Anthroposaurus
Were Dinosaurs Primitive, Cold-Blooded, Lumbering Beasts with Walnut Sized Brains?
Abstract
Contents Updated: Wednesday, December 15, 1999
"No non-dinosaur larger than a turkey walked the land in the age of the dinosaurs."
Just Big Lizards
Many would say it is absurd to imagine intelligence in dinosaurs, cold-blooded, lumbering beasts with brains the size of walnuts. But is it so absurd? Were dinosaurs so primitive?
They were considered so for many years but in the 70s, two American professors, John Ostrom and Robert Bakker, drove a coach and four through previously held dogmas about these astonishing beasts. They argued convincingly that dinosaurs were not just big lizards, cold-blooded and sluggish—dinosaurs were hot-blooded. Like mammals and, especially, like birds they had a high metabolic rate and an active lifestyle. These new ideas provide a base for the reassessment of intelligence in dinosaurs. Hot-bloodedness changes the picture completely. The evidence for it is crucially important to the thesis presented here and justifies some detailed attention in this chapter.
The earliest large fossils that had been found were recognized by the authorities of the time like Baron Cuvier, Dr Mantell and the Reverend Buckland as being those of reptiles, similar to the monitor lizard and the iguana. These modern lizards are all cold-blooded. Strictly their blood is not cold but, unlike birds and mammals, they have no internal thermostat to keep their temperature constant. In cold weather their blood temperature drops and they become sluggish and inactive, but when the sun shines they bask in it until their temperature rises, then they become active and seek some sustenance. If the sun gets too hot they shelter in the shade or try to expose the minimum surface area to the sun’s rays to absorb less heat to prevent overheating.
When dinosaur fossils were first recognized, being lizard-like, they were classified as being of cold-blooded animals—though Richard Owen, who first categorized the dinosaurs, realized that, in many respects, they were more advanced than lizards and crocodiles. But it was the picture of a giant lizard that prevailed rather than that of a more advanced creature—until Bakker and Ostrom came along.
Owen’s definitive paper was written in 1841, yet almost 130 years later Robert Bakker could write that paleontologists still regarded dinosaurs as overgrown lizards in respect of their everyday behavior. Because crocodiles and lizards spent much of their lives in inactivity, sunning themselves on a convenient rock, the brontosaurus was pictured as a rotund, long necked crocodile moving about slowly and infrequently, and basking whenever it could in the sun.
Was this picture of such amazingly successful animals correct? Were they really just big lizards?
If they were, their physiology, the physical construction of their bodies, would have meant that they were severely handicapped creatures. Let us take a look at these handicaps.
Lungs
Lungs are spongy air sacs with millions of tiny, thin walled lobes containing circulating blood to absorb oxygen from the air. The smaller the tiny air lobes, the greater the surface area exposed to the inhaled air and the more efficient the absorption of oxygen. In a lizard these lobes are not miniscule as they are in mammals, they are larger and correspondingly inefficient.
In the human mammal destruction of the walls between the tiny air cells , by smoking cigarettes, for example, reduces the surface available for the exchange of oxygen causing a chronic and totally debilitating disease, emphysema. People with emphysema suffer from oxygen deficiency and have to move slowly. Pure oxygen inhalers help a little. Lizards effectively have to live their whole lives with emphysema: they are unable to take in much oxygen, their blood does not get much and their activity is correspondingly curtailed.
In contrast, birds, which have some lizard-like features (witness the scales on their legs, for example), have highly efficient lungs to support their active lifestyle. Yet many dinosaurs had hollow bones containing additional air sacs connected to their lungs just like birds. Did they have bird-like lungs with subsidiary air sacs pushing air steadily through in one direction, much more efficient than mammals’ lungs in which the air travels first one way then the other. This alone could have ensured the dinosaurs’ superiority for millions of years. But did it also make them more prone to gaseous or particulate pollution contributing to their eventual extinction?
Heart
A lizard’s heart is particularly inefficient because it has only one main pumping chamber or ventricle whereas mammals have two. Two ventricles are more efficient. The oxygenated blood from the lungs can be kept separated from the oxygen depleted blood returning from the body.
In humans “blue babies” are born with a hole in their heart, not a hole to the outside of the heart—blood does not escape—a hole between two of the chambers of the heart allowing oxygenated and spent blood to mix. Consequently, the capacity of the blood for oxygen is not fully utilized. The blue baby’s arteries always carry some spent blood, which is dark in color unlike oxygenated blood which is bright red, giving it a blue look instead of the normal pink.
