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
1825 Shady Creek Court
Mississauga, Ontario, Canada L5L 3W2 mailto:ornstn@home.com