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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"
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