In a lizard the two bloodstreams have to enter the single ventricle where they mix just as they do in the heart of a blue baby. Lizards are like mammals with a hole in the heart.
A zoologist, Roger Seymour of Adelaide University, researched into the connection between the difference in height between the heart and the brain of sauropod dinosaurs and the blood pressure needed to push the blood up to that height. If the animals were browsing from high branches then high blood pressures would obviously be required (a giraffe’s blood pressure is twice that of a man) and to generate such high pressures the heart would have to be sophisticated. It would certainly need four chambers not two like lizards.
High browsing dinosaurs, if no others, must have had four-chambered hearts like this and, if they had them, it is probable that the others had too.
Bone
Bone is not simply a dead inorganic material which only provides the internal framework of a vertebrate’s body, it is also an important living organ equivalent to the heart and the lungs. The bone marrow is where blood cells are made and the bones act as a reserve of calcium, a mineral that is essential in the functioning of muscle.
Naturally the efficient working of bone in these tasks depends upon a liberal supply of blood to transport the organ’s products to the sites in the body where they are needed. Mammals and birds have a complex system of blood supply through the bones but in reptiles the blood supply is much more feeble. Growth is slow in the warm days of summer and stops altogether in the winter giving annual growth rings like those in trees or on tortoise shells.
The exchange of calcium between the bones and the blood occurs in structures called the Haversian canals which are relatively lacking in reptile bones. In 1988 Robin Reid of Belfast University published an extensive survey of bone structure. He found that reptiles could have a mammalian type of bone structure, particularly if they were large or grew in hot temperatures.
What of dinosaurs? Sometimes they had reptilian and sometimes mammalian types of bone but most dinosaur bone is of the warm-blooded variety, whereas it is unusual for reptiles to have it. Reptilian physiology obliges reptiles to be slow and to conserve their energy. Compared with mammals they suffer from almost crippling defects of lungs, heart and bone. Unlike mammals their bodies simply cannot sustain continuous effort. That is why they move jerkily and spend long periods totally motionless.
Adrian Desmond, author of The Hot-Blooded Dinosaurs, says the monstrous carnivorous dinosaur, tyrannosaurus rex, could have reached no more than three or four mph if it were cold-blooded. You or I would have little need to fear such a creature—if we jogged away from it, it could not catch us moving even at its fastest. Yet it gives every appearance of being built for speed as well as strength. Indeed for almost a hundred years dinosaurs have been illustrated as active creatures. Just after the turn of the century, Charles Beecher of Yale University constructed a mock-up of a hadrosaur, claosaurus, in a running posture.
From analyzing dinosaur tracks we know a dinosaur’ normal walking pace was faster than would be expected for the maximum speed of an equal sized lizard. McNeil Alexander of Britains’s Leeds University has a formula for working out travelling velocities from imprints of an animal’s tracks. Carnivorous dinosaurs seemed to travel commonly at speeds of between five and ten miles per hour. Herbivores sauntered around apparently at about four miles per hour. A fast walking pace for a human being is about three miles an hour so these speeds are respectable for large animals no more likely to be making haste than a grazing cow. And, like the cow, when the occasion demanded it they could move a lot faster.
“Even a cursory glance at dinosaurian limbs and joints”, says Adrian Desmond, “should be evidence enough that many dinosaurs were built on the same lines as modern hoofed mammals”.
A useful yet simple guideline in this respect is the length of the shinbone to the thigh bone. Relatively slow moving animals have longer thighs than shins whereas swift animals have evolved longer bones in their lower leg giving them a longer stride to increase their speed.
Take a look at some modern animals: the elephant rarely has need for speed and its shins are shorter than its thighs, the ratio being about 0.6. A gazelle needs speed as its means of avoiding predators—its shins are 1.25 times longer than its thighs, the highest such ratio of all animals.
What do we know from the fossil record of the shin to thigh ratios of dinosaurs? The brontosaurus had short shins compared with its thighs similar to an elephant’s—both are heavy, slow moving, browsing animals. What of the hadrosaurs? Their shins were between 0.8 and 0.9 times the length of their thighs, much longer than the brontosaurus’s (and the elephant’s) but much smaller than the gazelle. It seems that, relatively, they were faster than an elephant but not as fast as a gazelle. So, what modern animal is comparable? The racehorse! A racehorse’s shinbone is about 0.9 of the length of its thighbone showing that the hadrosaurs occupied a similar ecological niche. Finding similar leg measurements on modern animals and dinosaur fossils suggests similar lifestyles.
