From these measurements they calculated which parts were warmest. As phosphate precipitates from the body's fluid, the weightier, less energetic oxygen-18 is preferentially trapped in the solid mineral. The technique cannot reveal absolute body temperatures because the water the animal drank also affects the isotope ratios in the bone. This is the pattern found in large birds and mammals, whose bodies keep themselves at a constant temperature. Cold-blooded animals such as monitor lizards rely on heat from their surroundings to regulate their temperature, so the temperature of their extremities fluctuates much more than their main body parts.
Some scientists are sceptical of the isotope work.
Yehoshua Kolodny at the Hebrew University in Jerusalem doubts whether the isotopic make-up of the T. rex skeleton is the same as it was when the animal was alive. However, Showers says the skeleton is particularly well preserved. Found in Montana in 1990, it has been de-scribed by its excavator, Jack Horner, as 'mummified'. It is the same skeleton from which red blood cells are rumoured to have been extracted. Showers says that comparisons of isotope signatures in the bone carbonate with after-burial calcite deposits points to the isotope ratios that he used to calculate temperature being those from life.