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A question concerning the BCF hypothesis



Hi!
 
I tried to find some information of the BCF hypothesis from the internet, and found the following address:
 
http://www.isgs.uiuc.edu/dinos/de_4/5c60bbe.htm
 
There is a text. It says:
 
"A sprawling diapsid such as Mesenosaurus effectively becomes an upright animal when climbing a tree and will suffer a momentary disadvantage until its blood pressure is able to partly compensate for the change. Any adaptation, such as a four-chambered heart, that removes this disadvantage will enable the animal to climb faster and easier and escape from a predator more effectively."
 
Why should an erect stance be an adaptation to arboreal life? There are many, many climbing lizards today and I think all of them sprawl. Well, there are chameleons, but then again, they have the usual heart.
 
I don't think the logic here is good at all, really. Is this really how the BCF goes?
 
Henri Rönkkö