Dinosaurs (meaning fearfully-great lizard – Richard Owen 1842) are reptiles, so are therefore cold-blooded. Yes? Well maybe not….
A quite aptly named theory suggests that dinosaurs were neither cold blooded or warm-blooded, but instead “dinosaur-blooded”. Combining elements from both cold-blooded and warm-blooded strategies with a changing metabolism over the animal’s lifetime. By using Annuli, which are concentric rings of growth used to age individuals (like the rings of a tree) scientists were able to deduce the metabolism of a number of species (including Tyrannosaurus, Allosaurus, Tenotsaurus and a range of present-day species) and found that their growth rates were not characteristic of either warm or cold-blooded animals.
It was concluded that their strategy resided somewhere in the middle and were dubbed mesotherms. Of course it is important to note that the dinosaurs studied sit in the Mesozoic era, a time spanning 180 million years, when you consider how much our climate has changed in only the last 2000 years a great deal of environmental analysis should be done alongside to confirm these theories. For instance increasing temperatures would improve the metabolism of a cold-blooded animal. It is also important to remember that birds evolved from a group of dinosaurs known as theropods, whose members include Tyranosaurus and velociraptor species, and birds are warm-blooded. So where exactly did this warm-bloodedness originate from? But on the other-hand there are very few species living today that inhabit the middle of this scale, suggesting that there is an adaptive advantage of being either cold-blooded or warm-blooded.
One such example that radically challenged the cold-blooded theory was the finding of Ugrunaaluk kuukpikensis in Alaska. This dinosaur is a member of the Hadrosaur family, a family of duck-billed dinosaurs that were very common in the late cretaceous period of North America and Eurasia. But this finding was the furthest north ever found and due to the tilt of the Earth at this time would have lived in darkness for months at a time. Gregory Erickson says that this finding “casts doubt on everything we know about dinosaur morphology – How did they survive up there?”. We know from today that the species found in these extreme northern climates are usually mammals with a number of adaptions allowing them to liver here, such as a thick blubber layer, fur coats with hollow hairs and a (usually) small surface to volume ratios to reduce heat loss to the environment. So how could a cold-blooded animal have lived here?
Of course a great-deal more research needs to be done before it can be definitively concluded whether Dinosaurs were warm-blooded, cold-blooded or somewhere in-between. We may never know the truth, but what is clear is that these dinosaurs inhabited a range of climates and environments showing incredible diversity – So maybe they could have been all three.