Another sensitive measure is the length of the middle metatarsal (middle toe) to the thighbone, because fast running animals tend to lift themselves on to their toes to increase their stride still further.
The brontosaurus had a middle toe to thigh ratio the same as that of an elephant (0.13). Dinosaurs evidently built for speed were the coelurosaurs which had a middle toe 0.77 of the length of its thigh comparing favorably with a horse (0.78).
Sometimes the similarities went further. Indeed dinosaurs could be so astonishingly similar to modern beasts that it is difficult to accept they did not have the same ecological role. The coelurosaurs occupied the biological niche now held by the ostriches and similar flightless birds. Characteristic of them was the struthiomimus, so called because it was felt to mimic the ostrich (Struthio camelus).
- It is aptly named because its similarity to the ostrich is quite remarkable.
- It was seven feet tall, slightly taller than an ostrich.
- Its back was short and its hind legs were long.
- It had a shin to thigh ratio of 1.0 with its leg length effectively increased by an additional extended metatarsal bone.
- Its mouth was toothless and beak-like.
- It might even have had a flap of skin from its arms to its body to perform the same role as the ostrich’s wings when it was running.
- There is only one essential difference between the two creatures—the dinosaur had a tail which was more than six feet long.
The functional anatomist can only read into such a remarkable resemblance the closest similarity of lifestyle. Convergent evolution has led these animals to look alike. In Desmond’s words, “They have come to look like one another because they have come to live like one another”. The conclusion is that struthiomimus behaved similarly to Struthio camelus.
The ostrich is capable of sustaining speeds of over 45 mph. From its physiology, the dinosaur equivalent looks capable of the same, yet Bakker calculates that if struthiomimus had the constitution of a lizard it would be capable of sustaining hardly two mph!
The kneebones of animals provide another measure of their speed. Fast running creatures have strong muscles attached to the knee to snap the leg straight to provide much of the thrust while running. A protrusion at the knee, the knee crest, is the point of attachment of these muscles. Fast runners have larger muscles and need a stronger knee crest to withstand the stresses.
Elephants are large animals with big muscles but they do not run at more than 22 mph. Their knee crests are relatively small. Similar arguments can be used for the ceratopsian dinosaurs like triceratops, the plated three horned dinosaur looking much like a rhinoceros. These animals were herbivorous and with their powerful armor one might be forgiven for thinking they had no need to run fast. Today the rhino which occupies the same ecological niche is able to run at 30 mph even though it has no natural predators. But the ceratopsians had genuine reason to be able to run fast—they were sought as a nourishing feast by the voracious tyrannosaurus rex. Sure enough, both the prehistoric creature and the modern one have pronounced knee crests and triceratops can be assumed to have moved as fast as the rhinoceros.
What of predators? The arch enemy of the horned dinosaurs, the tyrannosaurs, had exceptionally large knee crests suggestive of a speed of 40 mph. McNeil Alexander’s formula supports this view indicating that some types, especially of smaller bipedal dinosaurs, did reach speeds of 40 mph.
Quite fascinating is the deinonychus, the eight feet long carnivorous dinosaur, living in the early part of the Cretaceous period, discovered in 1964 by John Ostrom. Ostrom describes his find as being…
…a fleet-footed, highly predacious, extremely agile and very active animal, sensitive to many stimuli and quick in its responses. These indicate an unusual level of activity for a reptile and suggest an unusually high metabolic rate.
The interesting features of this dinosaur were:
- its legs were long marking it as a fast predator
- marks on the backbones, reminiscent of those in ostriches, were channels for ligaments which held the spine horizontally as it ran
- parts of its vertebrae had grown to lengths of about 18 inches and functioned as struts to brace the tail rigid when deinonychus gave chase
- its feet had three toes but only two of them were used for running, the third having become adapted as a villainous hook for gouging its prey
- its arms were long and its hands ideally suited to grabbing hold of its victims.
Plainly, deinonychus hunted by giving chase to its intended victim, caught hold of it using its highly developed hands then struck at it with its vicious claw. In so doing it had to balance on its other foot with its tail as a counterpoise to keep its balance.
It must have borne some resemblance to the cassowary, a most dangerous modern ground bird weighing 200 pounds. The cassowary also has a large hind claw with which it strikes its enemies using the full power of its strong thigh muscles. Considering that there is reason to believe deinonychus might have been feathered, the resemblance could have been closer than one might initially imagine. We shall have more to say about this remarkable creature later.
