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Re: 3 1/2 vs. 4 chambered hearts
Ken Kinman writes:
But what most interests me right now is the evolution of the chambers
of the heart. Is it not possible that ALL reptiles except Ornithodira had
1/2 chambered hearts (including crocodyliforms) and that true
hearts (other than the separate evolution in the mammalian lineage) only
evolved in Ornithodirans----perhaps all ornithodirans (including
pterodactyls?), or just the dinosaur-bird clade, or possibly even just
certain branches of dinosaurs (including those that gave rise to birds).
Is there any reason at all to assume that all archosauromorphs had
four-chambered hearts, and crocodyliformes reverted back to a 3 1/2
chambered heart? This doesn't seem very parisimonious to me, unless I am
unaware of evidence that would make it more parsimonious. A four-chambered
heart in birds, plus all (or some) dinosaurs makes more sense, and all
archosauromorphs with some version of 3 1/2 hearts.
Well, crocodiles do have a four-chambered heart. They have two seperate
atria (the top chambers) and two seperate ventricles (the bottom chambers).
Unlike the condition in mammals or birds where there is one large artery
called the aorta from which all the subsequent blood vessels begin to
diverge, crocodilians retain two aortae with a shunt in between that can be
closed or opened. It is the right aorta that contains oxygen rich blood and
sends it (via many blood vessels of course) to the head and brain. The left
aorta (from recollection, will check facts tomorrow and post an update)
contains a mixture of oxygen-rich and oxygen poor blood. The shunt between
the aortae opens or closes depending on the activity of the animal and how
much oxygen needs to be conserved.
Interestingly, in mammals, our aorta rises from the heart and dives
inferiorly (down) in a J-shape to the anatomical left, sending off three
major arteries toward the head -- the brachiocephalic, left common carotid,
and left subclavian. We see the reverse in birds: their single aorta rises
from the heart and dives inferiorly in a J-shape to the anatomical RIGHT.
As I stated above, it is the RIGHT aorta in a croc that sends the oxygen
rich blood to the brain. Therefore, we have a potential model system in the
croc of an ancestral four-chambered heart with two aortae that may have
given rise to a bird-style heart in ornithodirans in which the right aorta
became dominant. Apparently from these observations, we might guess that
synapsids (ancestors to mammals) had a similar (though not, perhaps,
identical) setup (two aortae), but their LEFT aorta delivered the
oxygen-rich blood the brain, not the right. Therefore, the LEFT aorta in
mammals became dominant.
So, crocs are probably not reverting to a 3 chambered heart (technically,
most "reptiles" have five chambers -- something for another post!) but
rather retaining a potentially primitive archosaurian character: a definite
four-chambered heart. Add to this their paternal instincts and their
ability to draw their forelimbs and hindlimbs out of a sprawl and underneath
their body, and again we have a nice model animal for the earlier
NOW BEFORE ANYONE JUMPS UP AND SAYS, "BUT, MATT, CROCS ARE DERIVED!" I say,
"Yes I know." This is very simplified: crocs have a very interesting,
unique, and cool evolutionary history of their own, and some of the
characters they have are definitely modified from the basal stock of
archosaur to be sure. In fact, for locomotion, much recent study and
evidence suggests early crocs had upright limbs that were later re-modified
for a sprawling gait: hence their ability to switch between a sprawl and
what is usually called the "high walk" with the legs drawn under the body.
So, there is a functional reason to suggest that the ancestor of both birds
and crocs (or ornithodirans and crurotarsi) had four-chambered hearts. And
another functional reason why crocs appear to be more closely related to
birds than to the other "reptiles."
Confused yet? =)
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