Dinosaur Thermoregulation

(Picture from here.)

I spoke about thermoregulation a while back. Well, now there's an interesting way to figure out if fossil animals were endotherms or ectotherms-- maybe.

Every element has a unique atomic number-- the number of protons in the nuclues. Carbon has 6 protons. Oxygen has 8. And so on. However, the number of neutrons are variable within each element, each numbered by the atomic weight-- the sum of neutrons and protons in the nucleus.

There are 15 known isotopes of Carbon and seventeen isotopes of Oxygen. Many of these isotopes are unstable: they undergo radioactive decay over time. Carbon-14 is a famous member of this dysfunctional family.

Carbon-13 is one of the stable ones. So is Oxygen-18.

Now, let's talk about bioapatite.

Apatitite is a collection of phosphate minerals. Bio-apatite are phosphate minerals that are produced and used in biological systems. Hydroxylapatite, found in bones and teeth, is hard-- Mohs 5 hard. It's why when you bite down on the European walnut, the walnut breaks and not your teeth. But it's only Mohs 5, which is why when you bite down on the black walnut your teeth break and not the walnut.

The chemical process by which bones and teeth are built in vertebrates is highly conserved and fairly similar across tetrapod classes. Hydroxylapatite is used pretty much across the board. Couple this with the fact the teeth are so hard that often they are the only component of a dead animal that escapes decomposition. Teeth have therefore been used to discern diet, behavior and physiology of extinct species.

Now, they can be used to tell body temperature.

Hydroxylapatite has a chemical formula of:

Ca₁₀(PO₄)₆(OH)₂

There's no carbon present: Calcium, Phosphorus, Oxygen and Hydrogen only.

However, bioapatite has a crystal structure that contains carbonate:

CO2−
3

This is a Carbon atom connected to three Oxygens with a net charge of -2. It turns out that ¹³C and ¹⁸O have a natural affinity to one another that is related to temperature. This is called isotope clumping. The amount of clumping of ¹³C and ¹⁸O together is directly related to temperature. Could you use this to determine an animal's temperature?

This was determined in 2010 by a lab in University of California at San Diego in a wonderful paper in the Proceedings of the National Academy of Sciences. (See here for the abstract. Here for the full paper.)

First they calibrated the ratios for various animals: white rhinos, Indian elephants, Nile crocodile, alligator, sharks, etc. This gave them a good understanding on what the ratio meant.

Then, they looked at fossil mammoth tooth bioapatite. The average temperature found for the mammoth was 38.4 degrees. The temperature found for the Indian elephant was 36.9. The error bars on each measurement was about 2 degrees so the difference between these two values is well within error. Since mammoths are 1) mammals and 2) related to the elephants, the similarity further served as evidence that the technique worked.

Now, a year later, Eagle and Eiler have extended that research into determining the ¹³C/¹⁸O in species that have no real modern analogue: non-avian dinosaurs. (Abstract here.) They used two saurpods, Brachiosaurus and Camarasaurus. The data indicates that the body temperature of these two animals ranged between 36 to 38 degrees Centigrade. This is similar to most modern mammals but 4-7 degrees lower than some models that scaled for body mass.

So, what does it mean?

There are several possibilities:

1. The sauropods investigated thermoregulated and kept the temperature lower than our models suggested.
2. The sauropods did not truly thermoregulate but retained significant heat by sheer body mass. Tuna and other large mass ectotherms have been shown to do this.

If the animals thermoregulated at a lower temperature than expected it means that the animals used some mechanism to cool themselves or just operated at a lower "setpoint." This does not appear to be the favorite mechanism as I read the literature but I'm at a loss as to why. The operating temperature of an organism is important. Often, proteins cease to function outside specific ranges. These ranges dictate the require temperature range of the animal. There's a reason a high fever can be fatal.

Non-avian dinosaurs have no close living relative so we don't have a biochemical analogue, such as birds or crocodiles, with which to compare them. We also no that somewhere in dinosaur evolution some dinosaurs developed thermoregulation-- they became birds. So that presumes at some point the animals did not thermoregulate. Mammals and dinosaurs share a common heritage but it's generally accepted at the point they separated they were ectotherms. Consequently, thermoregulation evolved in the two groups independently. It does not strike me as unreasonable that the biochemistry of those dinosaurs that have no modern analogue required a different temperature than predicted by models of those animals with which we are familiar.

On the other hand, it is common for large mass ectotherms to be able to maintain a body temperature higher than ambient. Tuna do it. Large crocodiles do it. The earth was warmer back then so the temperature difference would be less. And these were incredibly large animals-- there is no modern equivalent to a sauropod. Whales are not a good comparison tool since live in such a different environment. Who is to say that once an animal's mass increases past a certain point that metabolic heat alone is enough to sustain temperature. We know it is possible. We don't know if it happened that way. It's distinctly possible that sauropods essentially got higher body temperatures for free.

There are profound differences in behavior depending on which is true. What is the nature of the thermoregulation of sauropod young? They don't have the mass to support it. But if they have endothermy, then they would need to temper it as they grow to retain that lower temperature. Could the temperature of the young and temperature of the adult sauropod be different? Could endothermy be a product of development? Young sauropods have it; older sauropods don't need it.

If they have endothermy as adults, would they have to have elaborate mechanisms for cooling to keep from overheating because of their gigantic size? If they are essentially ectothermic as adults, how would they manage in some of the antarctic sites. Antarctica was warmer then but it still had winter. And how did this relate to smaller sauropods? Ones that had significantly lower thermal mass?

It's like a really elaborate mystery novel. I can't wait for the end.