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Re: sauropod lung collapse



The main point is that the inability to use the neck as a snorkle in no way excludes semi-aquatic lifestyle from the list of plausible sauropod lifestyle choices. It is known that relative to metabolism, dive-time scales positively to mass. If a 1 ton croc can hold it's breath for 30-45 minutes, then a large sauropod might plausibly have gone much longer than an hour on stored O2...

Well, all of the other problems with the idea of aquatic sauropods aside for the moment, that still means that the animal must exit the water entirely every time it wants to breathe, which is vastly different from a whale, seal, or other aquatic air-breather allowing its lungs to collapse at depth, then allowing re-expansion upon surfacing and taking a breath at the surface. Not only is this not feasible in and of itself, but the animals would have to consistently find very deep pools with conveniently sloped sides. Unless water-filled craters were common, it seems unlikely that a sauropod coming from say, a flood plain away from the coast, would find bodies of water 80 feet deep all over the place.


Also, their ability to store and conserve O2 may have allowed them to deflate their lungs prior to submerging, reducing bouyancy to the point that walking on the bottom was practical.

Collapsing the lungs first (I'm not sure how this would be accomplished) would leave a very small amount of available oxygen. Remember, when the lungs of an aquatic animal collapse, the gas hasn't just disappeared, it's mostly just taking up far less volume. Yes, blood O2 storage is important and some of the gas under pressure does leave the lungs and get stored in the blood instead in derived aquatic species. Just the same, the collapse of the lungs is a result of reduced gas volume (from increased exterior pressure), not muscular action, and the gas stored in the lungs is still important.


During deep diving, the lungs of seals and sea lions are totally
collapsed.  Which is normal.  They evolved that way.

Yes, but the lungs are not collapsed when the animal is trying to breath. Breathing is accomplished at the surface, without a long 'snorkle' so the gas takes up standard volume.


One last question:  Do penguin lungs collapse during dives?  What role
does their air sacs play during diving?

The lungs will decrease in volume some, but they are very dense and spongy, unlike the lungs of a mammal or lepidosaurian. The air sacs will collapse under pressure, which helps to reduce the density of the diving penguin, and thus helps it stay down. The same goes for air trapped in the feathers. I do not know how inflated the air sacs are before descent, but there's a paper by Sato et al. from the last year or two that looked at air intake modulation prior to dives in penguins.


    A decent test may be to study the morphology of the ribs for
continuous
submersers like aquatic species like sirenians and dolphins versus
those of
temporary submersers like hippos and crocs, and rare submersers like
horses,
etc., and determine the variabilities. The presence of dense bone to
counteract
bouyancy, rounded ribs to counter external pressure, a round
thoracic cavity
rather than "slab-sided" for equalization of pressure around the rib
case, etc.


Actually, the bones of sirenians and cetaceans are essentially polar opposites with regards to overall element density. Sirenians have thick, high-density bones. Cetaceans have bones that are extremely 'spongy' and porous. The overall density of the elements is very low in cetaceans. I'm not sure that a round thoracic cavity would actually equalize pressure, though it might equalize stress due to high pressure around the perimeter of the thorax (at least until you get to an animal of sufficient size such that pressure differences are high at the top and bottom of the body).


Of course, the problems with respiration are not the only problems with aquatic sauropods. In fact, there was never really any evidence for them being aquatic in the first place. The original spark that started that idea was that they were too large to be terrestrial. This wasn't stupid, but it was misinformed. With better knowledge of biomechanics (and a better knowledge of animals past and present), it becomes quite apparent that a sauropod would be able to support its weight on land. At that point, there is no further reason to model them as aquatic. The habitats they inhabited were often dry (or at least not full of 100 foot water pits), the limbs show terrestrial adaptations, the food availability underwater for herbivory would have been pretty minimal (compared to the amount required), and trackways indicated prolonged overland travel. Of course, I'm guessing these last bits are recognized by nearly everyone on the list, I suppose it's just the snorkle bit under debate.

Cheers,

--Mike Habib