Giganotosaurus, at 12.5 m in length and ~8000 kg estimated mass (Coria and Salgado 1995) is similar in size to Tyrannosaurus rex (~6000 kg, Anderson et al., 1985) and is the largest theropod known. The type specimen of Giganotosaurus carolinii was recovered from the Upper Cretaceous fluvial deposits of the Rio Limay Formation (Albian-Cenomanian), Neuquen Group, deposited at southern mid-paleolatitudes in the eastern part of Neuquen Province, Argentina (Coria and Salgado, 1995; Scotese, 1997). It has been assigned to the Carcharodontosauridae, a group of advanced allosauroids also known from the middle Cretaceous of Africa and North America (Sereno et al. 1996; Harris 1998). The large size of this theropod suggests biological comparisons including thermal physiology and food requirements with the northern hemisphere tyrannosaurs.
Barrick and Showers (1994) used oxygen isotope values to delineate the thermal physiology of Tyrannosaurus rex. They concluded that T. rex was a homeotherm (maintained a pattern of stable core body temperatures) by virtue of a metabolic rate elevated above the modern reptilian level. It was argued that the data were the result of diagenetic alteration (Kolodny et al. 1996) and that homeothermy in dinosaurs is the result of gigantothermy (Paladino et al. 1997). The case against diagenetic alteration has been outlined in Barrick et al. (1996) and Barrick (1998). Two papers (Barrick and Showers 1995; Barrick et al., 1996) have shown that patterns of homeothermy may be found in juvenile and small adult dinosaurs (20-150 kg) as well as in large individuals (2000-4000 kg). These dinosaurs did not live in a completely homogenous environment with respect to temperature, thus simple mass homeothermy or gigantothermy (patterns of stable core body temperatures maintained in bradymetabolic animals utilizing large body size and homogenous environmental temperatures) is not an adequate explanation for isotopic patterns seen in these dinosaurs. Barrick et al. (1996) and Reid (1996) have suggested intermediate metabolic levels for the Dinosauria. However, the isotopic patterns in a large theropod have yet to be repeated. In this paper we report the results from a Giganotosaurus carolinii individual and compare them to the pattern of isotope distribution found in the previously studied T. rex individual.