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mammals are? We can never tell from the fossils.
And did those species which best developed hairiness and
sweatiness become what we call mammals and did they survive
where the less advanced other therapsids did not?
Let's look in another direction. In reptiles, the nostrils open
into the mouth just behind the teeth. This means that reptiles can
breathe with their mouths closed and empty. When the mouth
is full, breathing stops. In the case of the cold-blooded reptiles,
not much harm is done. The reptilian need for oxygen is relatively
low and if the supply is cut off temporarily during eating, so what?
Mammals, however, have to maintain a high metabolic rate at
all times if they are to be warm-blooded, and that means that the
oxidation of foodstuffs (from which heat is obtained) must con-
tinue steadily. The oxygen supply must not be cut off for more
than a couple of minutes at any time. This is made possible by
the fact that mammals have a palate, a roof to the mouth. When
they breathe, air is led above the mouth to the throat. It is only
154 THE PROBLEM OF THE PLATYPUS
when they are actually in the act of swallowing that the breath is
cut off and this is a matter of a couple of seconds only.
It is interesting, then, that a number of late therapsid species
had developed a palate. This might be taken as a pretty good in-
dication that they were warm-blooded.
It would seem then that if we could see therapsids in their liv-
ing state and not as a handful of stony bones, we would see hairy,
sweaty creatures that we might easily mistake for mammals. We
might then wonder which hairy, sweaty creatures were reptiles
and which were mammals. How would we draw the line?
Nowadays, it might seem, the problem is not a crucial one. All
the hairy warm-blooded creatures in existence are called mam-
mals. And yet, are we justified in doing so?
In the case of the placentals and the marsupials, we are surely
justified. They developed their placentas and their pouches about
eighty million years ago, after the mammals had already existed
for some hundred million years. The early mammals must have
been egg-layers and so, therefore, must have been their therapsid
forebears. If we want to look for the boundary line between
therapsids and mammals, we must therefore look among the hairy
egg-layers.
As it happens, there are still six species of such hairy egg-layers
alive today, existing only in Australia, Tasmania, and New
Guinea, islands that split off from Asia before the more efficient
placental mammals developed, so that the egg-layers were spared
what would otherwise have been a fatal competition. The egg-
layers were first discovered in 1792 and for a while biologists found
it hard to believe they could really exist. It took a long time be-
fore they got over suspecting a hoax hairy creatures that laid eggs
seemed a contradition in terms.
The best known of the egg-layers is the "duckbill platypus" (the
last part of the name means "flat-foot" and the first part refers to
the horny sheath on its nose that looks like a duck's bill). It is
also called "Omithorhynchus" from Greek words meaning "bird
beak."
HOLES IN THE HEAD 155
These egg-layers have hair, of course, perfectly good hair, but
so (very likely) had at least some therapsids. The egg-layers also
produce milk, although their mammary glands have no nipples
and the young must lick the hair where the milk oozes out. How-
ever, some therapsid species might also have produced milk in
that fashion. We can't tell from the bones.
In some respects, the egg-layers lean strongly toward the side
of the reptiles. Their body temperature is much less perfectly con-
trolled than that of other mammals and some of them possess
venom. The platypus, for instance, has a homy spur at each ankle
which secretes venom; and though a number of reptiles are
venomous, no mammals (other than the egg-layers) are.
Then, too, because they are egg-layers, they have a single ab-
dominal opening, a "cloaca," which serves as a common passage-
way for urine, feces, eggs, and sperm. All living birds and reptiles
(also egg-layers) possess cloacae, but no mammals, other than
those few egg-layers, do. For this reason, the egg-layers are called
"monotremes" ("one-hole").
To most zoologists, the hair and the milk spell mammal un-
mistakably, but the eggs, the cloaca, and the venom are sufficiently
reptilian so that the egg-layers are placed in a subclass "Proto-
theria" ("first beasts") while all other mammals, marsupials and
placentals alike, are in the subclass "Theria" ("beasts").
The question arises, though: Are the monotremes really the
first of the mammals, or are they rather the last of the therapsids?
Are they really reptiles that have the outer appearance of mam-
mals, as did, perhaps, a number of late therapsid species; or are
they mammals that have retained some reptilian characteristics?
This may sound like a purely semantic matter, but zoologists
must make decisions in such matters and, if possible, come to
agreement over it.
An American zoologist, Giles T. Maclntyre, has recently en-
tered the fray, using skeletal characteristics as the criterion. (We
have only the skeleton as direct evidence in the therapsid case.)
He has concentrated on the region near the ear, where some of
the reptilian jawbones became mammalian ear bones and where
156 THE PROBLEM OF THE PLATYPUS
you might expect some useful distinction between the two classes.
There is a "trigeminal nerve" which leads from the jaw
muscles to the brain. In all reptiles, without exception, it passes
through a little hole in the skull that lies between two particular [ Pobierz całość w formacie PDF ]

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