Photo credit: Smokeybjb, Wikipedia
Modern birds are the only living dinosaurs remaining, as their reptile ancestors, termed non-avian dinosaurs, went extinct at the end of the Cretaceous period ~65 million years ago. Their evolutionary relationship can be seen in a number of similar skeletal features and perhaps most notably in feathers which are possessed by all modern birds and have been found preserved in fossils of dozens of dinosaur species. Despite this we still don’t fully understand the evolutionary steps which lead from non-avian dinosaurs through early pre-modern birds to the birds we see today; this is particularly true in the case of thermophysiology. In other words: were non-avian dinosaurs warm-blooded endotherms like modern birds or cold-blooded ectotherms like modern reptiles? One recent study suggests the answer may be neither, and that at least some dinosaurs used intermediate strategies .
A recent analytical technique called clumped isotope analysis has been used in a new study by Robert Eagle et al. from the University of California, LA. They used it to try and determine more information about non-avian dinosaur thermophysiology, i.e. what body temperatures did these dinosaurs have and how did they maintain them?
Isotopes are atoms of any given element, each with a different mass. Each element has a range of isotopes going from relatively heavy with a higher mass to relatively light with a lower mass. This type of analysis is particularly interested in the use of 13C and 18O which are relatively rare heavy isotopes. However, clumped isotope analysis is not just interested in the composition of isotopes in a molecule, but also how those isotopes are arranged in carbonates which are compounds found in certain fossils and also crucially in eggshells and bones. Carbonate crystals have a lattice structure in which 13C and 18O isotopes arrange themselves. It has been determined that at low temperatures these heavy isotopes preferentially ‘clump’ together, while at higher temperatures there is less clumping . By using this method it is therefore possible to determine what the temperature was when a carbonate-containing substance was formed. Luckily for us, that includes fossilized eggshells, bone and sometimes even teeth .
Eagle et al.  determined the degree of heavy isotope clumping in the fossilized eggshells of two different dinosaur groups: oviraptorids, a type of relatively small bipedal dinosaur closely related to modern birds, and titanosaurids, quadrupedal dinosaurs of great size, possessing long necks and tails which are more distantly related to modern birds. Because eggshells are formed within the body of the mother, their formation temperatures reflect core body temperature of the mother.
Body temperature worked out for the oviraptorid dinosaur from their eggshells is around 32°C which is ~6°C warmer than temperature estimates of warm months during the Cretaceous when these dinosaurs were alive (~66-100 million years ago). What this means is that the oviraptorid was able to raise its internal body temperature above the ambient environmental temperature even at the warmest times of the year. Therefore, the oviraptorid is warmer than most reptiles suggesting that it does not possess the same cold-blooded (ectothermic) thermoregulation that they do, however it is also colder than almost all modern birds and mammals. 32°C is significantly lower than the uniformly high, 36-43°C, temperatures of modern birds which means the oviraptorid is also unlikely to possess the warm-blooded (endothermic) thermoregulation strategy that modern birds have. This comparison illustrates that at least some non-avian dinosaurs had a more variable approach to dealing with internal temperature maintenance than we see in modern birds or reptiles. It is possible that they were cold-blooded with thermoregulatory behaviours or other adaptations, or perhaps drivers of their thermoregulation, such as metabolism, were at an intermediate level between cold and warm-bloodedness. The titanosaurid dinosaurs temperature was determined to be between 38°C and 40°C, quite a bit higher than most modern reptiles as well as the oviraptorid dinosaur, once again illustrating a wide range of temperature regulation strategies and body temperatures in non-avian dinosaurs. This high temperature might be an indicator of warm-bloodedness like in modern birds, however another explanation is the phenomenon of gigantothermy, whereby very large animals can more easily maintain high body temperatures because they have a smaller surface area to volume ratio .
Photo credit: LadyOfHats, Wikipedia
It is difficult to tell how representative these dinosaur groups are of thermophysiology in other non-avian dinosaurs; we can however see that these dinosaur groups had quite a variety of temperature regulation strategies, some of which don’t match up exactly with the strategies we see in modern birds. Because of the close relationship between the oviraptorid group of dinosaurs and modern birds we can assume that the high temperatures and true endothermic regulation we see in modern birds originated quite late in their evolution . These new methods are generating new and fascinating ideas about dinosaur thermophysiology and it is certainly an area with countless opportunities for future research.