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Were dinosaurs slow and lumbering, or quick and agile? The answer depends largely on whether they were cold- or warm-blooded. When dinosaurs were first discovered in the mid-19th century, paleontologists thought they were plodding beasts that had to rely on their environments to keep warm, like modern-day reptiles. But in the last few decades several lines of evidence have suggested that they were faster, nimbler creatures, more like the velociraptor stars of Jurassic Park. Maintaining such levels of activity in turn implies a warm body whose tempe rature is self-regulated. But how to know for sure?
New work by a Caltech-led team essentially lets us “stick a thermometer in an animal that has been extinct for 150 million years,” says postdoc Robert Eagle, the lead author of the Science paper describing the research. By measuring the levels of rare isotopes in dinosaur teeth, the team has shown that some sauropods—a group that includes the biggest land animals to have ever lived—were about as warm as most modern mammals.
“The consensus was that no one would ever measure dinosaur body temperatures, that it’s impossible to do,” says coauthor John Eiler, the Sharp Professor of Geology and professor of geochemistry. And yet, using a technique pioneered in Eiler’s lab, the team did just that.
The researchers analyzed 11 teeth, dug up in Tanzania, Wyoming, and Oklahoma. Three of the teeth belonged to Brachiosaurus brancai, which at some 25 meters from nose to tail tip was as big as the much better-known Apatosaurus, a.k.a. Brontosaurus. The other eight teeth were from Camarasaurus, a smaller creature that averaged some 15 meters long. The results showed that Brachiosaurus had a body temperature of about 38°C (100°F) and that Camarasaurus was about 36°C (97°F), warmer than crocodiles but cooler than birds.
Previous studies of dinosaur metabolism and thus body temperature have, of necessity, consisted of chains of inference—for example, figuring out how fast the creature ran based on the spacing of its footprints, studying predator-to-prey ratios, or measuring the growth rates of bone. But these various lines of evidence were often in conflict. “For any position you take, you can easily find counterexamples,” Eiler says. “How an organism budgets its energy—there are no fossil remains for that. You just have to make your best guess based on indirect arguments.”
The new method relies on direct measurements of so-called clumped isotopes, eliminating the guesswork. The researchers analyze the concentrations of carbon-13 and oxygen-18 in bioapatite, the mineral that makes teeth hard and bones strong. Carbon-13 and oxygen-18 atoms are heavier than their run-of-the-mill brethren, carbon-12 and oxygen-16. When a carbonate ion (CO3–2) ion forms in a warm liquid, the heavy and light isotopes intermingle at random; but as the solution cools the heavier isotopes tend to seek one another out. Then, when the carbonate-containing mineral—oh, say bioapatite—precipitates from the liquid—blood, for example—to, perhaps, form the enamel of a tooth, the isotope clumps are locked into the crystal structure to create a permanent record of the temperature of their surroundings, which might be the inside of a dinosaur’s mouth.
Eiler says, “What we’re doing is thermodynamically based, and thermodynamics, like the laws of gravity, is independent of setting, time, and context.” In other words, thermodynamics worked the same way 150 million years ago as it does today. “We’re getting at body temperature through a line of reasoning that I think is relatively bulletproof, provided you can find well-preserved samples.”
Identifying well-preserved dinosaur teeth was indeed a challenge, and the researchers used several ways to find the best samples. One method compared the bioapatite in the tooth enamel with the more easily altered dentin in the tooth’s interior. If both materials had the same concentrations of the heavy isotopes, the odds were that the enamel had been compromised.
So, were the dinosaurs warm-blooded? It’s hard to say. Huge sauropods would retain their body heat very efficiently. “If you’re an animal that can be approximated as a sphere of meat the size of a room, you can’t be cold unless you’re dead,” Eiler explains. So even if dinosaurs were cold-blooded in the sense that they depended on a sunny afternoon to heat themselves, they could still be warm-bodied—so-called gigantotherms that stay warm through sheer bulk. In fact, some physiological models based on the meat-sphere approximation predict temperatures four to seven degrees higher than those measured. This might be a hint that these dinosaurs had a slower metabolic rate than assumed by the models, which in turn would raise a fresh set of questions.
The team hopes to find out whether body temperature does indeed increase with size by looking at teeth from young sauropods and from adults from species that might only get as big as a St. Bernard. The researchers also intend to take the temperatures of the feathered dinosaurs that gave rise to birds, which may shed light on when warm-bloodedness evolved.
The paper appeared online in the June 23 issue of Science Express. The other authors are Thomas Tütken from the University of Bonn, Germany; Caltech undergraduate Taylor Martin; Aradhna Tripati, an assistant professor at UCLA and a visiting researcher in geochemistry at Caltech; Henry Fricke from Colorado College; Melissa Connely from the Tate Geological Museum in Casper, Wyoming; and Richard Cifelli from the University of Oklahoma. Eagle also has a research affiliation with UCLA. The work was supported by the National Science Foundation and the German Research Foundation.—MW
Rob Eagle (left) and John Eiler with a new kind of oral thermometer: a dinosaur tooth used to take the creature’s temperature.
