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(long) Still more on respiratory turbinates



I just got back from the Smithsonian, where I had a chance to examine some
theropod and bird skulls in the search for those pesky respiratory turbinates
(RT). At SVP Ruben gave an interesting talk on RT in theropods, but some
things bothered me by the time I got home. At this time (have not seen his in
press paper yet), his argument that theropods probably lacked RT appears to
be difficult to sustain.

It should be understood that in birds RT are usually made of and set entirely
on cartilage, and leave no bony trace of their existence. The form and
placement of avian RT is also extremely variable. So is size and shape of the
nasal passage that contain RT in birds. Further, a significant number of
terrestrial mammals and birds have small but apparently effective RT. In the
end, the only way to disprove the presence of effective RT in an extinct
tetrapod is if the nasal cavity is too extremely small to contain them, or
entirely closed off. The only way to prove their existence is by the finding
of ossified examples.

>From what I recall of Ruben's SVP talk, he asserted

 that Dromaeosaurus had a short, vertical nasal passage that could not
contain RT. Problem

, the skull is incomplete, and does not include the nasal passage. What is
preserved of the snout is similar to most other theropods. In most theropods
the anterior nasal passage is a long, rather narrow tube set above the
maxillary sinus complex. The posterior passage (excluding the olfactory
region) then arcs strongly downwards and expands laterally before it exits
through the internal nares. 

Ruben seems to be concentrating on the narrow anterior nasal passage.
However, in some birds (particularly of prey) the only RT is set in the
posterior nasal passage, directly above the internal nares. There is room for
well developed RT in the posterior nasal passage in theropods. A simple
scroll can also fit into the anterior passage without hindering airflow, but
this is not critical.

Another problem. From what I have seen, Ruben is not adequately taking into
account the problems of comparing small headed birds to big headed theropods.
In most theropods, and crocodilians also, the nasal passages appear small
because the head is so big. In ratites, the nasal passages appear to be large
because the head is so small. However, the nasal passages of big headed birds
(shoebill storks, phororachids, diatrymids) are vertical, and very short and
small compared to the skull. What needs to be compared is the size of the
nasal passage with body size equalized. The over all nasal passage of a
crocodile is larger than that of an ostrich of similar size, and the diameter
of the passages is similar in the two forms. The somewhat crushed juv. T rex
skull (=Nanotyrannus), the ostrich-mimic Gallimimus, and an elephant bird all
weighed about half a tonne. The last two had small heads, and the over all
volume of the nasal passage is roughly similar (the passage is narrower but
longer than in the big bird). Although the nasal passage of the baby
tyrannosaur looks small, in fact it was many times larger than that of the
giant bird!

So far I have not been able to find any dinosaur that clearly did not have
room somewhere in its main nasal passage for at least small but effective RT
that left no bony trace of its existence. I am NOT saying that any dinosaur
 did have RT, only that they cannot be ruled out now, and probably never can.
It is therefore improbable that RT can be used to falsify or support any
metabolic rate in dinosaurs. The Rosetta stone was important because it was a
big rock, and therefore preserved. Positive identification or rejection of
delicate, often cartilage RT is so difficult, and probably impossible in many
cases, that they do not meet the criterion of a Rosetta stone.

I also find it notable that in the Science article Nagy, one of the
preeminent herp field biologists, appears to express doubts about the
significance of RT. This brings us to the interesting subject of Emperor
penquins. At the Smithsonian I confirmed that they have closed nares, so they
cannot use RT to save water. The big birds spend up to four months each year
out of the reach of liquid water. Metabolic rates are suppressed during the
fasting period, but energy budgets are still well above reptilian levels.
Eating snow costs energy. I am not aware of studies showing whether and how
these penguins conserve water. 

On a related subject, Ruben suggested that the narrow anterior nasal passages
of theropods are not compatible with high rates of ventilation. Along similar
lines, Hengst in the DinoFest volume asserted that diplodocid sauropods had
nasal passages no larger than those of horses. However, the passages in the
biggest land mammal, the elephant, are shockingly small, being only about 1.5
inches in diameter! Even the nasal passages of diplodocid sauropods were
about as large as those of mammoths. (The extremely narrow airways of
elephants and sauropods probably minimize the dead airspace of the long
airways.) The anterior nasal passage of an adult T rex was much larger than
that of an elephant. 

I believe that Hengst gave a talk at GSA on respiration in sauropods, does
any one have any info on the presentation?

Rowe, please forward to JR. 

GSPAul