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Re: [Re: 3 1/2 vs. 4 chambered hearts]
As a follow up to this, the Journal of Experimental Biology did a wonderful
anatomical study of the crocodylian heart and how it works when breathing
normally and when diving.
Judging from all the derivations required to achieve a 3 chambered mode while
underwater, I'd say the evidence seems to clearly point to a primitive
crocodilian condition of 4 chambers.
The ref is:
Axelsson, M., Craig, F.E., LÖFman, C.O., Nilsson, S. and Grigg, G.C. (1996).
Dynamic anatomical study of cardiac shunting in crocodiles using
high-resolution angioscopy. J. exp. Biol. 199, 359-365
The abstract can be read online at:
And the full version (for those with Adobe Acrobat reader) can be read in PDF
"Matthew Bonnan" <email@example.com> wrote:
> 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
> >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? =)
> Matt Bonnan
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