|The Evolution of Mammalness
During the Jurassic and Cretaceous periods – which had a total duration of over 120 million years – mammals formed a small element of the terrestrial fauna (Clemens 1970).
All modern mammals arose from a group called cynodonts. These advanced mammal-like reptiles of the middle and late Triassic were somewhat dog-like predators. For lack of any better point at which to split the reptile-to-mammal continuum, mammals are defined as those in which the early articular-quadrate jaw joint has been superseded by a new articulation between the dentary bone of the lower jaw and the squamosal bone of the skull. (Macdonald 1995)
With the extinction of the dinosaurs at the close of the Cretaceous, approximately 64 M.Y.B.P., the mammalian fauna underwent a major radiation and for the first time began to occupy niches available only to large terrestrial vertebrates (Clemens 1970). The first major adaptive radiation of reptiles consisted of the Synapsida, or mammal-like reptiles, which are now totally extinct, but which dominated the terrestrial fauna from the late Pennsylvanian, throughout the Permian and for much of the Triassic periods (Kemp 1980). Dicynodonts are in the order Therapsida, which is in the class of Synapsida. These animals ranged in size from tiny beasts, no larger than mice, to animals like Triconodon of late Jurassic age, that were as large as cats (Colbert 1980). The Dicynodonts were, as stated by King (1990), a kind of missing link group once thought to bridge the evolutionary gap between mammals and reptiles. Mammals evolved from their therapsid reptilian ancestors in the Late Triassic, only a few million years after the first dinosaurs appeared (Crompton 1968). Today and throughout the Cenozoic mammals have been the dominant class of land vertebrates (Bakker 1971). Bakker (1971) also stated:
The success of mammals probably is due in large measure to their high metabolic rates, great scope for increasing metabolism during activity, and their ability to maintain constant body temperatures by complex mechanisms of heat production, insulation, and heat loss.
If it were not for these reasons, mammals probably would not dominate the terrestrial fauna today. The problems that the early mammal-like reptiles faced and the solutions are the subject of this paper.
There are three major problems facing an animal living on land. The first problem is the vast variation in temperature, either daily or seasonally. The second is the tendency to lose water because of huge water gradient between the air and the animal’s tissues, and the resulting difficulties of maintaining osmotic, and ionic balances (Kemp 1982). And the last problem as stated by Kemp (1982), is the gravitational problem arising from the absence of buoyancy in air.
Maintenance of a constant, relatively high body temperature is of obvious selective advantage (Spotila 1980). Mammals, according to King (1990), differ from present-day reptiles and amphibians in having an elevated body temperature and higher rate of metabolism, both while resting and during activity. This constant rate of body temperature is accomplished by endothermy, which is defined by Kemp (1982) as, internal heat production from a high cellular metabolic rate, typically some seven times that of a similar sized ectothermic reptile.
For the early reptiles:
If the environment became too hot, they were forced to hide from the sun’s rays; or, if it became too cold, they became sluggish and even torpid. Clearly there were fresh fields and golden opportunities for any forms that could control heat loss or heat gain. A variable, insulatory coat was the answer and it was developed independently by two groups of reptiles. In one case it evolved as feathers, giving rise to birds; in the other case as hair, leading to the mammals. (Morris 1965)
Mammalian endothermy must be seen as a highly successful, if somewhat extravagant, means simply of maintaining a constant body temperature within fine limits and under a wide range of environmental conditions (Kemp 1982).
There are two questions that arise when asking what a thermostatic setting for an animal should be.
On the one hand, the cost in terms of food requirements rises as the proposed thermostat setting rises. On the other hand, the difficulty of dispersing the excess heat produced by the muscles when the animal is fully active requires that the thermostat is not set too low, and therefore a reasonable temperature gradient from the animal’s body to cooler surroundings exists. (Kemp 1982)
There are considerable costs due to the benefits of endothermy. The direct cost is in the greater amount of food that has to be collected and assimilated (Kemp 1982). Since the early mammal-like reptile had to eat a lot of food, and often, the structure of the mouth and nose had to change. The incisors probably integrated, lower between the upper, in order to dismember the prey, and the jaw muscles were enlarged but still arranged basically as in the sphenacodontid pelycosaurs (Kemp 1982). When writing about the mammal-like reptiles Kurtén (1971) stated:
Their dentition shows the beginning of a differentiation into front teeth or incisors for nipping and grasping, enlarged eye-teeth or canines for piercing, attack or defense, and expand cheek teeth for chewing. This suggests a need for rapid food utilisation, which makes us suspect the therapsids wee active beings, perhaps warm-blooded or nearly so. This is also suggested by the construction of their palates (roof to the mouth), which reroutes the air tubes around the mouth cavity, so that breathing can go on undisturbed by chewing. In the nasal cavity there may be turbinal bones, as in the mammals; covered with mucous tissue, they serve to clean, moisten and perhaps warm the air before it enters the lungs, as well as being used for smelling. All this may suggest metabolic processes at a much higher rate than in normal reptiles.
