Comparison of Thermoregulation of Four Ornithischian Dinosaurs and a Varanid Lizard from the Cretaceous Two Medicine Formation: Evidence from Oxygen Isotopes

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Abstract
The delta(18)O values of vertebrate bone phosphate are dependent upon the delta(18)O values of their body water and the temperature of bone deposition. This allows the measurement of the temperature fluctuation during bone deposition for individual skeletal elements within any individual. delta(18)O results for bones from a varanid lizard and four Late Cretaceous ornithischian dinosaurs (Orodromeus makelai, Hypacrosaurus stebingeri, Montanaceratops sp., and a juvenile Achelousaurus horneri) from the Campanian Two Medicine Formation in Montana are compared. Results from the varanid indicate that bone deposition occurred over a range of 10-15 degrees C, as expected for an ectotherm. Results from the dinosaurs indicate that bone deposition, occurred over a range 2-7 degrees C within the body core and 3-8 degrees C in the extremities. The intrabone and interbone pattern of delta(18)O values suggests that these dinosaurs were primarily homeotherms with all but Orodromeus exhibiting regional heterothermy Homeothermy over the size range of the individual dinosaurs suggests that they had metabolic rates elevated above those of modern ectotherms and the range of isotopic variability suggests intermediate endothermy.