Re: no marine dinos/no viviparous dinos.

["David Marjanovic" <david.marjanovic@gmx.at>]

> Flying seabirds certainly can and do cross the equator.

Loons have done it only once, BTW.

> Since viviparity has evolved a large number of times and in essentially
> every major vertebrate group except birds it seems reasonably that for
> some reason viviparous birds are impossible. Two main explanations have
> been suggested:
>
> 1. Bird embryos require calcium from the eggshell (this has already been
> mentioned in this thread).
>
> 2. The birds' peculiar "stream-through" breathing system is incompatible
> with viviparity.
>
> If 1) is true, then probably no birds have ever been viviparous. 2) on
> the other hand may not necessarily have applied to some or all
> non-neornithean birds, and in that case some Mesozoic birds might
> conceivably have been viviparous.

On the contrary, 2) applies at least to Saurischia and Pterosauria;
pneumatic features can be seen on vertebrae throughout the clade
(Rhynchosauria + (Prolacertiformes + Archosauriformes)), so perhaps the
crocs have lost it secondarily, and none of said clade was viviparous.

But why _should_ the flow-through lung + air sacs be incompatible with
viviparity? I can't imagine that.