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Respiratory turbinates - again
Having at last managed to get a copy of the Ruben RT paper from Science, I
have a few comments on some aspects of the study and its conclusions.
The first comment, which I don't think anyone esle has noted yet (?) relates
to the measurement of nasal cavity cross sectional area. As Reuben et al
point out in their description of the function of RTs, they serve to
increase the surface AREA of the nasal passage. This increase is related not
only to the cross-sectional area of the RTs as measured by Ruben, but also,
and possibly more importantly, to the length of the RTs, which are
orientated with their long axis parallel to the length of the nasal passage,
and, perhaps even more importantly again, to the extent of surface
convolutions of the RTs. This latter would be extremely difficult to
measure, but if we are going to use an approximation of RT surface area,
surely a crude measure of RT VOLUME would be far more useful than just
cross-sectional area? Presumably water and heat exchange in RTs is related
not only to the surface area, but also to the length of contact time with
the passing air volume, so that a "short, fat" RT may actually be less
effective as an exchanger than a "long, thin" RT of equal area?
My second comment relates to the selection of skulls used in the study.
While I appreciate that finding fossilized skulls in pristine condition is
not always easy, and may be particularly difficult with smaller birds, are
there really no suitable bird or mammal candidates other than animals
existing today? If you are going to draw inferences from this study to
dinosaurs, you would be on much stronger ground if the study material
included bird and or mammal results from late Cretaceous or as soon after as
possible. The greater the temporal distance the weaker the inference, IMHO.
Now for Figure 3 - the plot of nasal passage cross section against mass.
Firstly, this is basically a plot of one measure of size against another, so
the fact that the points lie along a line should come as no great surprize -
it would be odd if you could not fit a good regression line to such data
unless there was some reason to expect gross disproportionation between the
rate of increase in size of the two variables. What we then are left with is
the 2 lines for birds/mammals and reptiles. These appear to the eye to be
separate lines and may well be so, but the statistical assurance is missing.
I am not a statistician, but I'll bet there is a test that will demonstrate
the difference between 2 such lines at a particular degree of confidence.
This test should have been done and the result included in the paper.
Assuming that the lines are in fact different, what does this tell us. Ruben
has already presented evidence that none of the reptiles have any sign of
RTs, and that nasal cavities in birds and mammals have had to expand to
contain RTs. Therefore, if we choose to plot those animals with RTs and
those without RTs as separate groups, this selection process of itself will
lead to different lines. This says nothing more than "an animal with an RT
will have a larger nasal cavity than a same-sized animal without RT" and
does not contribute greatly to information elsewhere in the paper, other
than to make it more visually obvious(?). Note that all of the animals
(birds, mammals and reptiles) could have been considered as a single group
and a third regresssion line obtained - hence the importance of knowing
whether the 2 lines presented really are different or not.
Finally, we come to the lung ventilation rate arguement. Ruben claims that
the increased nasal size is consistent with increase lung ventilation rates
seen in extant endotherms over ectotherms. However, as he also notes, the
increased nasal cavity size is in order to contain the RT. Depending on how
much space is taken up by the RTs, there may or may not be any real increase
in nasal cavity SPACE. In fact, for the RTs to function at maximum
efficiency as exchangers, they would need to come into contact with as much
as possible of the air flowing past them, and so should fill as much of the
cavity as is possible, conducive to getting enough air flow to supply
adequate oxygen to the lungs. In the absence of any measurement of nasal air
space, the lung ventilation arguement appears mere speculation, and not
particularly well founded at that. In addition, and as has been mentioned in
other posts to this list, nasal anatomy is only one of a number of possible
determinants of lung ventilation.
The conclusions I would draw from this paper are:
1) there appears to be good evidence that extant ectotherms do not have RTs,
while almost all extant endotherms do have them.
2) it appears, on the basis of a small sample, that dinosaurs did not have
RTs, but we still have no evidence either way regarding ANY other extinct
animals. If contemporaneous birds and mammals did not have RTs either, RTs
would be of no use in distinguishing different types of extinct animals.
3) the most important function of RTs in terms of the endo- ecto-therm
debate appears to centre on respiratory water loss (lung ventilation rates
yet to be demonstrated as significant). To answer this question definitively
in respect of their absence in a particular animal requires a detailed
biochemical, physiological and anatomical understanding of water handling
and conservation, which we simply do not have for any extinct creature,
4) a positive finding of RTs in dinosaurs would have contributed another
piece of circumstantial evidence in favour of endothermy. The apparent lack
of RTs in dinosaurs can similarly be used as circumstantial evidence against
endothermy, but is weakened by the 60MY speculation gap.
5) I would accept the argument that RTs have a direct relationship to
control of respiratory water loss. Ruben's paper does not demonstrate a
direct connection with lung ventilation rates, nor with endothermy per se.
In fairness, and terry Jones comments to this list notwithstanding, my
reading of the paper does not indicate that Ruben et al make that claim. To
quote their conclusion
"...a variety of ...dinosaurs....., possessed....relatively constricted
nasal passages, devoid of sufficient cross-sectional area to have
accomodated respiratory turbinates and endothermic lung ventilation rates.
These observations do not necessarily either preclude or support the
possibility that some ...maintained routine metabolic rates somewhat greater
than those of extant ectotherms."
I don't think that this translates as "dinosaurs were not endotherms".
Endothermy is connected by them with lung ventilation rates, but I believe
this is an incorrect conclusion based on the evidence presented.
I would welcome any comments on these observations
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