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Warm or Cold-blooded? Dinosaurs’ Bones Could Hold the Answers

"A paper published in Nature may hold the key to understanding dinosaurs’ thermal regulation. Read more here at Ancient Beasts!"

Paleontologists have been perplexed by the question of dinosaur blood temperature for decades. While someone not invested in the field may think it strange to be quibbling over whether a class of animal that no longer walks the earth in its ancient form was ectothermic or endothermic, the temperature of dinosaurs’ blood can tell us a great deal about how they lived. 

Luckily, a paper in Nature seeks to end the confusion once and for all by proposing a brand new method for studying the metabolic rates of dinosaurs – through their bones. Here are some details of what they discovered and what it could mean.

The Question Of Dinosaurs' Thermal Regulation May Soon End

Can dinosaur bones tell us if they were warm or cold-blooded?

Jasmina Wiemann, Nature’s lead author, is particularly excited about its findings. 

‘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 is a postdoctoral researcher at the California University of Technology. Matteo Fabbri, a postdoctoral researcher at Chicago’s Field Museum, had the following to say about Wiemann’s paper: 

‘The new proxy developed by Jasmina Wiemann allows us to directly infer metabolism in extinct organisms, something that we were only dreaming about just a few years ago. We also found different metabolic rates characterizing different groups, which was previously suggested based on other methods, but never directly tested.’

People usually consider metabolism as an indication of how easily someone stays in shape. This makes sense because your metabolism has a lot to do with your nutrient intake and how easy it is for your body to use up those nutrients. 

However, Wiemann points out that ‘metabolism is how effectively we convert the oxygen that we breathe into chemical energy that fuels our body.’ 

As Wiemann explains, warm-blooded or endothermic organisms have a high metabolic rate. This means that they take in mass amounts of oxygen and consume vast calories in order to keep their body temperature under control and protect themselves from the elements. 

Ectothermic organisms, or those typically regarded as cold-blooded, breathe far less than those of the endothermic type and also have to consume fewer nutrients in order to support their core functions. Their lifestyles are also less expensive in an energetic sense than that of the endothermic group. 

However, this means that ectotherms, like reptiles, are completely reliant on their environment to regulate their body temperature, whereas endothermic mammals and birds are able to control this temperature themselves. An excellent example of this mechanic at work is lizards’ propensity to bake in the sun. 

The Question Of Dinosaurs' Thermal Regulation May Soon End

Is the debate over now?

The incredible decade-spanning debate of whether dinosaurs were cold or warm-blooded likely spawns from the identification of creatures like alligators and crocodiles as the genetic descendants of these magnificent beasts. For years many researchers have held the notion that if these descendants are cold-blooded reptiles, the organisms they genetically originated from must have been too. 

However, not everyone holds these notions as the unequivocal truth as new descendants of the dinosaurs have been identified to be birds such as the chicken, among various mammals. Thus, the debate rages on. Or does it?

During their research, Wiemann’s team found that, overall, dinosaurs had relatively high metabolic rates. However, there exists a distinct difference between the metabolic rates of saurischians and ornithischians. These are the two main groupings that dinosaurs are divided into according to taxonomy, with the former being lizard-hipped dinosaurs and the latter those with the presence of bird-like hip bone arrangements. 

Ornithischians like the Stegosaurus and Triceratops were found to have had lower metabolic rates comparable to those present in modern cold-blooded organisms. On the other hand, the saurischians, further divided into the sauropods and theropods, were found to have had metabolic rates so high that they equal those of modern birds. Among the most well-known dinosaurs, all but confirmed to have been warm-blooded, are the Tyrannosaurus Rex, Brachiosaurus, and Velociraptor. 

The findings echo the results of previous independent observations and investigations, but the latter had a lack of direct proxy, and therefore the metabolism rates of the dinosaurs could not previously be inferred. 

The Question Of Dinosaurs' Thermal Regulation May Soon End

So where does this leave us?

Wiemann says that information about dinosaurs’ metabolic rates can give us much-needed context into their lives. For instance, ‘Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioral’ thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live.’ 

In reference to the saurischians, Wiemann says: ‘The hot-blooded giant sauropods were herbivores, and it would take a lot of plant matter to feed this metabolic system. They had very efficient digestive systems, and since they were so big, it probably was more of a problem for them to cool down than to heat up.’ 

Wiemann points out that this class of information is integral to our survival: ‘It is important for us to understand how modern and extinct animals physiologically responded to previous climate change and environmental perturbations so that the past can inform biodiversity conservation in the present and inform our future actions.’

News source: FieldMuseum

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Russ Kidson

Russ Kidson

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