June 27, 2011

"Warm-blooded" dinosaurs and "warm blooded" fish*

At 80beats (@Discover blogs) there is a post about a method of inferring the body temperature of large dinosaurs by looking at the temperature that would be needed for the enamel of Brachiosaurus and Camarasaurus teeth to form. The post references a recent study published in Science. The conclusion is that big dinosaurs were as warm as mammals, but that's not to say that they had the same temperature regulation as mammals.

Here, we used clumped isotope thermometry to determine body temperatures from the fossilized teeth of large Jurassic sauropods. Our data indicate body temperatures of 36 to 38°C, which are similar to most modern mammals. This temperature range is 4 to 7°C lower than predicted by a model that showed scaling of dinosaur body temperature with mass, which could indicate that sauropods had mechanisms to prevent excessively high body temperatures being reached due to their gigantic size.

It's always been a pet peeve of mine to note when the terms "warm-blooded" and "cold-blooded" are used indiscriminately - in popular use they are incredibly widespread - or even when the more scientific terms endothermy and ectothermy are put against each other. This doesn't actually give the right view of the diverse temperature-regulation strategies that different animals can have. At Deep Sea News, one of my favourite blogs, there is a great post about those strategies that are somewhere in between the notions of warm- and cold-bloodedness, highlighting the really interesting strategies in pelagic fish*, such as lamnid sharks, tunas, billfishes and several others.

One important pattern that emerges from these observations is that body-warming is not a taxonomic thing: it has evolved several times in several different lineages, at least twice for sharks and once each for rays, tuna/billfish and opah. Rather, body warming is an ecological thing because it occurs in many species that are not related but all share pelagic migratory habits. Doubtless a closer look at other pelagic species will show that it has evolved in quite a few other species of the open ocean too.

Considering this, it's certainly not far-fetched to assume that "warm-bloodedness" has evolved several times in land-living tetrapod vertebrates as well. To me there is little doubt that the small theropod dinosaurs that birds arose from were able to regulate their body temperature internally, but perhaps sauropods, or some other dinosaur groups, were able to do it as well to some extent.

* I use the term fish very loosely here. Lamnid sharks are as related to us humans as they are to tunas and billfishes.

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