Measurements of Δ can be used to better understand the physiology of extinct organisms, and to place constraints on the early development of endothermy, the process by which organisms regulate their internal body temperatures.  Prior to the development of clumped isotope analysis, there was no straightforward way to estimate either the body temperature or body water δO of extinct animals.  Eagle et al., 2010 measure Δ47 in bioapatite from a modern Indian elephant, white rhinoceros, Nile crocodile and American alligator.  These animals were chosen as they span a wide range in internal body temperatures, allowing for the creation of a mathematical framework relating Δ of bioapatite and internal body temperature.  This relationship has been applied to analyses of fossil teeth, in order to predict the body temperatures of a woolly mammoth and a sauropod dinosaur. The latest Δ temperature calibration for (bio)apatite of Löffler et al. 2019 covers a wide temperature range of 1 to 80 °C and was applied to a fossil megalodon shark tooth for calculating seawater temperatures and δO values.