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On Mon, 5 Jun 1995, Warren Raddatz wrote:
> On the subject of live birth in large saurapods, as they are grazers
> and need to eat everyday wouldn't live birth make more sense for them...
> ...Any one have any ideas,
> refferences etc.?
As far as I can see, viviparity would have to have evolved before gigantism.
If eggs are large, they must be laid immediately: it would be impossible to
provide enough oxygen for the embryo if they were retained within the body.
Also, large eggs develop more slowly, so it is more costly to retain them
until they are ready to hatch. Small eggs (by terapod standards, they are
large compared to other animals') can be retained and placentation is then
possible. For example, monotreme eggs absorb more nutrients from the
mother after fertilisation, and it is not hard to see how the therians might
have evolved viviparity from this state.
Similarly, a large viviparous animal would not be able to produce a strong
enough egg to revert to oviparity, even if this is possible for smaller
tetrapods (and I know of no examples). So I think either all or none of
the sauropodomorphs were viviparous, and probably no ornithischians.