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Re: Unidirectional [...] Science

>> Even whales and crocs cannot inhale with their lungs well below
>> the surface, and elasmosaurs probably had to bring their lungs
>> near the surface to inhale.
> These seem like solid assumptions, w/ the caveat that elasmosaurs
> might have had some variant of the avian-style system. I note in
> passing that penguins have air sacs, yet there is apparently little
> or no _skeletal_ evidence of that.

Keep in mind that the lung + air sac system of birds is as a whole homologous to the lungs of crocodiles. The penguins (and most other diving birds, including the Hesperornithes) have jettisoned the weight-saving parts of the air sac system (those that replace fat or bone), but not those that actually ventilate the lungs!

 A problem is, it seems to be more parsimonious to suppose
 sauropterygians they had the primitive cul-de-sac system, because
 they are more related to lepidosaurs than archosaurs as far as I
 know. David Marjanovic may tell better.

Well, no, I won't. :-) There is no fossil evidence for anything other than a standard sauropsid lung (...itself a fairly broad concept, but never mind...) in euryapsids. Phylogenetically, the last couple of analyses have found them as lepidosauromorphs, but all published analyses are too small for my taste, many or all don't order any characters, many or perhaps all contain correlated characters, and so on and so forth. In short, I'll try to figure this out as part of my postdoc.