Dinosaurs were tachyaerobic endotherms

[GSP1954 <GSP1954@aol.com>]

Rey and Troutman have a nice little discussion going here on dinoenergetics.
Some points. 

RT. No fossil bird has yet been shown to have RT: Ergo they were not
endotherms! Or, is it because archosaur RT are usually entirely cartilage?
Albatross and kiwis have extremely slender anterior nasal passages as narrow
or narrower than those of reptiles, yet there is an RT in there. A few other
birds and mammals have little or no RT. In general, the posterior nasal airway
of theropods may have been broad enough to accomodate large RT. That this part
of the passage is short is not a problem, in a number of birds the airway is
short, and the larger RT sits in the portion of the airway immediately above
the internal nares. Some other dinosaurs may also have had large nasal airways
- we will never know because soft tissues lining the nasal cavity walls are
not preserved. (You see, Ruben et al compared the broadest section of the
nasal airway containing RT in birds, to the narrowest sections in theropods -
naughty naughty! As for their restoration of a short reptile nasal airway in
Dromaeosaurus, sheer fantasy based on an EXTREMELY incomplete snout. Well
preserved dromaeosaur nasal airways are nice and long like in other
theropods). So, no one knows whether or not some or all dinosaurs had
cartilage RT, so RT are of no value at this time. 

Big size and overheating: The biggest rhinos, indricotheres, weighed about
15-20 tonnes and they did obviously did not overheat under the blazing
central-southern Asian sun despite the lack of long necks and tails. Having a
low surface area to mass ratio minimizes heat gain from the environment when
it is too hot to unload heat, and can help prevent overheating in hot climes.
The long necks of sauropods were for eating among the tops of trees, the tails
were for defense, display and as props, the long legs for walking. Any
thermoregulatory purpose was probably secondary. 

Erect legs and bipedalism: Erect legs probably force elevated aerobic exercise
capacity because they force walking speeds to be too high to be sustained by
reptilian AEC. But one must be careful about this. The earliest dinosaurs -
your eoraptors, staurikosaurs, herrerasaurs - had erect legs and were bipeds.
Yet they had small ilia that suggest a rather small musculature, and lack
sophisticated respiratory adaptations. This combination suggests that although
they were no longer reptilian in aerobic capacity, they were only barely out
of the reptile zone. Ergo, erect legs do not indicate the presence of high,
avian-mammalian energetics. 

Aerobic capacity, resting metabolism: activity and endothermy: Most people
badly misunderstand the issues involved here. There is no such thing as a
truly highly active reptile on land. They can either move for a long time but
only very slowly, or they can move fast but only for a very short period of
time. Birds and mammals can be highly active for long periods of time. 
     Nor is it possible to have a high aerobic capacity yet be a low resting
metabolic rate nonendotherm at the same time if you are a vertebrate. For
reason that are not entirely clear, the centralized respiro-circulatory system
of vertebrates seems to require that when AEC goes up, so does resting
metabolism. Certainly there are no exceptions to this correlation living today
(and flying insects do not count because they have a dispersed respiratory
system). 
     Also, in order to be truly highly active for extended periods of time
requires burning lots of oxygen, which produces lots of heat. Such creatures
will therefore not be dependent upon external heat, they will be endotherms
strictu sensu. 
     
Dinosaurs: The evidence that dinosaurs had elevated aerobic capacity is quite
good, the evidence they did not is absent. Ergo, they should have had elevated
resting metabolisms, and produced lots of heat, so they were endotherms. Short
hipped early dinosaurs seem to be just out of the reptilian grade, armored
ornithischians may not have been very energetic either, otherwise dinosaurs
had strongly elevated AEC etc and were approaching or at the avian minimum.
They may not have had a strong metabolic reponse to cold, hard to tell at this
time. 

There, solved the whole problem, am sure all will agree. What a relief.

GSPaul