The plethora of thermoregulatory strategies seen among living and extinct animals have evolved within the constraints imposed on animals by their body sizes and the physical characteristics of their environments (Spotila 1980). When talking about the Dicynodonts, King (1990) considered that these forms conserved heat by having a compact body and short tail (to minimize the surface area to volume ratio) and possibly by having insulation.
Large body size insures constancy of body temperature and favors selection for a low metabolic rate. Small size demands a high metabolic rate if there is selection for an elevated body temperature. (Spotila 1980)
When looking at the cellular component, enzymes and cellular components play a vital role in the evolution of endothermy.
All the enzymes of the body can be arranged to work at their optimal levels throughout the life of the animal. This means that complex multi-enzyme systems can evolve and function, because of the action of each individual enzyme within the system is constant, and therefore predictable. If the enzymes did not behave in this way, then the probability of a complex system malfunctioning would be high. It would not be exaggerating to claim that the constancy of body temperature is the sine qua non of the highly complex biological organisation that characterises the mammals compared to the reptiles of today. (Kemp 1982)
The rise in metabolic rate seems to involve little more than an increase in the number of mitochondria in the cells, and presumably minor alternations to the pattern of hormonal control of metabolism (Kemp 1982). When endothermy was finally formed in some organisms it had to be regulated. Neither alterations in the rate of metabolic heat output during differing levels of activity, nor variations in ambient temperature are allowed to cause a change in body temperature (Kemp 1982).
Along with size constraints, hair or fur was another mechanism evolved to deal with heat retention. This was geared, not to sudden minute-to-minute fluctuations, but to the major seasonal changes, a generally heavier coat growing as winter approached and then a lighter one replacing it in the spring (Morris 1965). Hair also played another role for the early mammal-like reptiles. Hair may protect the skin from the sun’s rays or from freezing wind, slowing the escape of watery sweat in the desert or keeping aquatic mammals dry as they dive (Macdonald 1995).
Mechanisms of taking care of the hair soon had to evolve, Morris (1965) wrote:
It seems likely, therefore, that the early reptilian forms which gave rise to the mammals must still have retained, from their amphibian ancestors, some sort of skin secretions and that these were modified and perfected in new roles as sebaceous or sweat glands.
These sweat glands secreted sweat, which, when evaporated cooled the animal down. A mechanism was also created to enable the hair or fur to be raised from the skin, allowing air to reach the skin, thus, cooling the animal down when needed. The sebaceous glands were, according to Morris (1965), associated with each hair that secreted an oily substance that lubricated the fur and waterproofed it. The heat retention properties of the insulating coat of hair, combined with this heat reduction system, gave the early mammals the great advantage of a high, constant body temperature.
The increasing and stabilizing temperature inside the organisms body needed to be compensated.
Thus, hair, sweat glands and specialised skin blood vessels must evolve. More indirectly, but equally important in the functioning of endothermy are several other aspects of the biology of mammals. The locomotory apparatus must become capable of carrying the animal about in search of its some tenfold increase in food requirements. The feeding apparatus has to ingest at this greater rate, and also assist in the breakdown of the food, a process which would be far too slow if left solely to the intestinal processes. The diaphragm is needed for the greater rate of external gas exchange that occurs. The potential increase in water loss that would result from the higher temperature and greater breathing rate must be combated by the kidney, and finally the sense organs and central nervous system must be designed to organise and control all these activities. (Kemp 1982)
This problem with the increase in water loss was the second greatest terrestrial problem faced by the early mammal-like reptiles. The kidneys play one of the most important roles in the conservation of water in the body. According to Kemp (1982) the kidney is more elaborate in mammals than in any other vertebrate. Kemp (1982) also stated:
The blood pressure in the renal artery supplying the kidneys is high and the number of kidney tubules is large. The first point about the mammalian kidney, therefore, is that there is a very high ultrafiltration rate of the blood. The second point is the very long loop of Henle, which is associated with the production of a concentrated, hypertonic urine, the main means of water conservation. The third point of importance is that by producing hypertonic urine, sufficient water is conserved that the animal can afford to excrete liquid.
Because of the modifications made to the urogenital system, the early mammal-like reptiles were able to sweat and excrete urine without dehydrating. Environmentally, the regulation mechanisms free the animal from dependency upon excessive external water supplies or specialised diets (Kemp 1982). So without having to remain by water all the time the early mammal-like reptiles were allowed to take up other niches in the terrestrial fauna that the larger reptiles couldn’t (Macdonald 1995).
