Dinosaur warm-bloodedness: The meaning of 'warm-blooded'

 Being ‘warm-blooded’ has nothing to do with the temperature of the blood. It is a possibly misleading colloquial name for a certain physiological condition. What it means cannot be explained in one sentence, and one could write several books about the vast implications for the biology of an animal.

To really appreciate the debate about whether some dinosaurs were warm-blooded or not, it is of course important to have an understanding for what warm-blooded actually means.

The formal meaning of ‘warm-’ and ‘cold-blooded’

There are three pairs of contrasting terms that can be equivalent of warm- vs. cold-blooded. Each pair relates to a certain aspect, and usually people really refer to different aspects when they just say ‘warm-blooded’. All three are however very intimately linked, so the only time you need to distinguish between them is when you want to be really precise about one specific aspect.

I will first give you a simple table that summarises these six terms, but do not get stuck on it if you do not understand: it will be explained in the text that follows.

 

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

Stigmaria – fascinating fossil from Arran

The field trip to Arran was disappointingly devoid of interesting fossils. But, on the last day, the paleos (plus a group of honorary geologists) went (on an invigorating trip over a mountain) to work on an Arthropleura trackway. Arthropleura was the largest land vertebrate we know, a colossal millipede (or maybe centipede… the actual relationships are not clear) that could grow to over two metres in length! It lived in near-coast areas of the Carboniferous period, some 300 million years ago, and was a heavily protected herbivore that would have fed mostly on plant litter or detritus. 

The group was too large for us all to work on the short preserved trackway, so I was delegated to examine the other fossil on the site: Stigmaria, a plant root belonging to one of the colossal Carboniferous club mosses (which are not mosses at all: they are closer to true trees, but distinguished by having leaves attached directly to the stem, i.e. no branches in between).

Stigmaria is a morphotaxon, a group of body parts that is too poorly known to know its evolutionary origin, but is still given a taxonomic name so that it can be easily referred to. Morphotaxa are common practice in paleobotany, the study of extinct plants, since you rarely find fossils of entire plants, but usually only leaves, pollen, trunks or branches and nothing more. It is then the challenge of paleobotanists to puzzle certain morphotaxa together into a whole organism, if possible (which rarely is the case). Stigmaria is considered to be the roots of Lepidodendron, a 40 metres tall club moss, or lycopod. (So, Stigmaria is part of Lepidodendron, but only Lepidodendron is an actual, classified organism; Stigmaria is just the roots.)

Reconstruction of Lepidodendron with its Stigmaria roots.

 Image from http://commons.wikimedia.org/wiki/File:Stigmaria_Heimans.jpg

No post on the Arran fieldtrip

I am sorry to tell that during the fieldtrip to the island of Arran in Scotland the assessed work we had to do was too much to be able to take any useful photos (or fossils), and none of it makes a story worthwhile to tell here.

Our "adventures" consisted mostly of measuring angles on rocks, trying to pinpoint different rock types on messy, obscure maps, and in general trying to guess what the point of all of this is (since they wouldn't tell you what you were expected to do in the field until after you had already been there...), all under considerable time pressure leaving no time for thinking twice about anything... In other words, it was generally confusing and stressful (until the work had been handed in, marked and returned to us, at which point they told us what we should have seen and done – we understood it in hindsight, but by then it was of course already too late...).

Apart from the poorly organised formal assignments, the trip was really pleasant. The weather was nice and sunny all week, and the company was fantastic! On fieldtrips, you spend so much time with your coursemates and build stronger bonds of friendship and solidarity.

Also, this trip reminded me soooo much about the fell hiking trip in Sweden from last summer: the curvy paths interrupted by bogs and mud pools, the hilly landscapes, the abundant streams of meltwater (Arran had suffered a heavy snow storm just a week ago), the long walks, the brisk air, etc. It was great, because I will sadly not be able to go with the guys on the hike this summer.