I must not have seen Fantasia until I was a few crucial years older, because I remember loving "Night on Bald Mountain" -- the drama! the music! Whereas a lot of the other parts were a lot more candyfloss to me, though some of it was beautiful -- the mushrooms (though now erk, of course), the goldfish, etc. And I had totally forgotten the amoebas-through-dinosaurs bit, but you've called back a dim memory of them collapsing under the broiling sun. I think I mostly liked that section, being the science-minded kid I was, but got upset about watching all the dinosaurs die.

(I suspect the dinosaurs were at least reasonably accurate by 1940s understanding, if not necessarily cutting-edge, based on a 1954 encyclopedia we had when I was a kid. Definitely heavy on the "cold-blooded, lumbering, sluggish, dragging their tails everywhere as they heavily trundled along" interpretation.)