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Thecodonts versus protomammals
Bob Bakker wrote in his book "The Dinosaur Heresies" that thecodonts were able
to beat protomammals because thecodonts were warm-blooded.
Some people think that crocodilians were originally warm-blooded and
secondarily evolved their sluggish, dowdays lifestyle. Some have also suggested
that their diaphragm originally evolved to provide better lung ventilation, a
useful character for an endotherm that requires great quantities of oxygen to
burn glugoce and stuff at an elevated rate.
Gregory Paul has said that the metabolic rates of earlier theropods, such as
Coelophysis, were not very higher than those of ordinary reptiles respectively.
The evidence comes from bones, which don't show much evidence of pneumacity.
Now, a few questions come in my mind.
If thecodonts, including the ancestral crocodilians, were warm-blooded, it
obviously means that a vertebrate can have an high metabolism even though it
has septate lungs. It is most unlikely that crocodilians evolved septate lungs
Bakker has drawn a picture showing simultaneous archosaurian diversification
and protomammal desimation. I wonder if the protomammals had alveolar lungs.
And also, did they really have a diaphgram? At least some dicynodonts don't
seem to have a lumbar region at all, in my eyes. In case of crocodilians, the
diaphgram evolved first, in fact it was the only aerobic enchancement that
evolved in their case; which came first in with mammals, diaphragm or alveoli?
If, in turn, even the earliest dinosaurians were still cold-blooded, how come
that they succeeded so much better than mammals at that time (true, the urine
of extant birds is solid except for emus, but many extant mammals have very
impressive kidneys)? And if this is correct, it also seems that evidence of
dinosaur warm-bloodedness based on the nature of the particular ecological
niches the dinosaurs occupied is of no value.
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