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Re: Science (1997;278)



Colette H. Adams wrote:
> 
> With regard to the article in Science (1997;278), I hesitate to comment
> without seeing it, but I don't really understand how it was concluded that
> particular theropods lack avian-type lungs.  Greg Paul has commented on a
> number of occasions about the lung structure of theropods and gives
> evidence for avian-type lungs in most.  But they are arguing that
> Archaeopteryx was an ECTOTHERM?  I'm afraid my credulity has limits.
> 
> With regard to endothermy and running, note that some ectotherms can sprint
> quite nicely, and big monitors can maintain quite a speed for some time.
> 
> Best regards,
> 
> Dave

Here we come to the problem of defining terms. We tend to assume
that 'ectotherm' and 'endotherm' are at opposite poles, when in fact
they tend to overlap somewhat. A bear is an endotherm, a monitor
lizard is an ectotherm, and yet the metabolism of a hibernating
bear is probably lower than that of some of the active forms of
monitor lizard. There is even great variation in metabolic rate
within each definition. The platypus has quite a low metab.rate
for a mammal, certainly not comparable to a shrew. Metab.rates
can even vary within closely related groups. Some monitor lizards
are highly active, with complex convoluted lung systems (like the
Afrcian desert monitor), while some of the aquatic monitor species
have generally lower metabolisms with correspondingly simple bag-like
lungs more like those of most other lizard groups.

I agree that something covered in feathers and capable of some degree
of active flight (assumption made) was probably not as ectothermic
as your average lizard, but by the same token it may not necessarily
have required a metabolic system like a modern bird or mammal.
We tend to define 'ectotherm' and 'endotherm' based on extant creatures,
so it is no wonder that they begin to blur when considering extinct
animals whose biology we can only guess at. 
-- 
____________________________________________________
        Dann Pigdon
        Melbourne, Australia

        Dinosaur Reconstructions:
        http://www.geocities.com/capecanaveral/4459/
        Australian Dinosaurs:
        http://www.alphalink.com.au/~dannj
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