Which dinosaurs might have been warm-blooded?

I have a new idea to kick re-start my posting on this blog. (I know I have been lazy/busy, but it is only an excuse for so long.) When the exams get closer, I will probably try to make entries on topics related to the things I will be revising, but for now, I want to write about something that has fascinated me for years: were the dinosaurs warm- or cold-blooded?

The question has been heavily debated about for about a century, and still is! The reason is that there is no simple answer (which makes it all the more interesting). There is a massive wealth of evidence for and against each view. Both sides have some evidence that should be conclusive (i.e. definite, final, end-of-discussion), and since both cannot be right at the same time, it has become something of a never-ending mystery.

To give two brief examples of arguments that make it difficult to decide on either side, consider the flying, bird-like theropod dinosaurs, which certainly must have been warm-blooded as it is absolutely necessary for flight in a vertebrate animal – in contrast to the colossal sauropods, which were so massive that they would simply be too hot to live if they were warm-blooded.

The tiny theropod Sinornithosaurus (left) compared to the gargantuan sauropod Camarasaurus. Images from: http://dinodiyar.wordpress.com/2012/04/01/bilinen-ilk-kus-benzeri-zehirli-dinozor-sinornithosaurus/ (left) and http://www.bhbfonline.org/Research/index.html (right).

There is, however, a beautiful solution to this dichotomy: what if some dinosaurs were cold-blooded and some warm-blooded? The dinosaur group was incredibly diverse, exploiting many different body forms and sizes, adapted to virtually any kind of environments, evolved numerous feeding strategies and behaviours. Therefore, it seems probable that there also were radically different physiologies across the group.

If the bird-like theropods had to be warm-blooded, that must not mean that the completely different sauropods were too. This is a nice, easy compromise. It makes sense of a lot of information, so it is a good idea in scientific eyes. Also, it makes the dinosaurs so much more interesting to study, because now we must investigate the obvious follow-up question: which dinosaurs were cold-blooded and which were warm-blooded?

I have been reading quite a lot about this topic over the years, and I stumbled upon an intriguing idea that seems to have been overlooked. In a chapter on dinosaur thermoregulation of The Complete Dinosaur (eds. Farlow & Brett-Surman 1997; see Book tips for more details), the now late R. E. H. Reid discussed the possibility that dinosaurs may have been some sort of intermediates between conventional cold- and warm-bloods. Moreover, Reid pointed out that different types of dinosaurs seem to have been closer to true warm-bloodedness than others, and at least the evolutionary sequence that gave rise to birds would have achieved full warm-bloodedness before taking flight. 

I found this proposition both plausible and intriguing. I like it because it makes sense and makes the question yet even more fun! It is this idea that I want to explore in the coming series of posts, where I want to discuss each major group of dinosaurs in the light of this idea. First, I will write about evidence for and against warm-bloodedness in the whole dinosaur group. I’ll break it down into smaller groups later to discuss the different ‘degrees’ of warm-bloodedness they might have had.

Dinosaurs!! from http://www.photofunt.com/tag/pictures-of-dinosaurs