Ecological boundaries have been of interest to naturalists since the time of Darwin and Wallace because they are transitional zones on the landscape across which distinct changes occur in constitution of plant and animal communities. In the xeric landscapes of the central Baja California Peninsula, fan palm (Erythea armata and Washingtonia robusta) oases are small (usually <1 ha) mesophilic islands of structurally complex habitats. We report new records of mesophilic reptiles and amphibians from the adjacent Californian biome in palm groves of the Sonoran region; these highly philopatric species provide evidence of earlier cooler and moister Pleistocene environments. The fan palm oases of the central Baja California Peninsula are natural laboratories for the study of evolutionary processes because they provide unique mesic habitats in a changing desert landscape.