The large long-necked dinosaurs such as brontosaurus and diplodocus have also been put through the sawmill of prejudice. It was felt that creatures weighing around 50 tons could not live on dry land. Water was needed to support such a bulk. The experts began to reconstruct them as long-necked crocodiles, their legs, like lizards’, splayed on either side of their body. Early reconstructions of dinosaurs such as those at Crystal Palace in South London, England, looked crocodilian. Yet the earliest restorations had given the brontosaurus massive columnar legs like an elephant’s. Nowadays physicists would agree that the thrust of the weight of huge bodies on dry land must be transmitted directly to the ground through sturdy, straight limbs.
Weight is related to volume which increases as the cube of the linear dimensions. The strength of the limbs however depends upon their cross section which increases only as the square of the linear dimension. As the weight of a body increases, its supports, its legs, have to thicken at a faster rate. Large terrestrial animals have to have legs like tree stumps. If the legs were splayed like a lizard’s so that, on dry land, the body weight was suspended between the limbs rather than supported directly by them, the animal would be unable to raise itself from the ground without breaking its own legs. Such a reconstruction would have required the sauropods to be purely aquatic. The “crocodile theorists” accepted that the creatures sometimes had to move on land. But that was no problem to them—the sauropods must have slithered through the swamps in which they lived.
Blindness! The experts had lizards in their heads and could see nothing else. Sauropods manifestly had a differently shaped thorax from a crocodile precluding them from walking on splayed legs. The deep rib cage of the diplodocus would have required it either to “slither” in a deep trench to make space for its thorax or to slide along on its chest with its legs protruding sideways making little or no contact with the ground. Pillar-like legs supporting the heavy body on top was the only rational reconstruction for the sauropods.
The discovery of brontosaurus tracks at Glen Rose in Texas in 1938 settled the controversy. The stride of the monster was twelve feet but the left and right prints were separated width-ways by only six feet. Brontosaurus made footprints much closer together than a crocodile of equivalent size. Its legs were upright not splayed.
Such large animals must have needed vast amounts of internal energy simply to keep them on their feet. How could they have processed the huge quantities of food required by their high metabolisms when it must have been of fairly low grade and they had relatively small heads?
“Paleontologists usually dismiss any thinking about the soft parts of dinosaurs ‘because innards do not fossilize’. All speculation about gastrointestinal tracts in the Dinosauria is futile”, mocks Bakker, mimicking the orthodox thinkers. Because the sauropods were supposedly partly aquatic, had small heads and had weak teeth, orthodoxy dictated that their diet was soft—duckweed or algae. But is it reasonable to believe that these 50 ton animals had to live off duckweed or algae? Surely a huge maw in a huge head like a baleen whale would be needed for them to scoop up enough algae to sustain such huge bulk.
It must be more reasonable to assume that these dinosaurs were terrestrial browsers and ground their food in gizzards or fermented it in their stomachs or both. Modern birds have no teeth and do no chewing but they process often tough food perfectly effectively in their gizzards, and modern ruminants like cows and sheep predigest poor quality grasses by fermentation in their pre-stomachs to prepare it for further mastication. Birds like ostriches, though they have small heads and long necks similar to many dinosaurs, have both gizzards and fermentation processes to maximize absorbed energy. This compensates for the slow food intake implied by their small heads whilst maintaining their high metabolic rate. Brontosaurs most certainly did the same, and, in fact, their gizzard stones, worn smooth by grinding, have been found.
If dinosaurs were warm-blooded and especially if they were naked, they would have had to grow quickly or die of heat loss. Immature dinosaurs are rarely found. Maturity must have been reached quickly and early growth must have been rapid. Birds and mammals, today’s warm-blooded animals, grow to maturity quickly. From a hatchling, an ostrich grows 150 pounds to adulthood in only nine months and an Alsatian dog is twenty times bigger at a year than it was at birth. (Humans are the exception, maturing slowly because of the time needed for them to fill their outsize brains with experience.)
Lizards have quite a different pattern of growth. They do not have a spurt of growth when young but increase in size constantly throughout their lives. Reptiles such as turtles and crocodiles take about ten times longer in the wild to put on the same weight as mammals.
The fossil record does not sustain such a growth pattern for dinosaurs. As bone grows it is supported by fibres of the protein, collagen. Fast growing bone has a random pattern of collagen fibres, rather like a felted fabric, whereas slow growing bone has a much more regular pattern of collagen put down in layers. The texture of all dinosaur bone is the irregular type typical of fast growth and warm blood.
