Dinosaurs—or at least two major groups of them—had body temperatures much higher than their environment, bringing paleontologists one step closer to resolving the debate over whether they were cold-blooded or warm-blooded. The new evidence from fossil eggshells shows some dinosaurs had body temperatures as high as those of today’s birds, their closest modern relatives. But it still isn’t clear how much of the boost in body temperature comes from increased metabolism versus insulation-providing feathers or behaviors like basking in the sun.
Paleontologists have long debated whether dinosaurs were cold-blooded like reptiles, warm-blooded like modern-day birds and mammals, or something in between. Cold-blooded creatures generally rely on the environment to regulate body temperature, whereas warm-blooded ones typically use increased metabolism for extra warmth. Previous studies have suggested scientists can determine the difference in dinosaurs by counting the carbon and oxygen isotopes in ancient bones and teeth. Depending on the temperature at which they formed, those materials contain different proportions of isotopes—atoms of the same chemical element that vary slightly in mass. But the technique works only if the material being analyzed is well-preserved, says Robert Eagle, an evolutionary biologist at the University of California, Los Angeles.
In the new study, Eagle and his colleagues looked at the eggshells of two different types of dinosaurs: large, long-necked herbivores known as titanosaurids and small, bipedal creatures called oviraptorids. Because the eggshells formed inside the body, they should chronicle temperature from the period of ovulation, Eagle says. Moreover, dinosaur eggshells—if they were similar to those of modern-day chickens—could have contained as much as 97% calcium carbonate. Their distinctive microstructure would also be able to suggest whether their chemical composition has changed since they were laid, thus offering a check on the quality of preservation. “This is a neat paper,” says Stephen Brusatte, a vertebrate paleontologist at the University of Edinburgh. Eggshells are “a new piece of evidence” that researchers haven’t tapped before, he notes.
Frankie Jackson, a vertebrate paleontologist at Montana State University, Bozeman, agrees. “The more directions we can come at the problem [of determining dinosaur body temperature], the better,” she says. The real advance in this team’s research, she adds, is its careful attention to preservation.
Of the 32 eggshells the team analyzed, only six were considered well-preserved enough to include in their results: three from oviraptorids in modern-day Mongolia and three from titanosaurids in what is now Argentina. To find out how warm the bodies were that laid the eggs, the team measured the proportions of bonds between two rare, heavy isotopes, carbon-13 and oxygen-18. The lower the temperature at which a mineral forms—in this case, the calcium carbonate in the eggshell—the more bonds between the rare isotopes carbon-13 and oxygen-18. The team then compared its measurements with data gathered from the eggshells of 13 modern-day species of birds and nine modern-day reptiles.
According to the team’s analyses, reported online today in Nature Communications, the egg-laying titanosaurids had body temperatures of about 37.6°C (99.7°F). The data lines up nicely with a 2011 analysis by Eagle suggesting that two different species of sauropods closely related to titanosaurids had body temperatures between 36°C and 38°C. Although titanosaurids and sauropods may have boasted some sort of enhanced metabolism, it’s also possible that their elevated body temperatures stemmed from their large size, which makes it difficult to shed body heat. In general, as a creature’s volume—which generates heat—grows faster than its overall surface area—which loses heat to the environment—the animal has a higher baseline body temperature.
The new study suggests that oviraptorids had a slightly lower body temperature than titanosaurids, about 31.9°C (89.4°F). That’s not as high as modern-day birds, showing, says Brusatte, that “avian warm-bloodedness obviously developed over time.” But the team’s analyses of carbonates formed naturally in the ancient soil suggest the average temperature in the environment was no more than 26.3°C (79.3°F). So oviraptorids were still able to elevate their body temperature above their surroundings. It’s not clear from the data, says Eagle, whether that boost came from an enhanced metabolism, behaviors such as basking in the sun, insulating layers of feathers or other material, or some combination of these factors.
Regardless, the team’s results are fairly compelling evidence that “dinosaurs were not cold-blooded tail-draggers,” says Gregory Erickson, a vertebrate paleontologist at Florida State University in Tallahassee.