Two Lost Worlds (1950)

Starring Laura Elliott, James Arness, William Kennedy, Gloria Petroff, Pierre Watkin
Directed by Norman Dawn
(actor & director credits courtesy IMDB.com)

The first mate of an American clipper ship is dropped off in Queensland to recover from an injury, where he falls in love with the magistrate's daughter, and tries to help the people prepare for a pirate attack.

An early pre-Gunsmoke starring role for James Arness comes in in a pretty well done historical drama, with plenty of action scenes and some nice chemistry between Arness and Elliott (later to be billed as Kasey Rogers in her career).  There's a love triangle, some western-flavor scenes on horses, and battles with pirates.  However, the film's best known for its climactic scenes, in which a party of survivors are stranded on a volcanic island with giant lizards, courtesy of footage from One Million B.C.  It's odd the filmmakers chose to go that way after staying within the realm of a period piece for the majority of the picture, but I love it that they chose the sci-fi angle, and probably wouldn't have paid much attention to the film without it.  The actors are well-integrated into the footage and it adds some excitement that otherwise probably would have been lacking in a more pedestrian finale.

Them! (1954)

Starring James Whitmore, Edmund Gwenn, Joan Weldon, James Arness, Onslow Stevens
Directed by Gordon Douglas
(actor & director credits courtesy IMDB.com)

Authorities in New Mexico discover atomic fallout has enlarged a colony of ants to gigantic size, and set out on a race to destroy them before they can migrate and multiply.

In my opinion, this was the best of the giant bug movies of the 1950s, and it holds up remarkably well, due to its excellent special effects and the serious performances of the distinguished cast, particularly Whitmore and Gwenn.  Although the ants are giant mocked-up puppets which are not animated and did not have a wide range of movement, they look very fearsome as manipulated by their handlers and photographed by Sid Hickox, with eerie and deafening sound effects.  But even before the ants appear, we're treated to a mysterious and suspenseful opening of the film which ranks among the best in any science fiction picture, as a young girl in shock wanders aimlessly through a desert and policemen follow her trail to a series of violent and gruesome murders.  There's also plenty of famous names in the cast to spot, from Gunsmoke's James Arness and Davy Crockett's Fess Parker to a very young Leonard Nimoy, and familiar character actors like William Schallert, Onslow Stevens, and Richard Deacon.

The Thing From Another World (1951)

Starring Margaret Sheridan, Kenneth Tobey, Robert Cornthwaite, Douglas Spencer, James Arness
Directed by Christian Nyby
(actor & director credits courtesy IMDB.com)

In the frozen Arctic, a team of scientists and Air Force men discover a spaceship and its monstrous pilot trapped under the ice.

One of the best science fiction films of the 1950s, the picture features a winning ensemble of a cast, and is highlighted by engaging dialogue and a memorable monster, as well as several eerie and suspenseful scenes.  This is likely the finest hour for both actor Kenneth Tobey as the stalwart Air Force captain, and Robert Cornthwaite as the determined scientist whose character became something of a sci-fi archetype.  Dimitri Tiomkin delivers an atmospheric and appropriately chilling musical score, and the film's direction is first rate, although per Wikipedia, there's been a great deal of contention over the years whether Nyby actually directed, or if producer Howard Hawks really helmed the movie.  Although John W. Campbell's original story "Who Goes There?" is named in the credits, the film is far from a literal adaptation of that story, but remains a gripping and impactful picture.