Reconstructions like those at Crystal Palace in London depicted all dinosaurs as quadrupeds. Yet in the middle of the 19th century Thomas Huxley realized that some of the dinosaurs were bipedal. The discovery of excellent fossil skeletons of the iguanodon settled the matter.
Iguanodons were large erect herbivores 35 feet in length. Their front limbs were much smaller than the massive rear limbs showing they could not have been used for support and evidently were used for grasping branches as the great beasts browsed the trees. The carnivore, tyrannosaurus rex, was so bipedal that its forelimbs had atrophied until they were apparently quite useless. They consisted of two claws which were too short to reach the creature’s mouth and so could not even have been used for feeding. Their purpose, it is surmised, was to provide leverage to lift the monster back on to its legs after it had been resting on its stomach.
Their tail however was important, stretching out backwards to provide a counterbalance to the creature’s trunk when it moved. These dinosaurs lunged forwards with a sort of waddling effect as the tail swung from side to side. Ancient tracks confirm this picture.
T. rex was an eight ton monster. For it to stand on two limbs, was an effort that required a large amount of energy. Its head was so massive, unlike that of its earlier predecessor, allosaurus, that it needed enormous amounts of energy just to carry it. Is it feasible that such a creature could have had the defective physiological system and intermittent metabolism of a cold-blooded lizard?
One concludes from all this evidence that dinosaurs could not have had the physical make up of present day lizards. They were not just lumbering beasts (though some had a lumbering lifestyle like some mammals), they were sophisticated creatures that kept mammals suppressed for over 100 million years!
There are, of course, counter arguments.
Nicholas Hotton III of the Smithsonian Institute claims that 80 per cent of mammals are smaller than the smallest dinosaur, claimed to weigh about 20 pounds. Only two per cent of modern mammals are heavier than two tons but 50 per cent of dinosaurs were. Bulk must have been advantageous to the dinosaurs. The reason claimed is that they had no internal mechanism for maintaining a steady temperature and had to retain their heat by being bulky.
A large body loses heat more slowly than a small one because it has less surface area per unit volume. It is the volume that holds the heat and the surface area that loses it. The greater the volume compared with the surface area, the greater the retention of heat. In technical terms, they were mass homeotherms. By growing to huge size they kept a more or less constant temperature giving them many of the characteristics of warm-bloodedness. Hotton concluded that the dinosaurs had reduced their dependence on the temperature of the environment at a much lower cost than mammals:
The activity of dinosaurs was more sedate than that of mammals. The basic strategy of dinosaurs in general was “slow and steady”, and what it lacked in mammalian elan, it made up in economy.
Yet this solution still leaves us with puzzles. Some dinosaurs “grew no bigger than rabbits or crows”: archaeopteryx was no bigger than a crow and compsognathus was no bigger than a chicken. Alan Charig of London’s Natural History Museum states that the smallest dinosaur was “no bigger than a mistle thrush”, weighing only a few grams. Pterosaurs were even smaller—some were tiny. How could a dinosaur the size of a thrush, or even a chicken, maintain an even temperature and how could hatchlings survive? Both would have a large surface area to volume ratio and would radiate heat rapidly.
Baby dinosaurs and pterosaurs could hardly have been other than genuinely warm-blooded, (mass homeotherms without the mass?) Why otherwise did some pterosaurs, if not all, have fur?
Fur is an insulator. It makes no sense for an animal to have fur unless it wants to keep heat in, implying internal heat generation and warm-bloodedness. If a small mammal had no insulation it would need to generate so much heat from its own activity to replace heat lost from its skin that it could never rest. If it did it would cool down, become comatose, be unable to forage and would starve to death. Small mammals need fur to live. Most pterosaurs were not bulky enough to be mass homeotherms. Yet, since they had evolved fur, they must have been warm-blooded. Their large eyes and brains also imply an active lifestyle, a characteristic of warm-blooded animals.
Archaeopteryx, the evolutionary link between dinosaurs and birds, was discovered because it was feathered. Feathers, like fur, act as an insulator suggesting that archaeopteryx had some reason to keep heat in—it was warm-blooded. Yet we noted above it was only as big as a crow, apparently counting out mass homeothermy. Its descendants, the birds, are hotter blooded than mammals. Yet archaeopteryx was living 140 million years ago, right in the middle of the dinosaurs’ reign. Is it reasonable to imagine that archaeopteryx had evolved warm-blood when the other dinosaurs were still tinkering with mass homeothermy or, according to many, were cold-blooded reptiles.
