Since dinosaurs were first named in the 19th century, palaeontologists have argued about whether they were cold-blooded like modern reptiles, or warm-blooded like mammals and birds. A novel technique suggests they might have had a foot in both camps.
of the University of California, Los Angeles, and colleagues estimated the body temperature of two types of dinosaur by analysing fossil eggshells. They found that Titanosaurus, a long-necked sauropod around 10 metres long and 13 tonnes in weight, had a body temperature around 38聽掳C, similar to modern mammals.
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On the other hand, Oviraptor, a theropod about 2 metres long and 35 kilograms in weight, had a body temperature around 32聽掳C. This is still warmer than crocodiles and their relatives, suggesting that oviraptors generated some heat internally to keep their bodies above the ambient temperature and allow them to be more active. But it also suggests their physiology was not fully warm-blooded, which would require much more energy to maintain.
The technique is based on measurements of the heavy isotopes carbon-13 and oxygen-18, which tend to bond together in a way that depends on temperature. If a mineral forms in a cold environment, these isotopes clump together more.
The team had previously analysed fossilised teeth and obtained , but this is the first time they have modified the technique for use on smaller dinosaurs, for which we have far fewer teeth to work on.
Eagle and his team looked at the presence of these isotopes in eggshells from 13 present-day bird species and nine reptiles, allowing them to identify a relationship with body temperature. Then they did the same with the best-preserved fossil eggshells and worked out the corresponding body temperatures.
Body mass effect
The finding is significant because larger animals are better able to retain heat and so would be expected to have higher body temperatures. Previous studies that suggested this was true of dinosaurs don鈥檛 necessarily indicate that they generated heat internally like mammals.
鈥淏y making these measurements in eggshells in much smaller species, we can eliminate this body mass effect on body temperatures and really get at their physiology in a better way than was possible previously,鈥� Eagle says.
Michael D鈥橢mic, a palaeontologist at Adelphi University in New York, says the technique has huge potential but cautions that it will have to be scrutinised by other researchers. If it stands up, 鈥渋t opens up a big new avenue of research鈥�, he says.
D鈥橢mic thinks we shouldn鈥檛 expect to be able to infer a 鈥渢ypical鈥� dinosaur physiology from any one species. 鈥淎s we learn more about the group, we鈥檙e learning just how diverse it is, metabolically and in terms of growth,鈥� he says.
Theropods are the dinosaurs most closely related to modern birds. Eagle is now keen to study the fossils of early birds to determine when they became warm-blooded. 鈥淚 think that evolutionary transition is the next big question we鈥檇 be excited to look at,鈥� he says.
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(Image credit: Joe Tucciarone/Science Photo Library)
