Now we know that many dinosaurs were actually bird ancestors. Birds are endothermic, and have super fast metabolisms.
So did some dinosaurs have bird-like metabolisms, and the hot blood to match? A study published Tuesday in Nature Communications claims to have found the answer in fossilized eggshells.
The basic findings line up with what most recent research in the area has concluded: Dinosaur metabolisms were all over the place.
"It's important to realize that there's actually a whole sliding scale of physiology," even in the modern animal kingdom, study author Robert Eagle of the University of California told The Post. Birds have metabolic rates that put humans to shame, he explained, making them arguably more "warm blooded" than we are. And then you have critters like sloths, that are on the slowest, coolest end of the warm blooded spectrum. "So the real question is where dinosaurs fell on that spectrum," he said.
That's where Eagle's work comes in. He and his colleagues analyzed the chemical makeup of ancient eggshells, using a technique previously perfected on teeth to estimate the temperature of the body they formed in. By measuring the abundance of chemical bonds between two rare, heavy isotopes (carbon-13 and oxygen-18) in calcium carbonate minerals, scientists can estimate body temperature. A mineral that forms at colder temperatures will have more of these bonds than the same mineral formed at a higher temperature. In the case of an egg, scientists can use this ratio to estimate the temperature of the mother's body when she formed it.
After showing that this measurement worked in eggs from modern animals, Eagle and his colleagues tested fossilized eggs. Many showed signs of decay that would alter any conclusions about temperature, but they were able to analyze two species successfully — and found signs of a range of metabolic rates.
One was a long-necked titanosaur sauropod, and it indicated a maternal body temperature of about 100 degrees Fahrenheit, comparable to large mammals today. Another species — a T. rex-like oviraptorid — indicated a cooler 90-degree body temperature, which is lower than most modern mammals.
But chances are that both of them were at least somewhat endothermic, Eagle explained. Analysis of the soil around the oviraptorid eggs indicates that the air temperature may have been lower than their body temperature.
"We can't take just body temperature and jump to the conclusion that they weren't cold blooded," Eagle said, "but combined with other data, it's consistent with them having some kind of intermediary metabolism. This suggests that maybe they were warm blooded, but hadn't developed the high level of temperature regulation seen in mammals and birds today. They were kind of part way to evolving endothermy."
Since oviraptorids like this one were close relatives to the earliest birds, Eagle hopes that studying the evolutionary lineage more closely will reveal when and how metabolisms sped up so drastically.
"There's just a massive spectrum of different questions we can ask now," he said.
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