An analysis of fossil eggshells may have settled a long-running debate about dinosaurs, suggesting that all species were warm-blooded.
This also means the ancestors of dinosaurs must have been warm-blooded too, says Robin Dawson at Yale University, who led the research.
It is now mostly agreed that the feathered dinosaurs called theropods that gave rise to birds were warm-blooded, but there is still a debate about whether other groups of dinosaurs were too. Until recently, we had only indirect methods of working out the body temperature of ancient animals, so there was no way to be sure.
There is a way to work out the temperature at which organic matter forms inside bodies based on carbon and oxygen isotopes. This technique can be applied to eggshells to reveal the body temperature of the mother when the shells formed.
In 2015, researchers applied this method to the eggshell of a theropod and a sauropod – a long-necked dinosaur – and found both were warm-blooded. Now Dawson’s team has applied this method to three more fossil eggshells.
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One belonged to a theropod called Troodon formosus, and another to a duck-billed dinosaur called Maiasaura peeblesorum. The researchers are confident the third eggshell belonged to a sauropod known as a dwarf titanosaur, although the dinosaur hasn’t yet been definitively identified.
The team’s analysis suggests the duck-billed dinosaur had a body temperature of 44°C, the troodon had a temperature up to 38°C and the dwarf titanosaur 36°C – all warmer than the environments they lived in.
Crucially, duck-billed dinosaurs belonged to a different group of dinosaurs from theropods and sauropods, which are more closely related to each other. This group includes animals such as triceratops and stegosaurs. It is much less likely that warm-bloodedness evolved independently in each of these three major types of dinosaur, says Dawson, which implies all dinosaur groups would have shared this trait, pointing to an ancestral origin.
“If these three major groups had the capacity to use their metabolism to raise body temperature, that is something that stands for them all,” she says.
However, it appears the troodon’s body temperature sometimes dropped as low as 28°C, says Dawson. So it may have been heterothermic: able to lower its body temperature to save energy, as many present-day birds and mammals can.
What these findings don’t tell us is why dinosaurs’ ancestors evolved to be warm-blooded, something that is still hotly debated.
Journal reference: Science Advances, DOI: 10.1126/sciadv.aax9361
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