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Re: cold-blooded dinos (revisited)



ORIGINAL MESSAGE
>Ruben and his colleagues (who are physiologists)  examined a fossil >of 
>Scipionyx samniticus,  a small meat-eating theropod.  They think the
>fossil - 
>which has traces of internal organs - shows that the diaphragm was >used for 
>breathing (like in mammals).   However,  Ruben et al.  state  that
>dinosaurs 
>(S. samniticus included) are cold-blooded.   In addition, James O. >Farlow
- a 
>paleontologist - is  quoted as saying that dinsoaurs were cold->blooded, but 
>that theropds were "as active as" warm-blooded animals.  These >scientists 
>(apparently) categorically state that dinosaurs were cold-blooded; >they
don't 
>even tip their hats (so to speak) in the direction of any other theories.  

Dear Barbara

Ruben's article about Skippy is very interesting. I think that most people
now agree that Skippy had a ventilation system along the lines of
crocodilians and mammals. Ruben maintains that theropods were cold-blooded
(i.e. had low resting metabolic rates) mainly because of an earlier study
conducted by the same team which failed to find evidence of respiratory
turbinates in a selection of dinos (many of us disagree). 

However, noting that other living reptiles seem to get by well enough
ventilating themselves just by moving their rib-cages in and out, the team
suggest that Skippy's piston diaphragm may have fascilitated the high rates
of gas exchange typical of mammals with similar anatomical features (most
biologists agree that the piston arrangement in mammals fascilitates high
rates of gas exchange). This would have given Skippy a lot of top-end power
for charging around.

The problem with this idea is that Ruben himself has written papers
suggesting that resting and maximal metabolic rates are linked in some way
so that selection for the latter naturally leads to increases in the
former. The reasons are complicated, but I can send you details of the
papers or explain off-list if you like. If early Ruben is right, then
Skippy must have side-stepped Ruben's own conclusion in some way. However,
althought the ratio of maximal to resting metabolic rate is remarkably
constant in living tetrapods at between 10 and 20, some mammals have ratios
in the 70s. So although Rubens ideas about Skippy do not sit *too*
comfortably with his earlier ideas, they are not without empirical precedent.

The remaining puzzle is why crocodilians have piston diaphragms when they
don't seem to need them. However, some early crocs were probably
terrestrial and possibly also highly active, so the pistons of modern crocs
may be a hangover from a more aerobic past.

Ruben's ideas about Skippy are something of an about-face, and I am waiting
for someone to criticise him for it. However, IMHO, the willingness to
change ones mind as new thoughts and data occur is what separates
scientists from advocates.

Best Wishes
Chris