The Lost World (1925)

Released just thirteen years after the book, this silent film is the earliest screen adaptation of Sir Arthur Conan Doyle's novel.  It is also the first American feature film to use model animation as a primary effect.  Said effects - which include numerous dinosaurs - were produced by Willis O'Brien.  O'Brien would go on to do the effects on some film involving a giant ape, as well as becoming the mentor of a fellow by the name of Harryhausen.

The original version of the film was 106 minutes, while the version on this DVD (and freely downloadable at archive.org) runs a mere 68 minutes, but what I saw pretty accurately matches the movie's synopsis on wikipedia; so either not much of note was cut, or that synopsis was based on this version of the film.

The film uses the same basic premise as the book: infamously hot-tempered scientist Professor Challenger organises an expedition to the Amazon in search of a plateau filled with supposedly extinct creatures.  He's accompanied by a skeptical colleague, a big game hunter, and a journalist.  The film also adds a young woman - the daughter of a vanished colleague of Challenger's - so as to shoehorn in a romance subplot.  Said subplot is pretty thin (but might be better fleshed out in the longer version), but at least the young lady doesn't become a damsel in distress, so let's count it as a win.

In any case, the expedition makes it to the plateau, witnesses all kinds of dinosaurs as they forage, hunt, fight and stampede away from a volcanic eruption, then returns to London with an Apatosaurus they've managed to capture.

The Apatosaurus gets free, of course, and smashes and crashes its way through London before falling into the Thames and swimming away.

You may have noticed that my synopsis of the film is all about dinosaurs, and not much about people.  There's good reason for that, as O'Brien's stop motion effects are most definitely the focus and primary attraction of the film.  This movie's effects were as transformative for their time as those of Jurassic Park nearly 70 years later.  Everything else: the romance subplot, the menace of a rather ineffectual 'apeman', and so forth, is just a sprig of parsley on the side of the plate: the meal is in the monsters.

If you have an interest in the history of film, this is worth seeing.  Even 90 years later, O'Brien's effects are impressive.