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Scientists can find answers in some pretty unusual places, and recently they found some evidence that dinosaurs weren't cold-blooded by looking at... eggshells?

Hosted by: Stefan Chin

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Sources:

https://jeb.biologists.org/content/jexbio/219/3/300.full.pdf
https://advances.sciencemag.org/content/6/7/eaax9361
(Intro)

Sometimes in science, the answer just isn't where you thought it was gonna be, and last week, in the journal Science Advances, an international group of researchers published insight into an age-old problem from a pretty surprising place.  They analyzed dinosaur eggs to get an idea of the mother dinosaur's body temperature when the egg was formed, and the results are giving us a much clearer picture of how dinosaurs evolved from cold-blooded reptiles to warm-blooded birds. 

Warm-bloodedness, AKA endothermy, is a huge part of how birds and mammals have been able to spread all across the world.  It seems to have evolved separately in each group, but the result is the same.  Instead of relying on the sun's warmth to maintain our body temperatures, we have the freedom to live in a whole range of climates and keep up fairly consistent energy levels at the same time.  

Many researchers are pretty sure that birds' warm-bloodedness evolved in their dinosaur ancestors, but for a long time, they've been trying to figure out exactly how and when that happened.  These days, a lot of researchers argue that many dinosaurs were mesotherms, basically somewhere in between warm and cold blooded.  Like endotherms, mesotherms burn energy to produce heat that raises their body temperature.  Now, we endotherms maintain our body temperatures at a set point, about 37 degrees in humans, but living mesotherms don't have a thermostat.  They just kind of turn on the heat and hope for the best.

To learn more about how temperature regulation might have evolved in dinosaurs, it's important to know what their internal temperatures were.  If they were warmer than the weather outside, that points to at least some control over their body temperatures.  Previous research on this has involved looking at different dinosaurs' growth rates, which you can calculate based on marks left in their bones, kind of like tree rings.

Cold-blooded reptiles, with their slow metabolism, tend to grow slowly, while warm-blooded animals tend to have much higher energy levels and therefore grow more quickly.  The problem is, the relationship between growth rate and metabolism isn't always that simple, which means looking at how dinosaurs grew might not be the most reliable way to figure out if they had control over their body temperatures.  

So the authors of this new paper used a different technique, one that's only come into use relatively recently.  It's called clumped isotope paleothermometry.  The paleothermometry part just means measuring temperature in animals, usually extinct ones.  It's the clumped isotope part of the name that really describes what the technique is.  Isotopes are basically versions of the same element with different weights.  Some are more common than others and their abundances can vary based on a variety of factors.  

So when the rarer isotopes clump together within groups of molecules, that can tell you a lot about how they got together and what the conditions were like when those molecules formed, like what the temperature was.  In the past, some researchers have used the clumped isotope technique on dinosaur eggshells, which allowed them to calculate the temperature inside the dinosaur when the egg was formed, but as the authors of this paper pointed out, the problem is that they've only done that for dinosaurs that lived in warm climates, which means it wouldn't matter if they were endothermic or mesothermic or whatever, because their temperature would have been close to that of their warm surroundings no matter what.  

When dinosaurs ruled the world from about 230 million years ago to around 65 million years ago, Earth was much warmer than it is today, but there were still some places with average temperatures below 30 degrees or so, and some dinosaurs were adapted to live in those lower temperatures, so the team applied the clumped isotope technique to eggshells of dinosaurs that lived in places with cooler climates, mainly ancient Canada.  They looked at three different species and in two of them, the temperature inside the dinosaurs when these eggs formed was much warmer than the temperature outside would have been.  In fact, at 36 degrees and 44 degrees, both temperatures were in line with what we see in endotherms today.  

So these dinosaurs probably had some control over their body temperature, although it's hard to tell from this data whether they were mesotherms or endotherms.  In the other species called Troodon formosus, they actually found a range of body temperatures, from about 27 to 38 degrees Celsius.  That could be a sign that they were able to raise their body temperature but not necessarily control it, which would indicate that, at the very least, they weren't cold-blooded and may have been mesotherms.

So just from looking at the chemistry of eggshells, we now understand a lot more about how certain dinosaurs might have controlled their body temperature, and if future studies apply the technique to more species, we could get a lot closer to understanding how endothermy evolved over time, which just goes to show what we can learn from looking in some pretty unexpected places.

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(Endscreen/Credits)