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RE: erect posture

Martin wrote:

>Perhaps vertical height (and mass?) is only possible with a four chambered

It is thought that having high pressure blood in the pulmonary artery would 
cause bleeding in the lungs.  (I don't know if this has been tested, or 
exactly what happens to 'hole in the heart' babies.)  A divided heart is the 
easiest way to solve the problem.  

(Other possibilities: keep the lungs at the top of the body, and never roll 
over.  Or have the one ventricle beat alternately into the pulmonary artery 
and the aorta at different pressures.  Or develop a valve which cuts off the 
blood at high pressures.)

Incidentally, there is a differrence between fish and amphibian hearts and 
'reptile' hearts.  Lizards and crocodiles can typically adjust the flow of 
blood to keep their pulmonary circulation separate.  But usually they don't 
want to send blood to the lungs every time it's been around the body: it 
generally has enough oxygen left.

>I suspect that a higher metabolism is advantageous in that it permits
>migration...How many modern reptiles can migrate long distances...turtles, 
>what else?

Saltwater crocodiles.  Perhaps sea snakes.  Some land snakes travel 
respectable distances.  

>I also suspect this idea is not new.

The heart idea is not new.  It has been claimed that some dinosaurs were 
ectotherms which avoided cold winters by migrating.

                                                        All the best,