Download The Evolution of Mammalness

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has been superseded by a new articulation between the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 mammallike 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
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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
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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
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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.
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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,
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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
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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)
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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)
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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
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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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Clemens, William A.
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Colbert, Edwin H.
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Crompton, A. W.
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