The above painting is by famed stop-motion animator Willis O'Brian for the 60s remake of Conan Doyle's The Lost World. As shown, it would have been a far different movie than the rather infamous version that got made. There is a pair of Triceratops, some Brontosauruses, one attacked by an Allosaurus, a Protoceratops, a Stegosaurus, even a Dimetrodon! All would have been stop-motion, no giant lizards. Below that, O'Brian and Irwin look over some designs for a stop-motion dinosaur movie that did get made, The Animal World.
Willis O'Brian, the famous stop-motion animator behind King Kong, the aborted Creation, Mighty Joe Young, and many others, very nearly made Irwin Allen's 1960s remake of Conan Doyle's The Lost World, one of the greatest dino-mation films of all time. The above production painting gives a hint to want it might have like.
Unfortunately, it was, of course, scrapped in favor of lizards and gators with fins and plates stuck on their backs. And worse even than that, one of the beasts, outfitted with plates and ceratopsian-like frill, was referred to as a "brontosaurus", and the fin-backed horned fire-lizard a tyrannosaurus (when Professor Challenger refers to a hatchling version, played by a gecko). Some of the footage of these beasts was replayed on some old Lost In Space episodes, mainly "The Keeper", which were more convincing on that already cheesy show, for virtue of the fact they were supposed to be alien monsters, not creatures from earth's past.
One of the reasons this was done (aside from budget constraints) was because there was a recent fairly realistic use of live lizards playing prehistoric beasts, in the sixties version of Jules Verne's Journey to the Center of the Earth, featuring rhinoceras iguanas as Dimetrodons, fitted with rubbery fins on their backs and a few small spikes along their flanks. The only trouble is, Dimetrodons had a sprawling lizard-like gait, very unlike actual dinosaurs!
Artist William Stout, however, published two books of convention pen and inks, celebrating the films of O'Brian, and he included three of the proposed scenes from the proposed stop-motion version, that never made to screen:
Left: a battle between two t-rexes, and a Pteranodon. That's obviously Challenger himself against the rock in the foreground. Right: Another scene involving a dino circus ac, with a trained Apatosaur and young Triceratops. : It's not known whether this was to take place in Maple White Land, or back in London, with the caveman played by an actor.
This would certainly have been cool: a tug-of war between a Brontosaurus, an Allosaurus and a Titanoboa!