One of the biggest questions concerning dinosaurs - were they "warm-blooded" like birds and mammals or "cold-blooded" like reptiles? - might finally have been solved.
Dinosaurs were reptiles - they have all the right bones in all the right places, so most early paleontologists believed that dinosaurs had physiologies similar to those of modern reptiles, which use behavior (basking in or finding shade from the sun) to control body temperature.
The terms "warm-blooded" and "cold-blooded" are not scientifically useful - "cold-blooded" animals can have blood and body temperatures that are warmer than many "warm-blooded" animals. Those terms also mix up two concepts that need to be kept separate: heat source and constancy of temperature.
"Warm-blooded" animals generate most of their body heat by burning large amounts of food - about 10 times more than so-called "cold-blooded" animals.
"Endotherm" is a better term than "warm-blooded" just as "ectotherm" is a better term that "cold-blooded."
At the same time, most endotherms can precisely control their body temperature. Such organisms are called "homeotherms."
Homeothermy also can be produced by large body size - it takes longer for a large organism to heat up or cool down than for a small one. The larger dinosaurs were almost certainly homeotherms, so the real question has been whether dinosaurs were endotherms or ectotherms.
But how do you take the temperature of an animal that has been dead for more than 65 million years?
New research analyzed isotopes of carbon and oxygen in apatite, the calcium phosphate mineral that makes up bones and teeth. Isotopes are atoms with the same number of protons, but a different number of neutrons.
They behave the same chemically, but have different weights. Most of the carbon in our bodies is Carbon-12, which has six protons and six neutrons. However, Carbon-13 has an extra neutron, making it heavier. The same goes for oxygen, which occurs as O-16 or O-18.
The relative amounts of heavier and lighter isotopes that get incorporated into bones and teeth change depending on body temperature. That strongly suggests that dinosaurs were endotherms.
The new research used a different approach. How much the heavier isotopes of carbon and oxygen unite or "clump" together also is dependent on body temperature. That can give more-precise estimates of that temperature.
The new research measured isotope clumping in the teeth of some sauropod dinosaurs, including Brachiosaurus and Camarasaurus. They had body temperatures of 92 to 101 degrees Fahrenheit, well within the mammal range, but higher than those of most reptiles.
Even more interesting, the body temperatures of those sauropods were about 10 degrees lower than predicted for animals of their size, based on their weights.
That suggests that, like modern mammals, they had sophisticated ways of controlling their body temperatures.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.