One of the reptile keepers at this fabulous museum/zoo/botanical garden told me that these iguanas had been deliberately introduced in the 1970's, and had since established a breeding population. We saw two of them basking on the boulders by the Bighorn Sheep exhibit.

An article on the Tucson Herpetological Society website describes DNA studies that show that this population appears to have developed from at least two lineages of different Ctenosaura species, Ctenosaura macrolopha and Ctenosaura conspicuosa.