Moreover archaeopteryx was not the only feathered dinosaur. A fossil dinosaur discovered in the Gobi Desert named avimimus (bird mimic) was described in 1980 by a Dr Kurzanov. Looking somewhat like compsognathus, a sort of miniature allosaur, the main feature about this dinosaur was that it showed signs of being feathered like archaeopteryx yet is much later, coming from the Late Cretaceous about 75 million years ago. The upper arm bone had low projections like those of birds for the attachment of flight muscles while the lower forelimb had a bony ridge similar to bony protrusions on the forearm of birds. The feathering was thought by Kurzanov to indicate warm-bloodedness especially as they were small. Interestingly the skeleton had no tail making it look particularly bird like.
Unless feathers evolved twice this could be evidence that all small therapods were feathered for insulation. Certainly deinonychus was so similar to archaeopteryx that Bakker presumes it too must have been feathered. Possibly many others also were, though the feathers would not have been flight feathers but rather a sort of down. If small dinosaurs were warm-blooded they could not have evolved without insulation. (Did Dinosaurs Have Feathers? Article and link to Dinosaurs On-Line.)
As in many disputes, not least scientific ones, the answer might not be at either of the extremes. One could argue that not all the dinosaur genera were fully warm-blooded. The huge, noble sauropods might have been stately homeotherms, needing less internally generated heat, because their bulk retained it. Natural selection in animals like these might have pushed them to vast bulks. Smaller dinosaurs and perhaps predators were warm-blooded.
But this idea is not supported by the absence of mass homeothermy today. If it offers such an evolutionary advantage over warm bloodedness, except for tiny animals, why are there no large mass homeotherms in the tropical regions where conditions are stable and ideally suited to them. The large animals are elephants, rhinoceroses and hippopotamuses—fully hot-blooded mammals.
Mass homeotherms must necessarily have been displaced by creatures that were actively warm-blooded irrespective of size. A ten degree Celsius drop in temperature approximately halves the rate of a chemical reaction including those that make the metabolism work. Consequently the activity of a cold-blooded animal roughly halves with every ten degree Celsius drop in temperature. That is why cold-blooded lizards get less active in colder temperatures. In cold climates only the animal with a body temperature regulated at the optimum level for its metabolism can keep active. Closer to the poles or up mountains a temperature must be reached where the mass homeotherms would lose body heat becoming less active whereas a fully warm blooded animal with its built in thermostat would remain active. In some such environment the warm-bloods would have had the advantage over the mass homeotherms. Over millennia, the advantage of always being active would translate into total dominance.
As Robert Bakker puts it:
In direct confrontation, high metabolism always conquers low metabolism.
A Bakker cartoon illustrates this idea wonderfully gruesomely in his book, The Dinosaur Heresies. The cartoon depicts a large sauropod, the mass homeotherm, comatose and unresponsive in the freezing rain, failing to notice tiny rat-like mammals, active despite the cold, eating him alive from the tip of his tail. It never really occurred, of course. Dinosaurs were quite capable of surviving in freezing climates near both Poles.
In 1987 a group of scientists led by Elisabeth Brouwers of the US Geological Survey reported in the journal "Science" that they had found dinosaur remains inside the Arctic Circle. A mixture of young, old, large and small dinosaurs were found near the Colville River. How could cold-blooded dinosaurs or even mass homeotherms survive here in the cold and darkness of winter—they had to be warm-blooded Migration after the fashion of the caribou is just a possibility but again implies a highly active animal. Dinosaur fossils have also been found in Australia which in the Triassic period was much closer to the South Pole having just broken away from Antarctica. Mass homeotherms seem unlikely to have lived in such places.
Furthermore, if dinosaurs were anything other than warm-blooded they could never have displaced the mammals, or rather the mammal-like reptiles, 200 million years ago and could only have survived by taking the role of our mainly small, present day reptiles. That they conquered and then suppressed the mammals, growing eventually to huge size shows that they were fully warm-blooded and not mass homeotherms.
A species evolves to fit different niches in its environment by the process of adaptive radiation. The speed of adaptive radiation depends upon the metabolism of the animal. Cold-blooded animals evolve more slowly than warm-blooded ones. Yet the dinosaurs on several occasions evolved explosively. In one small locality in the US, seven genera of hadrosaurs appeared in only ten million years, all plainly evolved from a known ancestor. Such rates of adaptive radiation have been typical of mammals since the extinction of the dinosaurs.
Evolution is now believed to occur in spurts separated by periods of stability when a species does not change much. This idea, proposed by Eldredge and Gould, was anticipated by an English paleontologist, Hugh Falconer who died in 1866, only seven years after the publication of Darwin’s theory. But by the time the arguments of the scientists against the religious bigots had been won the soft voice of Falconer had been forgotten and his message was not heard again until a century after his death.
