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In listening to this discussion (which I'm enjoying, at any rate), I am
wondering if there is any egg-laying mammals in the fossil record that
have been found with eggs?  Platypuses are river-dwelling, egg-laying
mammals after all, so have fossil eggs or embryos been found?  This
would be in roughly the same conditions needed for your mosasaurs,
plesiosaurs, or ichthyosaurs in inland streams.  Admittedly modern
platypuses dig (or perhaps borrow?) a burrow, I'm not sure what echidnas
do, but the soil or leaf mold of platypuses would seem to be similar to
what modern crocodiles tend to in covering an eggmound.  Sea turtles use
sand.  Shouldn't both be handy to preserving undisturbed nests in the
fossil record?

For that matter, would the egg-laying of echidnas and platypuses be more
like egg-laying of reptiles or more like egg-laying of birds in the
shell development and calcium needs?  Would these be hard-shelled (like
a chicken's) or soft-shelled (leathery like a turtle's)?  What would
have been beneficial to head the mammals in this direction?

-Betty Cunningham

Danvarner@aol.com wrote:
> P.S. I hope that most of you on the list don't object too much about
> occasional  mentions of marine reptiles from the age of dinosaurs. We marine
> enthusiasts should probably be on the vertpaleo list but we love dinosaurs,
> too, and we are afraid of mice, rodents, and other small mammals that imitate
> insects.