Were all dinosaurs cold-blooded?

If you are a paleontologist, then maybe you already know the answer to this question, but for the rest of us, we just thought dinosaurs to be cold-blooded, just like reptiles. I think what has triggered most people to believe this, is the fact that we might associate creatures that look vicious, heartless, and relentless to be cold, both in nature and so their metabolism should be the same thing as well. 

Warm-blooded creatures are, for the most part, associated with causing less or no harm. The old saying “never judge a book by its cover” is true, even in this case.

The Eggshells Project

Many might point out that they laid eggs that make dinosaurs 100% cold-blooded. According to Robin Dawson, the body temperature of dinosaurs was higher than the environment. We only have two types of metabolism, endothermic and ectothermic. Endothermic is the warm-blooded species that can regulate their temperature from within, while ectothermic is the cold-blooded species that need to regulate their internal body temperature with the outside environment.

This research consisted of determining the temperature of fossil eggshells with the temperature of that time. Three major dinosaur groups and those closely or slightly related to birds were chosen. The testing procedure is called clumped isotope paleothermometry, which states that the ordering of carbon and oxygen atoms in a fossil eggshell is determined by temperature. And once you understand the ordering of the atoms, you’ll be able to calculate the internal temperature of the mother dinosaur. 

After the testing, the results showed that the eggshells of a Troodon were at 38 degrees (100.4 degrees Fahrenheit), 27 degrees (80.6 degrees Fahrenheit), and 28 degrees Celsius (82.4 degrees Fahrenheit). Maiasaura eggshells were tested at 44 degrees Celsius (111.2 degrees Fahrenheit). Lastly, fossilized dinosaur eggs from classified species called oospecies, Megaloolithus, were tested at 36 degrees Celsius (96.8 degrees Fahrenheit).

The same testing was done on cold-blooded invertebrate shells in the same place as the dinosaur shells. This helped the research team know the environment’s temperature and if the dinosaur’s body temperature was higher or lower than this. 

As a final result, they found that the Troodon eggshells were 10 degrees Celsius (50 degrees Fahrenheit Celsius) warmer than their environment. Maiasaura samples were at 15 degrees Celsius (59 Fahrenheit Celsius), and the Megaloolithus results were at 3-6 degrees Celsius (Fahrenheit Celsius). Dawson concluded these results by saying, “What we found indicates that the ability to metabolically raise their temperatures above the environment was an early, evolved trait for dinosaurs.” 

Robin Dawson conducted this research while doing her doctorate in geology and geophysics. Together with Yale paleontologist Pincelli Hull and former Yale researchers Hagit Affek and Pincelli Hull.  

The Metabolism Consensus

Earlier research suggested that dinosaurs were both endothermic and ectothermic; as explained before, we have endothermic (warm-blooded) beings and ectothermic (cold-blooded) beings. Because we endothermic beings regulate our temperature internally, much energy is required, suggesting that we mammals are always active and grow faster. 

However, you will notice how some reptiles become very slow in cold temperatures because they can’t regulate internally; they usually bask in the sun to get their temperature up.  

But researchers found fast growth in dinosaur bones, which is quite confusing and abnormal for so-called ectothermic beings; where would they get all that energy? Upon this discovery, scientists started conducting comparative research, where they would compare the growth rate, adult size, and metabolism of 381 living and extinct vertebrate species with 21 dinosaur species.

It turned out that dinosaurs happened to sit right in the middle of the metabolism spectrum; some results even showed that dinosaurs overlapped some endothermic and ectothermic animals. With this result, scientists coined the term “mesotherms” as “meso” means middle in Greek.

Dinosaurs are not the only mesotherms as the tuna and leatherback turtle can regulate their internal temperature to a certain extent, but not the same way as endotherms. This research, however, gave scientists another ground for understanding ancient Mesozoic systems. 

The Oxygen Project

Jasmina Wiemann, who works at the California Institute of Technology, conducted the most recent research to find out if dinosaurs were also warm-blooded. She and her colleagues looked into waste products that form when inhaled into the body and react with proteins, sugars, and lipids. This would indicate whether an animal was warm-blooded or cold-blooded. 

These molecules are stable and don’t dissolve in water, indicating they are well-preserved during fossilization. Wiemann’s team took 55 femurs (thigh bones) from different species for analysis. In the 55, you had 30 extinct species and 25 modern animals. Pterosaurs represented the flying reptiles and Plesiosaurus for the marine reptiles. 

They applied a process called infrared spectroscopy, which looks at how molecules and light interact. This would allow them to have the number of waste molecules in the fossils. The results were then compared to known metabolic rates of modern species and used that information to conclude the metabolic rates of extinct creatures. 

The result showed that dinosaurs’ metabolic rate was higher in general than modern animals, whose normal temperature was around 37 degrees Celsius (98.6 Fahrenheit Celsius) and more than birds, with an average temperature of 42 degrees Celsius (107.6 Fahrenheit Celsius).

Wiemann said via email, “With our new evidence for an avian-level metabolism ancestral to all dinosaurs and pterosaurs, all hot-blooded dinosaurs likely had high body temperatures, comparable to those of modern birds.”

Early in paleontology, dinosaurs were grouped with reptiles, assuming that they looked and lived like reptiles. But after discovering feathered fossils in the 1990s, many paleontologists believed dinosaurs were closer to being birdlike. This discovery later led paleontologists to confirm the birds we see today come from dinosaurs.

There were some exceptions, dinosaurs classified as ornithischians (dinosaurs with bird-like hips) like the Triceratops or the Stegosaurus, who started with a high metabolic rate but then evolved to have low metabolic rates similar to those of today’s modern animals. 

After this discovery, Wiemann asked the question to know why and what this meant about 

dinosaurs’ ecology, behavior, and evolution.