The periods of equilibrium of species vary according to whether the animal is warm- or cold-blooded. Cold-blooded animals remain stable longer and evolve into new species much less often than warm-blooded ones. The reason is that the higher metabolism of warm-blooded creatures accentuates the competitiveness between them. The evolution of a new species changes the environment of the others which induces them to evolve in response. A positive feedback encouraging rapid evolution occurs and only warm-blooded animals can adapt so quickly.
Dinosaurs speciated at a rate comparable with the mammals and birds, not their supposed role models, the reptiles. An analogy might be a boxer who finds himself champion at a time when there are few contenders and they are of poor quality. He could remain champion for a long time. If there were more and better aspirants for the title, his reign would be shorter. The champion cold-blooded boxer remains champion for longer than does the champion warm-blooded boxer because there is a steady stream of new warm-blooded contenders but few cold-blooded ones. Where the warm and cold-blooded animals are in direct competition the cold-blooded one stands little chance. If a human boxer were fearful that one day he might leave the ring dead, he might be very glad to opt out of boxing and chose instead the life of a grocer or a publican. In a sense, that is what the cold-blooded animals that have survived till today have done. They have opted out of direct competition with the warm-bloods.
If the broader classifications of genera and families are considered instead of species, the differences in rates of adaptation are enhanced. Taxonomic families of genera are even more likely to be long lived. Indeed families of cold-blooded species often seem to go on indefinitely. The family of modern crocodiles has survived for a hundred million years and, according to Bakker, the average for cold-blooded reptiles is 55 million years. If dinosaurs were cold-blooded their taxonomic families should survive for a similar length of time. But “the dinosaurs evolved quickly, changed repeatedly, and turned out wave after wave of new species with new adaptations all through their reign”, according to Bakker. The average life of a family of dinosaurs was only 25 million years, virtually the same as that of families of mammals.
Desmond wrote:
Nobody before [Bakker and Ostrom] had demonstrated the inextricable relationship between high metabolism, stable temperature and erect posture, yet once explicitly stated this linking seemed obvious and natural. It resolved the long-standing contradictions inherent in the ludicrous sun-basking brontosaur model by scrapping the model altogether and substituting an endothermic dinosaur… The dinosaur’s suspected high metabolism and fast energy production places it not with cold-blooded lizards but warmer-blooded mammals and birds.
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.
Once warm-bloodedness arrived, it stayed. The cold-bloods were never able to gain control again. They evolved too slowly to fill the empty niches created by mass extinctions of the warm-bloods. Even the paleontological establishment accepts that the wolf-headed cynodonts were surely warm-blooded. So if dinosaurs were cold-blooded they could not have replaced them.
You might argue that even in “The Age of the Mammals” a lot of cold-blooded animals like tortoises and snakes have thrived though not as the dominant life form. Couldn’t the dinosaurs, though cold-blooded, have done the same, biding their time to eventually wrest control from the pre-mammals. It sounds feasible perhaps, but, though cold-blooded animals remain numerous in the world of the mammals, we saw above that they have occupied specialized niches in which they have avoided direct confrontation with the warm-bloods. Their survival has depended on that.
Tortoises evolved from turtles by adapting for a life on land about 50 million years ago, after the demise of the dinosaurs. On dry land the tortoise was in apparent competition with mammals, also free to occupy empty niches left by the dinosaurs. It succeeded to such an extent that one species called colossochelys grew as big as a small car. But the tortoise was not in direct competition with the mammals! It did not try to out-do the mammals at their own game! Instead it made the best of the qualities that it had—an armored carapace and the infinite patience conferred upon it by its slow, cold-blooded metabolism. Harried by a predatory mammal the tortoise, like a feudal baron, withdrew into its keep, pulled up the drawbridge and settled down to withstand the siege. And the tortoise could wait a lot longer than any mammal. The colossochelys survived for millions of years until recent times, able to withstand anything mammals could come up with—until they invented man. The intelligent mammal realized that not only was the giant tortoise helpless on its back but it also carried with it a conveniently large stewpot. Groups of men were soon levering the tortoises on to their backs and building bonfires underneath them. The result is that no colossochelys remains in the world today.
