Can Eggshells Crack the Case of Whether Dinosaurs Ran Hot or Cold?

Dinosaurs may not be cold or warm blooded, but somewhere in between

titanosaur eggs
A clutch of titanosaur eggs Luis Chiappe, LA County Natural History Museum

One of the hottest debates surrounding the dinosaurs is temperature: Were these "terrible lizards" who stalked, tromped and flew around the ancient world​ warm or cold blooded? A new study of dinosaur eggshells​ supports a third option—both. 

Since eggs grow their tough calcium carbonate shells inside their mother's body, looking at the chemical structures of those eggs can give scientists clues about the critter's temperature, according to the study recently published in the journal Nature Communications

Researchers analyzed eggshells from two groups of Cretaceous dinosaurs. One set of eggs belong to the behemoth titanosaurs—a group of long-necked herbivores that included the familiar Brontosaurus. The other eggs belong to the bipedal, omnivorous oviraptorid, which are smaller and more bird-like

Scientists can estimate the body temperature by looking for bonds between rare isotopes of carbon and oxygen (carbon-13 and oxygen-18) in the calcium carbonate minerals that make up eggshellsRachel Feltman reports for The Washington Post. More of those bonds means the eggshell formed at a cooler temperature in the mother’s body. 

This egg analysis suggested that tintanosaurs ran at a hot 99.7 degrees Fahrenheit, while the oviraptorid chilled at a cooler 89.4 degrees. But to tell if they were cold-blooded or warm-blooded scientist also needed to know the temperature of their environment.

The terms cold-blooded and warm-blooded are actually misnomers and are more properly termed ectotherms and endotherms. Like lizards warming in the sun, ectotherms need external heat sources to regulate their body temperatures, while endotherms generate their own heat through metabolism, explains a press release from the National Science Foundation. 

So to tell where the animals got their heat scientists examined fossilized soils that surrounded the eggs in present-day Mongolia and Argentina, which showed ancient temperatures of roughly 79 degrees Farenheit. Both dinosaurs were warmer than that, which could mean possible endothermy, but since the temperatures are so different, the researchers think a middle of the road approach was likely.

"This suggests that maybe they were warm blooded, but hadn't developed the high level of temperature regulation seen in mammals and birds today," lead study author Robert Eagle, of the University of California, Los Angeles, tells The Post. "They were kind of part way to evolving endothermy."

That puts dinosaurs in class with the catchy moniker mesotherms. Animals that subscribe to this strategy can generate and regulate their own body heat but don’t maintain as constant a temperature as mammals do. Modern animals including tuna, lamnid sharks and leatherback turtles are mesotherms, reports Alexandra Witze for Nature

Modern birds do fall squarely in the endotherm territory, with fast metabolisms and warm body temperatures to match. Since birds are the descendants of creatures like the oviraptoid, the researchers think their method might help track the evolution of endothermy in dinosaur lineages. 

As far as the dinosaur temperature debate goes, the results sound like a good compromise. 

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