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Re: buncha die-hards ain't ya? (Dino heart)

Caleb wrote:
> Gang,
> The morphology indicates that it had a high level of activity and yet it
> isn't endothermic? Well, you can't exactly get a high level of activity from
> ectothermic (and I have debated this at length before, if you check the
> archives, and don't incredibly care to again). And I don't think this thing
> was big enough to register for homeothermy (correct me if I'm wrong, and what
> else could it be?). But, regardless, I have to say this: I told ya so.

Hi Caleb. Don't confuse function with metabolism. This is a sticky issue. As has already been alluded to previously during this discussion, the possession of a four-chambered heart can only tell us something functional: that high pressure blood needed to go toward the head and body, while low pressure blood had to be shunted toward the lungs. The typical reptilian heart with "three" chambers (one could actually claim it has five) has a single ventricle (that's the bottom portion of the heart for those unfamiliar) which is highly convoluted. It does a pretty decent job of separating oxygen-rich blood from oxygen-poor blood through a series of contractions that shunt the blood into various "pockets" where it will then have a strong chance of either flowing to the body and head or to the lungs. Because the ventricle is not completely separated, the pressure of the blood going to the head and body has to remain somewhat similar to the pressure going to the lungs. If the BP gets too high, the delicate capillaries in the lungs would burst, killing the animal.

If you position your head far above your heart, or you become bigger so that you need blood to reach far away regions of your body, having a fully divided heart separates the high pressure blood going to the head and body from the low pressure blood necessary not to burst the capillaries in the lungs.

While most of the animals that have this four-chambered system today are endothermic, there are ectotherms that have a four chambered heart. Thus, this is a functional problem and not a linch-pin arguement for endothermy. In fact, it has long been predicted that dinosaurs would have had a four-chambered heart because of the problems of blood transport described above.

This having been said, perhaps possession of a large, four-chambered heart MIGHT indicate a level of activity above that of typical terrestrial ectothermic animals (tuna and sea turtles being weird exceptions because of their environs). Also, endothermy simply means that the animal possesses aerobic skeletal musculature (as opposed to the relatively anaerobic muscles of most ectotherms) and can internally control its body temperature. Therefore, an animal could be an endotherm but have a relatively low core temperature.

One of the vexing problems of dinosaur physiology (besides the fact that we have no living sauropods, ornithopods, etc., to run on a treadmill) is that both living outgroups have no sweat glands. How does a naked-skinned sauropod manage to cool itself effectively without an integumentary covering or a large mouth from which to pant out heat? Sure, the long neck, tail, and legs could have served as cooling towers, but even here this might not be enough.

On the other hand, a large ectotherm with no internal mechanism for temp control and anaerobic muscles might find itself in a worse predicament. Without aerobic muscles, a large ectothermic sauropod would have had a hard time moving around without quickly exhausting itself. Futhermore, without an internal means of temp control, just where does a large ectothermic sauropod hide when it gets too hot?

Hence, this is why finding the first concrete evidence that at least some dinosaurs had a four-chambered hear in no way helps us resolve the physiological abilities of dinosaurs, but mostly supports the functional hypothesis that dinosaurs had four-chambered hearts because of a necessary division of blood pressure.

I end by saying that I think all dinosaurs were endothermic, but only in the limited sense I describe above. Having very little concrete evidence to go on, we are unfortunately stuck in a stale mate for now. Beyond that, who knows?

Matt Bonnan
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