large and have massive shells, which you can see Torterra has a massive shell as it houses a mini-ecosystem on it's shell.
Serperior can be related to the Titanoboa in a sense, as it is a very large snake-like Pokemon, like the Titanoboa is a very large prehistoric snake. This one is a little flimsy in said theory, but still viable.
Chesnaught does take some inspiration from Glyptodonts. The shell being a chestnut shell doesn't really disprove the theory at all, it's a very tough shell that covers Chesnaught's back similar to that of a Glyptodont.
I guess it all depends on which side of the fence you're on when you get to the core of it. It's neither disproved nor proven, that's the beauty of theories I guess. x)
Could you explain your mairill argument? Cause It's not making sense to me given it's different gen changes (It was water then next gen it got a pre evo which was normal, and then later fairy got added to both) Also as said lavitar and pupitar were Not meant to be dark type the design dosen't fit, but the reason it replaced a type is that the design didn't fit the current critera of pure rock, it fit rock/ground with the ground being like a flying types normal typing. However rowlet could easily have been pure grass since it's well an owlet so it didn't have to fly. It's still possible of course but it just dosen't make much sense to me.
I may have went the wrong direction with my Marill
example. Why not give Azurill water typing rather than normal typing when it's just going to get rid of the normal type?
Larvitar and Pupitar definitely fit the criteria for a Dark Type. They're intimidating looking, not to mention in the anime Larvitar acts very self-centered and arrogant. They only went for Rock/Ground typing because of how they act ingame and it made sense from a design point of view.
Rowlet has been shown to fly though, just because it isn't flying when idle in battle doesn't mean it can't fly at all.