Plot hole: In the arial shot of the plateau, it's obvious its just a mesa. It's not at all large enough to support such big animals like those dinosaurs, who would have to eat great amounts of food. All small animals would be hunted into extinction, and all of the trees would have been cleared out long ago. Plus, there would be no place for the dinosaurs to hide, so the team should have seen the monsters right away.
Plot hole: When the group arrive on the plateau, they are "visited" by a dinosaur. Challenger states it is a brontosaurus. However, no one in the group ever actually saw the dinosaur. No one came in contact with the dinosaur at all except for the roaring sound (this is verified later in the movie). Summerlee doesn't even believe they were visited by a dinosaur at all! How could Professor Challenger even remotely theorize it was a brontosaurus? Plus, later we see it ourselves, and even I could say it wasn't a brontosaurus (it's low to the ground and is eating another dinosaur).