Now the mammal-like reptiles were not constrained by diet, water, or temperature.
All this led to a more active, alert and intelligent animal. But an animal equipped in this way had to have more efficient limbs to carry out its improved actions. The reptilian legs, sticking out on either side of the body, were no longer good enough. They had to be pulled round and tucked under the body supporting its weight more easily while propelling the animal forward. (Morris 1965)
By analogy with both temperature and water problems, the mammals can be regarded as having a locomotory system capable of operating in a wide variety of conditions, thereby extending the range of environments within which they live (Kemp 1962). New locomotion habits – evolved either in connection with avoiding predators or obtaining food – also influenced the structure of modern mammals in a number of ways, but principally the limb design (Morris 1965). When writing about therapsids, or early mammal-like reptiles, Colbert (1980) wrote:
The legs were generally “pulled in” beneath the body, with the elbows pointed more or less backward and the knees forward. The body was thus raised from the ground, so that the efficiency of locomotion was increased.
In comparing the physiology of the appendages of the lizards (reptiles) to the therapsids Bakker (1971) stated:
The critical biomechanical features show that in therapsids the humeral, and probably the femoral backswing were depressed no more than in living sprawlers. The GLA in even the most advanced therapsids was very critical as in lizards. In no known therapsids had the phylogenetic rotation of the GLA reached the point seen in semierect tetrapods where the GLA slants up and backward.
He also wrote:
Therapsids do show some peculiar specializations within the context of the Sprawling Gait which may be related to the origin of mammalian homeothermy. In typical sprawlers Humerus Long Axis Rotation is powered by the subcoracoscapularis muscles pulling up and forward on the posterior corner of the humerus, and pectoralis pulling backwards on the deltopectoral crest. (Bakker 1971)
The forelimb takes up a sprawling position so that the feet are far apart, thus producing a wide trackway (King 1990). This helped the animal easily maneuver in the sometimes-dense terrestrial landscape. No muscles are positioned to produce a powerful locomotory force at the forelimb but strong adducting and elevating muscles are present (King 1990). Since the forelimb was moved under the body, it had to be stabilized in some way. The muscles responsible for these actions are the pectoralis, scapular deltoid, scapulo-humeralis anterior and coraco-branchialis, and they would have helped to prevent dislocation of the forelimb and make the shoulder joint stable and strong (King 1990). Not only did the limb structure change, but the pelvic region changed as well.
The structure of mammalian limb girdles is, for the most part, rather distinctive and uniform throughout the class. This is particularly true of the pelvis; the ilium is rod-like and projects anterodorsally, whereas the pubis is more or less reduced to a narrow bar around the anterior margin of the large obturator fenestra. In contrast is the typical early synapsid condition in which the ilium projects directly upward, the pubis projects anteroventrally, and no obturator fenestra is present. (Crompton and Jenkins 1973)
One important note as stated by Colbert (1980), is that in the early mammal-like reptiles the pelvic elements are separate, however, in the mammals, the pelvic elements are fused. And one of the contrasting characters of reptiles and mammals is that in reptiles there is a generally small ilium and in the mammal the ilium is extended forward (Colbert 1980).
Among modern vertebrate animals, erect posture and gait occur only in endotherms – mammals and birds. Conversely, all ectotherms are sprawlers and incapable of maintaining a true upright posture. But, as some critics have correctly pointed out, no cause-and-effect relationship between posture and physiology has been established. However, the correlation between posture and endothermy or ectothermy is virtually absolute and surely is not merely coincidental. It may be that a metabolic regime subject to ectothermic temperature regulation imposes a major physiological obstacle that makes erect posture and locomotion impossible. We do not know whether erect posture could be achieved by an ectotherm, or whether the externally affected physiology of an ectotherm is so unstable that evolution from the primitive sprawling condition to an erect carriage simply could not take place. (Ostrom 1980)
Because of the ability to walk or run faster the early mammal-like reptiles were able to catch food, and escape predators. When they caught their food they were able to chew it into smaller pieces with their modified “molariform” (Clemens 1970) teeth. Which in turn allowed for quicker digestion, thus, getting energy faster for the increasing metabolism, which helped keep their internal temperature constant. Kemp (1982) stated it best when he wrote; no single characteristic could evolve very far towards the mammalian condition unless it was accompanied by appropriate progression of all the other characteristics. Finally, it should be noted that an important gap – both morphological and temporal – still exits in our coverage of the reptile-to-mammal transition (Crompton and Jenkins 1973).
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