There are about as many species of snake living on the land as there are mammals. Snakes are predators but ones that, like the herbivorous tortoises, make the most of their own special features including the infinite patience of the cold-bloods. The snake is able to poise unseen, because of its unusually extended shape, and totally motionless, because of its cold-blooded metabolism, until some careless victim stumbles upon it, whereupon it is swiftly poisoned or more slowly constricted. Snakes make the most of what they have and do not actively compete with warm-blooded mammals.
Yet during the Triassic period the confrontation between the mammal-like reptiles and the forerunners of the dinosaurs was head on. In Bakker’s words…
…a titanic ecological battle was waged between the advanced pre-mammals, led by the dicynodonts and the cynodonts, and the Erythrosuchidae[†]Erythrosuchidae. Crimson crocodiles, early predecessors of the crocodiles and dinosaurs. and their descendants… Two mighty evolutionary dynasties collided in direct competition.
What happened? The fastest and most efficient predators of the early Triassic, the warm-blooded cynodonts and their therapsid kin, lost to the supposedly cold-blooded crimson crocodiles and to their descendants, the thecodonts.
The only relatives of the therapsids surviving till today are the duck-billed platypus and the spiny anteater. Significantly enough these animals are only partly warm-blooded. A platypus’s typical body temperature is only 30 degrees Celsius, compared with 37 degrees Celsius for most mammals, and it is subject to wide fluctuations. It also lays eggs and has no nipples but instead has refined sweat glands that ooze milk into its fur for the neonate to lick. It is just what one might expect from a form intermediate between reptiles and mammals.
The therapsids probably became warm-blooded to protect themselves against the advent of colder conditions. Then, for the same reason they developed a furry exterior. But why did the crimson crocodiles and thecodonts become warm-blooded? The answer seems to be precisely because they were adapting for an active, bipedal mode of living—and, as we have seen that would not have been possible without internal heat regulation. Thecodonts evolved from swamp dwelling forms which developed strong rear limbs for swimming and steering themselves in the water, just like frogs and crocodiles. The swamps began to dry up in the middle of the Triassic period and many of a previous dominant life form, the amphibians, died out.
The thecodonts’ ancestors were somewhat more adaptable, necessarily learning to spend more time out of the water until they became independent of it. On dry land they found that their strong hind limbs allowed them to rear up and catch the insects that lived in profusion around the drying swamps. They found that they could run on their hind legs for short distances, improving their chances of survival. Simultaneously, through natural selection, they became warm-blooded thereby gaining the energy needed for running and hunting. Finally they brought their legs into a better position beneath their bodies to provide more efficient propulsion to exploit their faster metabolism. With long hind legs and an energy system to match, the thecodonts were ready to take on the world. Thus the thecodonts ousted the therapsids from their advantageous position.
They, in turn, were to succumb. The dinosaurs proper, descendants of the thecodonts, appeared on the scene by the end of the Triassic. The invasion of the thecodonts and their successors, the dinosaurs, put paid to the ambitions of the mammal’s precursors, the therapsids, and those also of the mammals themselves for a hundred million years or so.
Dinosaurs did not appear until 200 million years after the first vertebrates crawled on to the land. As we have seen, they were preceded by a series of other types of land animals including a period of several tens of million years about 250 million years ago when the pre-mammals dominated. They were warm-blooded but they were not the first warm-blooded animals. They were the culmination of a series of mighty dynastic struggles among previous warm-bloods. Bipedalism and possibly superior lungs gave them an advantage that others, including the mammals, could not match. In only five million years, short in geological terms, the dinosaurs sprinted to power.
It has taken about five million years from the emergence of mankind to world dominance!
For 140 million years mammals were forced into less desirable habitats especially where it was cold and barren. They lived in burrows, terrified of the dinosaurs and pterosaurs. Insignificance was an advantage. Tiny creatures like shrews and mice were less likely to attract attention. Because mammals at this time were very small—the largest was no bigger than a hedgehog—and rocky terrain is not suitable for fossilization, fossilized remains of these mammals are uncommon. Only their tiny teeth are commonly found.
Though small and insignificant, mammals were not free from danger. By day, though large dinosaurs would not have been interested in them, the smaller predatory dinosaurs were. And besides dinosaurs, true lizards were still sunning themselves motionlessly on the rocks waiting for a small animal to come unwarily by.
Mammals were nocturnal. They had to forage by night when it was too cold for lizards to be active, limiting their enemies to small nocturnal dinosaurs. The mammals’ niche was quite lowly for the millions of years of the supremacy of the dinosaur. They could not compete with a superior animal.
All was changed by the mass extinctions which ended the Cretaceous age. The mammals inherited the earth and eventually one of their species came to do remarkable things. It was the thinking mammal—mankind.
