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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 3
1/2 chambered hearts (including crocodyliforms) and that true four-chambered
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 other
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 archosaurs.

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? =)

Matt Bonnan
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