The two-legged dinosaurs (ceratosaurs) were rubber suits worn by actors in the desert of Palmdale, CA. When the explorers shoot grenades at them, one of the beasts falls down, apparently dead. The actor inside the suit actually fainted from the extreme heat, and the director decided to use the footage.
All the footage of the principal actors was shot on sound stages at General Service Studios (now Hollywood Center Studios). Only the second unit filming dinosaurs and sloth in Palmdale, Corriganville, and the beach, were on location. All shots of the principals and monsters in the same shot were achieved with rear projection effects.