Contents Updated: Thursday, August 05, 1999
Let us scotch another fallacy: that the mammals were superior to the dinosaurs and succeeded by out-competing them. Both mammals and dinosaurs evolved from earlier types at about the same time in the Triassic, about 200 million years ago, but it was the dinosaurs that established their superiority and for the next 140 million years mammals had to be content with harsh ecological niches that the dinosaurs could not occupy.
The fossil record testifies to the superiority of dinosaurs over mammals for twice the period that mammals have dominated the earth. Only when the dinosaurs mysteriously disappeared 65 million years ago did the mammals have the chance to succeed.
The story is quite amazing. The mammals' ancestors, the therapsids, had actually ruled the earth for 30 million years before the arrival of the dinosaurs. Then in a pre-historic world war they lost to the antecedents of the dinosaurs. Dinosaurs were not cold-blooded, as we have seen, but evidently not even dinosaurs were the first warm-blooded animals. Warm-blood evolved at some point in the 120 million years that therapsids and their predecessors, the pelycosaurs, dominated the earth. Not only are mammals warm blooded, their predecessors were. Not only were dinosaurs warm blooded, their predecessors were too.
The dimetrodon, a predatory pelycosaur was cold-blooded. It had a fin on its back probably for more efficient heat exchange between the animal and its surroundings. It would have turned this vane fully towards the sun when the animal wanted to be warmer and edge on to the sun or into the breeze when it wanted to be cooler. Capillary blood vessels in the fin then allowed the blood to heat up or cool down very effectively.
It was a more efficient way of doing what lizards do today and surely would have been retained by later generations. It might also have served as a sexual signal to attract mates rather like the salamander's tail. As we shall see sexual selection tends to enhance characteristics and such a sexual signal would be expected to be favored by subsequent evolution.
Remembering that cold-blooded creatures evolve slowly, the fin looked destined for a long life. Yet the dimetrodon's descendants had lost it. Why did it disappear so quickly? The answer could be that this was the point at which warm blood evolved. If the fin were a heat exchanger, dimetrodon's warm-blooded descendants had no need of it having developed a more efficient method of temperature control.
A signalling device implies a passive approach to sex but with the evolution of warm-blood active courtship (like the rutting of stags) had probably replaced the older passive form. The therapsids, undoubtedly used active methods of gaining sexual dominance, like head butting, which required surplus energy compared with that used by the earlier animals and which showed they were warm-blooded. The dinosaurs used both active and passive sexual strategies giving them a variety of courtships similar to that seen today among mammals and birds, both warm-blooded.
The pelycosaurs survived essentially unchanged for about 20 million years but then were replaced in only a few million years by the therapsids, mammal-like reptiles which eventually evolved into mammals.
Relevant here is the evidence Bakker found for warm-bloodedness in predator:prey ratios.
A warm-blooded animal needs a lot more energy than a cold-blooded one so a warm-blooded predator needs to kill more prey to supply it. A given number of prey animals of a given size will support a smaller number of warm-blooded predators than cold-blooded ones. The metabolic rate of the predators can be deduced from predator:prey ratios which can therefore give us an idea of when a more active metabolism and warm blood evolved.
The pelycosaur predator, dimetrodon, had a variety of prey and was often the most common animal in its environment, an unusual situation, the food chain usually narrowing towards the top. Its predator:prey ratio was 1:4, more or less the same as that of the wolf spider (naturally cold-blooded) and its prey. The predator:prey ratios of therapsids were typically about 1:14 indicating a significant move to warm-bloodedness. By comparison, Eocene mammals show a ratio of about 1:30 and modern mammals of 1:100 or even less.
But mankind has reduced the equilibrium numbers of predators so much that modern ratios are grossly untypical and the value for mammals in favorable environments is around 1:25—which is just the ratio found for the dinosaurs. The predator:prey ratio for therapsids indicated a marked move towards warm-blood and indeed the explosive adaptive radiation with which they replaced the pelycosaurs is typical of the warm-bloods. The therapsids' own rate of speciation matched that of modern mammals.
The earliest therapsids suffered a mass extinction after about ten million years and were replaced by new therapsids evolved from the old. All the evidence is that these new proto-mammals were warm-blooded. Again after a few million years they suffered a mass extinction and a fresh group of therapsids took over including the large cynodonts, with remarkably dog-like skulls and the herbivorous dicynodonts.
Analysis of their fossils confirms that these also were warm-blooded. The external physiology (bumps for muscle attachment like the knee crest) and the internal physiology (bone texture) of therapsid bones point to warm-blood. Walking speeds calculated from fossil footprints were in the warm-blooded range and finally some of the therapsids lived at extreme latitudes where it must have been cool if not cold, making it probable that they had already evolved hair and were looking quite mammal-like. Rapid cycles of extinctions and repopulation tell us that these animals were warm-blooded—through active competition prone to mass extinction but able to restock the environment with new species by explosive adaptive radiation.