RE: no marine dinos/no viviparous dinos.

["Tyrberg Tommy" <tommy.tyrberg@aerotechtelub.se>]

Some notes on constraints on the distribution of flightless seabirds and
bird viviparity

All the penguins that breed on continental mainlands (South America,
Africa and Australia) are small species that nest in burrows, the larger
"free-standing" species only breed on predator-free islands (no
non-avian predators, that is). 

The same was apparently true for Mancalla and Pinguinus. There is no
evidence that they ever bred on mainland coasts. Their dependence on
flattish predator-free islands proved fatal since there are no such when
humans with boats arrive. 

As for equator-crossing it is notable that no flightless seabirds have
ever been bipolar as far as we know. Hesperornithids, Plotopterids,
Mancalla and Pinguinus are strictly northern, Penguins strictly
southern. Probably the warm fish-poor tropical waters are simply too
inhospitable for flightless fish-eaters to penetrate since the area in
which they can forage is quite limited. As far as I know there is no
evidence that any cold current has ever crossed from the temperate areas
of one hemisphere to the other, certainly not in the Cenozoic.

Flying seabirds certainly can and do cross the equator. Gulls, skuas and
terns regularily do so on migration and so do the smaller procellariids
(shearwaters and storm petrels). Albatrosses can apparently only cross
the tropics with difficulty. Southern albatrosses do turn up in the
North Atlantic and North Pacific, but only very rarely. The problem in
this case is probably the tropical high-pressure zones (the "horse
latitudes") since albatrosses are unable to lift from water or maintain
sustained flight in windless conditions. However cross-tropics
colonization by seabirds is not very common though it does occur
occasionally. Seemingly clear and fairly recent examples are Fulmarus
and Catharacta (both from south to north). 

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. 

Tommy Tyrberg