The giant iguana was a large non-modeled Black iguana, that roamed the Prehistoric Deserts, hunting small easy prey. This is unlike a Modern Iguana, which are only vegatarians, however this factor was probablly overlooked in the making of the film. The prehistoric iguana was slightly smaller than a bus, and had a long body, a narrow tail, with spikes along the sides.
The giant iguana was a slow moving hunter, unlike ceratosaurus or allosaurus, and it probally hunted giant spiders, like modern lizards, or humans and other weak prey in the deserts. The giant iguana most-likely wasn't a tree-dweller, unlike modern iguanas (this factor was overlooked as well) and lived probally slept under large hill-sized rocks or in caves. Another factor making the giant iguana unlike modern lizards, was that modern desert lizards are nocturnal, and hunt at night to avoid the day's heat, while the giant iguana hunted durring the day. Many factors were overlooked in the making of the giant iguana, making it an unrealistic creature in the film.