CONCLUSIONS

Giganotosaurus and Tyrannosaurus were two of the largest terrestrial carnivores ever to walk the Earth. Living, respectively in South America and North America, these creatures represent separate end members of large theropod evolution (Sereno et al. 1996). Yet the two large theropods sampled here and in Barrick and Showers (1994) that lived at similar latitudes (~50°) on opposite sides of the equator exhibit remarkably similar bone oxygen isotope patterns. This would be a very difficult pattern for random diagenetic processes to replicate even without considering coexisting phase and dense/cancellous tests, which also indicate preservation of the biologic signal in these individuals. The _p values suggest that both individuals displayed (with the exception of the distal end of the tail) very similar heat distribution and thus thermoregulatory patterns. Given their body sizes, the thermoregulatory patterns (core homeothermy and greater limb heterothermy) seen in these two individuals strongly suggest to us that, as adults, they maintained metabolic levels midway between those of present mammals and reptiles. These intermediate metabolic levels would have supported homeothermy in the greenhouse world of the Cretaceous and would have supported very rapid growth rates in these theropods.