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Re: SPECULATION: pterosaur extinction versus bird survival



At 10:26 AM 06/02/01 -0600, Demetrios Vital wrote:
The breathing abilities of the animals at the end of the Cretaceous is, in
my mind, a more than major explanation for the extinctions of the respective
families.  Non-avian-dinosaurs and pterosaurs had less efficient breathing
systems than birds.

I do not see this as being much of an explanation. For one thing, there is far more to the efficiency of a breathing system than its gross physical structure; such things as the binding ability of hemoglobin in the blood, the extent of vascularization of the respiratory tissues and other physiological and cellular-level mechanisms have a great deal to do with the matter. I would suggest that we simply have no idea what the efficiency level of an extinct creature's respiratory system would have been; we can make some guesses about the efficiency of breathing, but that is not the same thing at all as judging the efficiency of respiration. There may have been all sorts of compensatory physiological mechanisms operating in species that did not have the physical structure of bird respiratory systems.


In addition, even if we could draw conclusions about the efficiency of oxygen respiration in air, that does not tell us anything about whether the creatures involved had a high tolerance for anoxia, as many living turtles do. For example, was there difference in this area among the various groups of marine reptiles? I don't see how we could tell, beyond noting that the presence of the shell in living turtles has given them some advantage because it can be a source of supply for buffering compounds to deal with lactic acid. However, this ability varies considerably even among turtles (and since only a few species have been studied, we do not know how widespread it is even among living species).

In short, without the physiological information we need we can only guess at how effective Mesozoic reptiles were at gas exchange, or at dealing with oxygen stress, and therefore -- since we cannot establish the differences among different forms with any accuracy -- this is useless as an explanation for extinction. At the very least, before you can claim that a difference among taxa explains why one group survived and another did not, you have to be able to prove to that difference really exists.


--
Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
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