Were dinosaurs hot or cold blooded? A new study settles an old debate

There’s an iconic scene in “Jurassic Park” where a velociraptor’s breath fogs up a window as it searches for its prey. If the so-called “terrible lizards” (as “dinosauria” roughly translates to) were cold-blooded – you know, like real lizards – this scene would have been inaccurate. Yet while many things about the “Jurassic Park” franchise are factually questionable (not the least of which is the idea that bringing dinosaurs back from extinction is even plausible), the concept of hot-blooded dinosaurs may very well have been realistic.

This is at least the argument of a new study published in the scientific journal Nature.

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Their crucial piece of evidence is, quite literally, stained – the paleontological team led by Jasmina Wiemann, a postdoctoral researcher at the California Institute of Technology, analyzed the dark spots that often appear in fossils. Sometimes these plaques are caused by the reaction of oxygen with lipids, proteins and sugars and the formation of waste products. The researchers inferred that more of these molecules would appear in warm-blooded dinosaurs because warm-blooded animals have a higher metabolic rate and therefore require more oxygen. All they needed were samples of ancient and modern creatures; for the former, they found femurs of pterosaurs and plesiosaurs.

“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.”

Even better, the waste molecules are stable enough not to deteriorate significantly during the fossilization process. When previous paleontologists tried to find out if dinosaurs were warm-blooded, they looked for growth rings or chemical isotopic signals in the blood, both of which are less reliable because they can alter during fossilization. Also, since the processes of extracting this kind of information can damage fossils, scientists have understandably been wary of accumulating too much data in this way.

In contrast, the scientists working with Wiemann used Raman and Fourier transform infrared spectroscopy, a method of probing fossils that can reveal incredible details about the biology of these animals. As Wiemann said in a press release: “These methods work like laser microscopes, we can essentially quantify the abundance of these molecular markers that tell us about metabolic rate. This is a particularly attractive method for paleontologists , because it is non-destructive.


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This method of probing fossils seems to have paid off. Using their approach, the researchers concluded that high metabolic rates consistent with endothermy (i.e., warm blood) existed “in pterosaurs, ornithischians, sauropods, and theropods long before the advent of energetically costly adaptations, such as flight in birds”. This means that a wide range of iconic dinosaurs were warm-blooded: velociraptors and tyrannosaurs rex were theropods, pterodactyls and so-called “monkeydactyls” were pterosaurs, triceratops and stegosaurs were ornithischia, and brontosaurs and brachiosaurs were sauropods.

That said, the study also notes that hot blood has different flavors than what we think of it today.

“Despite having ancestrally higher metabolic rates, ornithischians have reduced metabolic abilities toward ectothermy,” the authors wrote. They ruled out the possibility that larger dinosaurs like sauropods and T. rex used special forms of ectothermy (i.e. being cold-blooded) known as gigantothermy or mesothermy. “Giant sauropods and theropods were not gigantotherms, but true endotherms,” ​​the authors explain, adding that “the endothermy of many Late Cretaceous taxa, in addition to crown mammals and birds, suggests that attributes other than metabolism determined their fate during the terminal Cretaceous mass extinction.”

“It’s really exciting for us as paleontologists,” Wiemann said in a press release related to the study. “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’s views were supported by another co-author on the team.

“The new proxy developed by Jasmina Wiemann allows us to directly infer the metabolism of extinct organisms, something we dreamed of just a few years ago,” Matteo Fabbri, postdoctoral researcher at the Field Museum in Chicago, said in a statement. . “We also found different metabolic rates characterizing different groups, which was previously suggested based on other methods, but never directly tested.”

The paper’s conclusion pointed the way forward for future scientists examining these questions. The authors noted that because warm-blooded birds are “ancestors of pterosaurs and dinosaurs”, this suggests they were neither gigantothermal nor mesothermal. They also noted that “the major clades of ornithischian dinosaurs secondarily reduced their metabolic rates from those found in ectotherms today, suggesting that these taxa had lifestyles more similar to those of non-avian reptiles than to birds.” or modern mammals”. Overall, this paves the way for much research into how metabolism evolved in response to environmental factors.

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