Re: METABOLISM

["Ralph Miller III" <gbabcock@best.com>]

Matt Troutman writes:
> G. Paul (frequent posts on the list and an SVP abstract) has argued that 
> in Apteryx and many seabirds that the nasal passage proper and the 
> anterior vestibular region is narrow as in ectotherms and dinosaurs 
> (further discussed in Ruben et al. 1996) and still contains middle 
> turbinates and the anterior set.  It is also claimed that the region of 
> the nasal cavity proper is large enough to house turbinates.  
> 
> The faults with this correlation is that it ignores the specializations 
> of kiwis and seabirds.  Seabirds (most notably the tube-nosed seabirds 
> of the Procellariiformes) have small nasal passages and tiny turbinates. 

> However, the reason for this is not applicable to dinosaurs; tube-nosed 
> seabirds are predominantly aquatic and use their specialized bills to 
> capture food and reduce drag in seawater.  A large rostrum with 
> typically sized turbinates and nasal passages would be disadvantageous 
> because seawater would be more easily breathed in.  Frontal salt glands 
> are present that allow the birds to drink seawater; this may be an 
> adaptation to both the reduced turbinates and to get seawater.  Hence, 
> no water is lost and the metabolism of these birds is not effected.  

Quoting _The Mistaken Extinction: Dinosaur Evolution and the Origin of
Birds_ (1998) by Lowell Dingus and Timothy Rowe, pp. 188-189:

"The maniraptoran lineage is distinguished from other tetanurines by its
skull, arms, and tail.  The skull was simplified by the loss of a bone
called the prefrontal, which was previously situated above and in front of
the eyeball, along the rim of the eye socket.  This seemingly obscure
anatomical detail is significant because it may signal the presence of a
large salt gland or nasal gland above the eyeball.  This structure is
unique to birds among living species.  It helps maintain a balanced
concentration of blood salts preventing dehydration and death from a
prolonged or extreme imbalance.  In tetrapods, the kidneys also play a role
in this.  The kidneys of warm-blooded mammals and birds are enhanced,
probably to compensate for rapid salt buildups driven by their higher body
temperature and faster moisture loss.  In mammals, sweat glands help the
kidneys excrete excess salts.  But in birds, there are no sweat glands. 
Instead, the kidneys are augmented by a salt gland, which removes and
excretes salt from the blood in very high concentrations.  The salt gland
permits many birds to live in the arid deserts of the American Southwest
and Mexico, where they survive dehydration without developing poisonous
levels of blood salts.  Marine birds survive by drinking sea water because
they are able to excrete excess salt through the nasal gland.  It is this
gland that enables birds to exploit many of the diverse habitats that they
occupy today.  The subtle loss of the prefrontal bone in maniraptorans may
signal an important evolutionary step in avian physiology."

Perhaps Ruben et al. should examine recently recovered _Velociraptor_
material and report their findings.

-- Ralph Miller III     gbabcock@best.com