“It’s a little tricky to take a dinosaur’s temperature,” I’m envisioning a Jurassic Park veterinarian explaining to a visitor. Meanwhile, a scene unfolds before them in which one grad student distracts the Brachiosaurus with some sort of tasty vegetal treat while another tries to insert and read a thermometer without getting stepped on.

In the real world, where the only sauropods to be found have been dead for more than 100 million years, taking their temperatures is even trickier — but, amazingly, possible!

Caltech postdoc Rob Eagle, professor John Eiler, and their collaborators published a Science paper last year about their technique for doing so. I was surprised it didn’t seem to get much media coverage at the time, because it’s damn impressive (both from a “triumphs of human curiosity” and a “wow, they did a lot of work” perspective).

The overall “how to” (including both their latest paper and other, earlier work it was based on) goes something like this:

1. Start with the “thermodynamic preference of rare heavy isotopes of carbon and oxygen to bond with each other, or ‘clump,’ in carbonate-containing minerals.” Yes, “clump” is the technical term. And as for the other terms in there:
   • rare heavy isotopes of carbon and oxygen: Most carbon atoms have a nucleus with six protons and six neutrons, and are known as carbon-12 (¹²C). But a small fraction have seven neutrons; about 1 percent of all carbon atoms on Earth are carbon-13 (¹³C). Similarly, about 0.2 percent of Earth’s oxygen atoms are oxygen-18, rather than oxygen-16. (Those percents can vary, however, from one location to another.)
   • carbonate-containing minerals: Carbonate (CO₃^2-) is an ion made of one carbon atom bonded to three oxygen atoms. It’s found in a variety of minerals, including minerals formed geologically and minerals produced by living organisms. One such mineral is bioapatite, which is found in bones, teeth, and scales of animals and their remains.
   • thermodynamic preference: The rate of a chemical reaction depends on the temperature of the environment. But each reaction is different. At a given temperature, one reaction might already be going quickly while another is barely doing anything at all. Raising the temperature might speed up the slower one more than it does the faster one, changing the relative rates of the two different reactions.

   So in other words, the cooler the conditions in which bioapatite forms, the more carbon-13 and oxygen-18 in that compound you’ll find bound to one another, rather than to the far more prevalent carbon-12 or oxygen-16. Therefore, by studying the ratio of ¹³C-¹⁸O pairs to ¹²C-¹⁸O or ¹³C-¹⁶O pairs in a sample, you get a measurement of what the temperature was when the bioapatite formed.

2. Use this technique to measure the temperature at which, for example, crocodile teeth form, and show that it’s representative of the temperature of the rest of the crocodile.
3. Acquire dinosaur teeth.
4. Do a bunch of chemistry to extract bioapatite from the dinosaur tooth enamel, while not getting any other stuff mixed in. Then use a mass spectrometer to determine isotope ratios — both total amounts of the isotopes and the extent of the clumping.
5. Do a heck of a lot of additional work to assess to what extent the amounts of isotopes may or may not changed since the dinosaur died. This involves, for example, repeating the tooth enamel analysis for tooth dentin (which is more likely to have changed than the enamel), bone, and surrounding rock.

Eagle and Eiler’s article is only 2.5 pages, but has 48 pages of Supporting Online Material detailing all of their procedures and analyses. All that work seems like it was worth it, though: they have now taken the temperatures of three types of sauropods:

• Giraffatitan brancai (previously Brachiosaurus brancai): 38.2 +/- 1 °C;
• Camarasaurus sp. (one or more species from the genus Camarasaurus): 35.7 +/- 1.3 °C;
• and another that’s tentatively identified as being from the subfamily Diplodocinae: 32.4 +/- 2.4 °C.

So there you have it: the temperature of animals that lived more than 100 million years ago, measured to within a couple degrees. Incredible.

These temperatures (about 90-100 °F), by the way, are higher than those of modern reptiles but lower than those of modern birds. Interestingly, they’re also lower than a recent model — based on scaling laws and dinosaur growth rate analysis — that suggested dinosaurs were ectotherms (cold-blooded) but could still be high in temperature due to mass alone. (Large things retain heat more effectively than smaller ones.)

This difference could be accounted for by some sort of cooling mechanism. There was mention of “a tracheal surface and air sac system” as possibly having this purpose, as well as dissipation of heat from sauropods’ long necks and tails. But would such mechanisms have provided enough cooling even for dinosaurs to have been endotherms (warm-blooded)? The question remains open . . .

For more information:
Be sure to check out the new podcast “Measure Twice, Cut Once: A podcast about discovery,” with Nic Perez and Dr. Laurence Yeung, which devoted its first episode to the topic.

The Caltech press release also has a lot of good description, as well as a video and a bunch of images.