Were dinosaurs cold or warm-blooded? For 150 years, palaeontologists have argued the point. Now a new piece of evidence comes from an 80 million year old dinosaur eggshell.
Robert Eagle at the University of California Los Angeles and his colleagues found the mother dinosaur’s body temperature leaves a chemical imprint in the shell of her eggs that can still be read today. The results were published in Nature Communications in October. Their technique is a “revolutionary breakthrough” says palaeontologist John Long, at Adelaide’s Flinders University.
Cold-blooded animals such as reptiles heat up and cool down with their environment. But warm-blooded animals, such as mammals and birds, ramp up their metabolism to generate their own heat. Palaeontologists initially believed dinosaurs were cold-blooded, because their fossils resembled lizards. But in the late 19th century, English anatomist Thomas Henry Huxley compared the bone structures of the two-legged theropod Compsognathus with Archaeopteryx, an ancient beaked bird. He proposed dinosaurs were the ancestors of birds – and hence that warm bloodedness may have first evolved here.
While palaeontologists now agree birds evolved from dinosaurs, the evidence for when warm-bloodedness arose has been sketchy. Researchers have tried to look at the spacing of growth rings in dinosaur bones for clues – cold-blooded dinosaurs should have grown more slowly in the colder months of the year. Some palaeontologists have argued they do see evidence for warm bloodedness in the dinosaur bone, but the method has not provided a definitive answer.
So rather than relying on the structure of bones, Eagle turned to chemistry. An animal’s body temperature affects how rare, heavy forms of oxygen and carbon atoms are incorporated into tooth enamel. The colder the temperature, the more these rare atoms clump together. By measuring the number of these clumps in the sauropod teeth, he was able to calculate a temperature.
Using that technique, in 2011 Eagle and his team reported that a titanosaur sauropod, one of the largest dinosaurs to walk the Earth, had a cosy body temperature of between 36 and 38°C.
But the giant sauropod, with its fearsome teeth rather than a beak, is not one of those dinosaurs that gave rise to birds. The question of where the bird’s warm-bloodedness evolved remained.
So Eagle decided to ask that question in an oviraptor, a small bird-like dinosaur with feathers and a beak that once roamed the Gobi Desert. Problem was their beaks are not made of carbonate, the substance in teeth that contains carbon and oxygen. But their eggshells were! So Eagle’s team turned to fossilised oviraptor eggshells, another carbonate structure, and applied the same method.
Eagle’s team accessed a nest of fossilised eggs from New York’s American Natural History Museum. First task at hand was to check that the minerals were an intrinsic part of the eggshells, rather than contaminants that had seeped in from the surrounding soil over the millennia. Intrinsic minerals can be identified by patterns in their structure. “Sea shells or snails have these amazing spiralled structures,” Eagle explains. And eggshell, when viewed under the microscope, has a distinctive fan-like structure.
By closely examining the oviraptor eggshells, Eagle’s team was able to prune away sections of the eggshell contaminated by other minerals.
The remaining eggshell were then dropped into acid, to vaporise the carbonate into carbon dioxide gas. The gas was injected into a mass spectrometer, which counts the number of carbon dioxide molecules that contain heavy oxygen and carbon. The team also gathered eggshells from 13 modern birds and nine types of reptiles, measured how much clumped heavy carbon and oxygen these eggs contained and graphed that against the animals’ body temperatures. Then they compared all the data.
They calculated the oviraptor female must have formed her eggs at 32°C. That’s six degrees hotter than the average environmental temperature at the time – the team used the same analytic trick on surrounding soil to check.
That means the oviraptor could raise its own body temperature, though not by as much as today’s birds, some of which maintain body temperatures of more than 40°C. “While [the oviraptor] had a degree of warm-bloodedness, it hadn’t developed the full metabolic rate that you see in modern birds today,” Eagle explains.
Eagle next plans to test his technique on even older eggshell fossils to see if he can track how dinosaur body temperature evolved over time.
Eagle’s work confirms the theory first proposed by Huxley more than a century ago, says Long. “Palaeontologists actually got it right,” he says.