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I find Carlo's suggestion about some sauropod nostrils being placed
superiorly to facilitate cerebral oxygenation provocative, but I must
say I have some concerns with this hypothesis.
1) Oxygen flowing through the nostrils would have to get to the brain.
I do not know the distance between the nostrils and the brain case, but
oxygen does not diffuse well through bone, and directly oxygenation of
the brain this way seems very unlikely to me.
2) For nasal respiration to occur, oxygen would have to diffuse through
specialized respiratory epithelium in the nose. This would have to be
very thin, very moist, and very vascular. Human alveoli are less than
0.2 microns in diameter; frogs (which can get about half their needed
gas exchange through their highly vascularized skin) have an epidermis
only 5 to 8 cell layers thick. The problem for sauropod nostrils would
be drying out. Oxygen is only utilized by biological forms when
dissolved. Lung alveoli are coated with a small layer of water, and are
kept warm (warm air holds more moisture than cold air) and placed
internally to prevent evaporation. Frogs are either restricted to moist
microhabitats or have behavioral adaptaions (burrowing underground) to
avoid dessication. Nostrils on the top of the head with vascular, moist
epithilium would be at great risk for dehydration.
3) Oxygen which dissolved in sauropod nostril capillaries would likely
then flow into the venous system of the head and then to the heart, and
not directly to the cerebral arteries and brain.
My own best guess is in agreement with Dr. Holtz, that the nostrils were
placed posteriorly to get out of the way of foliage so these beasts
could eat almost continuously.--Ken Clay, M.D.