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It is indeed a mystery why birds and (so far as we know) other 
archosaurs have never evolved viviparity. Does this reflect a true 
historical constraint, or is it just chance that living birds and 
crocs don't include viviparous taxa? John Bois wrote...

> I forget who it was who claimed that the lack of
> viviparity in birds _needed_ an explanation; that the inability to
> provide a plausible hypothesis for this phenomenon was an 
> embarrassement for evolutionary theory.  

There is a lengthy and highly informative paper by Blackburn and 
Evans (1986) on this very subject. They review all proposed reasons 
for the lack of viviparity in birds (the most popular of which - one 
of which has been used for turtles (by Samuel Tarsitano) as well - 
include the embryonic calcium budget and the avian air-sac system) 
and conclude that birds could not have evolved viviparity. 
Embarrassingly, I forget exactly _why_ they conclude this. The paper 
was in _American Naturalist_.

At times it has been suggested that various dinosaurs - viz., 
sauropods and pachycephalosaurs - may have been viviparous. Both 
Phillip Currie and Alan Feduccia have speculated that 
hesperornithiforms may have been viviparous. None of these 
suggestions are compelling, and they are discordant with some data. 

"She doesn't want it""Munch munch munch"

School of Earth, Environmental & Physical Sciences
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