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Were Dinosaurs Endotherms or Ectotherms?
There is a question that has plagued palaeontologists interested in dinosauria for as long as the animals have been studied; were dinosaurs endotherms (warm-blooded) or ectotherms (cold-blooded)?
Today, endothermy is seen in mammals and birds but not in reptiles and is actually little to do with the temperature of the blood. An endotherm generates heat, climate-controlled internally to produce perfect conditions for the biochemistry inside the animal. This is because most biochemical reactions are catalysed, usually by enzymes, and those enzymes operate most efficiently in a narrow temperature range. Unfortunately, this level of control uses an enormous amount of energy so endotherms need to eat a great deal more than ectotherms.
Terrestrial ectotherms use the warmth from the sun to heat their bodies to optimum temperature and use shade, water or other means to lose heat should that become necessary. Consequently, ectotherms are not 'all-weather' animals and are only active in a fairly narrow range of conditions. The huge advantage of ectothermy is that it requires vastly less energy and therefore vastly less food.
This entry is not intended to answer the question, merely to describe the evidence for both sides.
Overview of the Question
Dinosaurs clearly evolved from reptiles - there is little debate about that. In addition, it is known that mammals and birds also evolved, directly or indirectly, from the same group. It is believed, in fact, that the mammals evolved from a group known as Therapsids and the birds and dinosaurs evolved from a group called Thecodontids. In either event it is known that warm-blood evolved in reptiles at least twice. The question is whether it could have done so a third time. Moreover, birds are essentially defined by their feathers and those, all agree, evolved as insulation, something only required in a warm-blooded animal. If we are therefore looking for a warm-blooded animal that could have developed feathers and evolved into birds, Theropod1 dinosaurs are a major candidate. In this instance we are back to two appearances of warm-blood, which suits Occam's razor (the principle that the simplest explanation is usually the correct explanation), but only if there is evidence for this line of descent.
If you had asked this question a century ago there would have been no doubt at all. At that time dinosaurs were believed to be cold-blooded. The reason for this stems from an historical viewpoint more than from evidence. When the first dinosaurs were classified, they were described as giant extinct reptiles, due to their bone structure. Dinosaur skeletons are very reptilian in form, having tails not unlike a lizard, for example. The word 'dinosaur' means 'terrible lizard' and there seems little doubt that when Owen2 coined the term he believed that that is exactly what they were. Socially, indeed, it was eminently suitable to categorise them as such. Darwin had published his opus The Origin of the Species in 1859 and Owen had been vehemently opposed to its precepts. Owen is, somewhat unfairly, remembered more for his fruitless struggle against the theory of evolution than for his outstanding work in the field of palaeontology. Clearly he saw no social gain in any perception of the dinosaurs as lizards; some of his contemporaries, however, did.
Owen, bolstered by the Church, encouraged the thought that the dinosaurs represented the pinnacle of reptilian evolution. This could demonstrate that evolution did not work since modern reptiles showed degeneration, rather than progress. In fact evolution had no inherent requirement of progress, merely of change, and change had certainly occurred. Some of the proponents of evolution made increasingly wild cases for dinosaurs being less impressive than the fossil evidence seemed to show. They could be seen as lumbering, archaic giants, slow witted and slow moving. Fortunately for science, the evolution supporters won the arguments and Darwin's work became an important part of biology. Sadly, this did no favours for the dinosaurs.
Now that evolution was accepted, it was seen as 'inevitable' that dinosaurs were primitive and poor animals. Attacks came from all quarters; an effort was made to redesign dinosaurs so that they sprawled like lizards. Sauropods were described as being too heavy to even support their own weight; the braincase of the Stegosaurus was picked on as an archetype of stupidity. A century ago this was clearly the perception. Not only did this support the more ignorant among the evolutionists, it also found eventual favour with the Church, adept at changing sides mid-argument. Possibly their very weakness was what caused a kind God to put them out of their misery and permit their extinction, the argument ran. Throughout this entire debate, however, the actual evidence of the fossils themselves had been largely neglected.
Evidence for Ectothermy
Approaching the subject from a new position, unaffected by the beliefs of those earlier palaeontologists, we must look for evidence for cold-blood.
Evidence for Endothermy
Due to this inequity between the evidence, almost all contemporary palaeontologists support the warm-blooded position, outlined first to a popular readership by Adrian J Desmond in The Hotblooded Dinosaurs and based principally on the sea of change caused by the discovery of Deinonychus (a fast and aggressive Dromaeosaurid dinosaur)7 by John Ostrom.
On the Internet
Dinosauria Online is filled with information and excellent articles concerning dinosaur research.
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