Chelonian Conservation and Biology, 2002, 4(2):263--287

The desert tortoise (Gopherus agassizii) has traditionally been viewed as an archetypal desert-adapted vertebrate. However, evidence from historical ecology, phylogenetics, anatomy, physiology, and biogeography qualifies this view significantly. Ancestors of G. agassizii stabilized as an essentially modern morph some 17-19 million yrs ago ago, perhaps 12 million yrs before the formation of major regional deserts in North America. Some physiological mechanisms for avoiding or accommodating desert stressors may be symplesiomorphies, primitive character states common to most ectothermic amniotes. Prominent among these are slow metabolic rates and high tolerances for osmotic flux in body fluids. Other functional characteristics for accommodating contemporary aridity are exaptations shared with forest-dwelling batagurid and manourine chelonian antecedents, originally evolved for terrestrialism, not aridity. Large brittle-shelled eggs, herbivory, and a generalized and expansive digestive tract may all be among these symplesiomorphies, at least relative to the gopherine clade. Other anatomical and behavioral features are associated with a fossorial life style which may have developed in sandy habitats within grasslands and along forest edges, where microclimates were semi-arid, but at a time North American landforms had not yet experienced desert aridity. Burrow excavation may have evolved in response to the stress of intense insolation in exposed scrub, grasslands, and meadows, only later serving as protection against cold, heat, and predators. Modern climate and vegetation typical for contemporary populations of G