THE CLIMATE IMPLICATIONS OF GIANT TORTOISES IN PLEISTOCENE NORTH AMERICA

Tortoises (family Testudinidae) are common components of terrestrial fossil assemblages dating back as far as the Paleocene. Over the past 61 MA the family has repeatedly evolved species of giant size (100+kg). While extant giant tortoises are found only off the coasts of South America and Africa, at least two giant tortoise species persisted in North America until the terminal Pleistocene. These tortoises, Hesperotestudo crassiscutata and Gopherus hexagonatus, were distributed across the American south and could reach carapace lengths of 1+m. Their presence in fossil assemblages is regarded as evidence of ‘frost-free’ conditions of the local environment. This assumption is based on observations of the climate tolerances and habits of modern species of giant tortoise. In addition, experimentation has shown that the modern Galapagos giant tortoise, Chelonoidis nigra, cannot survive at northern latitudes -- such as the US -- without human aid. The assumption that giant tortoise remains are an indicator of frost-free conditions is so pervasive in the literature that their discovery at a paleontological site is often considered evidence that the site dates to an interglacial oxygen isotope stage. However it is possible that their large body size resulted in high thermal inertia which provided protection from below-freezing temperatures. A good thermal model of the North American giant tortoises is needed to test this hypothesis. Developing such a model, however, requires accurate measurements of the animal’s body mass. Currently available body mass estimates for extinct taxa are based on qualitative comparisons with modern giant tortoises, and are insufficient for modeling purposes. The weight and 68 linear measurements from each of 113 tortoises of 20 different species were obtained and compiled in order to develop linear measurement-versus-weight regressions for tortoises based on carapace shape and taxonomy. The resulting weights were entered into a thermal model to determine rates of heat loss under various scenarios. Results indicate that even with the most conservative model parameters, giant tortoises could theoretically survive several hours of sub-zero temperatures. These findings suggest that the presence of giant tortoises in a fossil site should not be taken as evidence for frost-free conditions.