Taking the temperature of a dinosaur would have been dangerous in the Cretaceous – and difficult today – but some scientists think they have done it, estimating the body heat of many famous species. Their conclusions could alter the way we think about ancient animals' lifestyles, possibly making aspects of Prehistoric Planet out of date before it's fully released.
One of the longest-running debates in paleontology, whether dinosaurs were warm- or cold-blooded, has a new answer: it depends. According to this view, the immense diversity among dinosaurs was reflected in their approach to warming their bodies. Some, like modern mammals and birds, burned a lot of calories to maintain a constant body temp, while others depended on external warmth to get started in the morning.
The very name dinosaur means “terrible lizard” and they were initially seen in this way: as (usually) larger versions of modern reptiles, and therefore dependent on environmental heat. Subsequent evidence has called this into question, particularly the discovery that birds are actually dinosaurs, and the debate has gone back and forwards. A third explanation proposes the larger dinosaurs had so much thermal mass they maintained a steady temperature without needing to warm themselves.
A paper published today in Nature challenges the assumption all dinosaurs should be lumped in together on this question, and provides evidence for widespread variation. That in itself is not hard to believe. The discovery some dinosaurs lived in what was then below the Antarctic circle, experiencing months of near darkness, proved some dinosaurs must have been capable of supplying their own body heat. However, where some of the more famous species have been allocated is more unexpected.
Warm-blooded creatures burn fats and sugars in oxygen to raise their body temperature, requiring much higher oxygen consumption. That's part of the reason marine mammals need to come to the surface to breathe, rather than getting their oxygen through gills ( whatever the makers of some scam products will tell you).
Dr Jasmina Wiemann of Caltech and co-authors realized if they could identify how much oxygen dinosaurs consumed they'd have a very strong indication of how they kept warm. They consider this more reliable than previous methods which involved investigating growth rates or isotope ratios in their bones.
Oxygen consumption in the body produces waste, just as burning coal or wood does, some of which get fossilized as dark organic matter. The authors studied the leg bones of a wide array of animals to measure waste abundance, using surviving species as comparisons.
Most of the dinosaurs the team included in the study also showed signs of high oxygen consumption, but there was a wide range. At the lower end, the saurischians or "lizard-hipped" dinosaurs – famous examples include Triceratops and Stegosaurus – had oxygen consumption similar to modern reptiles.
On the other hand, dinosaurs like T. rex and Velociraptors were much bigger oxygen consumers. This may make it look like the division is based on diet, which makes intuitive sense. However, sauropods, including the largest herbivorous giants, were also warm-blooded according to the team's results, particularly Diplodicus. Like theropods, sauropods were 'bird-hipped”, and it seems members of this clade all provided their own heat.
The study also indicates many non-dinosaurs of the same period were also warm-blooded, including pterosaurs, which evolved high metabolisms long before they took to the skies.
“This is really exciting for us as paleontologists – the question of whether dinosaurs were warm- or cold-blooded is one of the oldest questions in paleontology, and now we think we have a consensus, that most dinosaurs were warm-blooded,” Wiemann said in a statement.
The work also challenges a hypothesis about why some creatures made it through the post-asteroid winter when others didn't. “Having a high metabolic rate has generally been suggested as one of the key advantages when it comes to surviving mass extinctions and successfully radiating afterwards,” Weimann said. If so many of the extinct families of dinosaurs were also warm-blooded, however, this couldn't be true.
“We are living in the sixth mass extinction,” Weimann added, making it crucial to understand previous ones.