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Re: feathers, fibers, endothermy...
Going over Miller's post I noted some misunderstandings that ought to be
cleared up. I have never asserted that kiwis lack respiratory turbinates.
They have the usual avian anterior and posterior set. What is important about
kiwis is that the anterior turbinate is simple, very long, and sits inside
the very long and extremely slender nasal passage that runs through the
straw-like beak. How slender? I got some live weights for kiwis at the
National Zoo, and measured some of the same (now dead) skulls at the
Smithsonian (this is superior to weighing pickled carcasses, because birds
often waste away before croaking according to the bird curator and the NZ).
The anterior nasal passage plots way below the reptile line. Ergo, it is
possible to have an RT in a very narrow nasal passage. Kiwis also have more
complex RT in a broader part of the nasal passage just before the internal
nares. Even this part of the passage is narrower than in birds in general,
albeit above the reptile line. Moa nasal passage cross-section was also
intermediate to reptiles and most birds (because their heads are extremely
small relative to the body even by ratite standards). What no one knows is
what minimal RT will serve for an animal with a resting metabolic rate in the
lowest avian range. Also, the posterior part of the nasal passage in most
theropods was potentially about as broad as in birds.
As for feathers, the whiskers of kiwis are about the same in size and shape
as cat whiskers. I could see no sign of any barbs along the mainshaft, could
not tell whether there were any at the shaft base. As far as I know, it's not
possible to tell if there are barbs at the base of Sinosauropteryx and
Protoarchaeopteryx bristles, because they are too dense. We may have to turn
to the Mononykus fibers to tell about that.
Enantiornithines are interesting for a number of regards. That they were well
insulated means that they were dependent mainly upon internal heat, so the
notion that they were ectotherms or near ectotherms must be rejected. Those
suckers were putting out lots of heat, perhaps as much as kiwis which have
the lowest resting MR yet measured among birds (about half the avian norm).
Obviously you do not need to have a really big sternum connected to the ribs
by ossified sternal ribs to be an avian endotherm of some sort. What is
interesting is that Late Cretaceous enantiornithines such as Neuquenornis
(JVP 14:230) have large, keeled sterna and ossified sternal ribs, so they
were probably running max aerobic exercise levels as high as modern birds.
Yet their bone histology has been considered to be semi-reptilian in type.
However, the thin walled bones did not retain the juvenile growth zones,
which was lost via reabsorption. The birds may have grown fairly rapidly like
kiwis, if not faster. The slow growth bone may represent slow and
indeterminate adult growth. This indicates that adult growth differed from
modern birds, but does not tell us much about metabolism because some
kangaroos grow throughout their lives, mate.