Here is the
continuation of my account of arguments for
warm-bloodedness in the whole of the Dinosauria.
2. The second main
argument is that dinosaurs are thought to have had four-chambered hearts, which may be coupled with warm-bloodedness. Both
mammals and birds have four-chambered hearts; crocodiles are the only
cold-blooded animal group that shares this feature (as far as I am aware!).
We can be fairly
confident that dinosaurs had four-chambered hearts because both their closest
modern relatives, birds and crocodiles, possess this feature. Modern crocodiles
represent the closest living thing to the shared ancestor between dinosaurs and
crocodiles, and birds represent what the dinosaurs became at the end, sort of.
The crocodiles suggest that the ancestral dinosaurs also had four-chambered
hearts, and the birds show that they were not lost during evolution (at least
not fully). This type of reasoning is common practice among paleontologists and
evolutionary biologists, and is known as the Extant Phylogenetic Bracket (EPB) principle. Of course, it is far
from certain, and there is no fossil evidence to support this. If memory
serves, the only dinosaur heart ever found was later shown to have been an
ordinary rock, deceptively shaped to be mistaken for a heart. But, let us at
least consider the implications for warm-bloodedness if the dinosaurs had had four-chambered hearts.
A four-chambered
heart is basically divided into two halves with two special chambers, an atrium and a ventricle, on each side. Blood enters the heart via the atrium, and
leaves through the ventricle. When the atrium muscles relax, the chamber
expands and blood is sucked in (just like with air in the lungs, see Part 1).
Next, when the ventricle muscles relax and the atrium muscles contract, blood
is forced from the atrium into the ventricle. Finally, the ventricle muscles
contract, forcing the blood out from the heart; at the same time, the atrium
muscles relax, sucking in new blood into the heart. That way, the two chambers
create a smooth flow of blood in and out of the heart.
If I am not
mistaken, this process is basically the same in two- and three-chambered hearts
as well, but the four-chambered heart is special in that it enables two separate blood circuits, with different blood pressures! This is
essential for an active animal, which needs a high pressure levels for the
blood to reach all parts of the body, and deliver oxygen and energy-storing
molecules (e.g. ATP; see the post on the meaning of warm-bloodedness). But,
the blood vessels in the lungs are very narrow and thin, a design to maximise
their ability to take up oxygen and give off carbon dioxide to the air inside
the lungs; these would explode if they were subjected to as high pressure as is
present in the other blood vessels of a warm-blooded animal.
In other words,
it is impossible to maintain a high enough blood pressure for efficient
transport of oxygenated blood in the body without destroying the blood vessels
in the lungs, which is where the oxygen is replenished. If all blood vessels in
the body are connected in the same circuit, the blood pressure would be limited
to what the lungs can handle without bursting.
A four-chambered
heart makes it possible to keep the blood circuit from the heart to the body
separate from the circuit from the heart to the lungs. We refer to them as the systemic circulation (heart-body) and pulmonary circulation (heart-lung).
Because the atria and ventricles are separated from one another by valves, it is possible to change the
blood pressure between incoming and outgoing blood.
So, if blood
coming from the body enters the right atrium with high pressure, when it is
pumped into the right ventricle, the right valve closes it off from the behind
it. The blood is then pumped out from the right ventricle, with more gentle pressure,
toward the lungs. The blood then goes from the lungs to the left atrium, and
then the left ventricle, from where it is pushed out to the rest of the body
with high pressure, until it eventually returns to the right atrium.
Thus, having a
four-chambered heart makes it much easier to be an active animal, which we from
previous arguments know is intimately linked with warm-bloodedness. I dare not
claim that a four-chambered heart is strictly necessary for a warm-blooded animal, but as with the parasagittal
limb posture, it would be highly advantageous.
However, a strong counter-argument to the link
between the four-chambered heart and warm-bloodedness is that very large animals would also need high
blood pressure in their systemic circulation for the blood to reach out, especially to its head. And many
dinosaurs were very very big! In fact, it is the smaller dinosaurs that are
most certain to have evolved warm-bloodedness.
Since the use of
a four-chambered heart can be explained by the size of the dinosaurs, that
feature is rendered weak as evidence for warm-bloodedness. We know the dinosaurs were big, but whether
they were warm-blooded or not is more doubtful, so the most rational thing to do is to assume that the four-chambered
heart was needed/used primarily because they were large animals.
Moreover, the
fact that the cold-blooded crocodiles also have four-chambered hearts casts
further doubt on the link to warm-bloodedness. I am not sure of why crocodiles
would need this, but I can imagine that they were at least not disadvantaged.
As you can see, there is yet more mystery here…
3. The final piece
of evidence for warm-bloodedness is
about special features in the microstructure of dinosaur bones: it has been discovered
that some had dense secondary Haversian
bone, which is a product of bone growth (or, bone remodelling, actually). A
lot of it indicates fast bone growth, and fast growth is characteristic of
mammals and, especially, birds. Dense secondary Haversian bone has been found
in fossil dinosaurs, pterosaurs and therapsids (mammal ancestors), all good
candidates for warm-bloodedness.
I have little
understanding for any of this, but apparently, from what I can gather from
various sources, it has later been shown that production of dense secondary
Haversioan bone is not as strongly linked to BMRs as previously thought, but is
more affected by age and size, among other factors. It seems, then, that the
relationship between this bone feature and warm-bloodedness is rather weak.
Moreover, it seems to be absent in small modern mammals and birds, which are
known to have the highest relative BMRs of all.
I hope you begin
to see a common pattern in the weaknesses of these arguments: they are based on
correlations that do not seem to hold all the way; there are exceptions, and
alternative explanations. The challenges should be met with special
explanations for the exceptions (if they are different from the rest, explain
why they are special, rather than abandoning the whole idea), or show that the alternative
explanations do not work, if that is the case. So, this is not the end!
Although these hypotheses have their weaknesses, they can all be strengthened,
and the counter-arguments can be challenged too!
Part 3 will be
about a few arguments against warm-bloodedness in the whole dinosaur group – well… except those closest to birds.
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