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Re: Mammal Reproduction
Lee J. Mclean writes;
>The authors of _Steropodon galmanni_ (the Cretaceous monotreme) were
>of the opinion that at its molars were tribosphenic. If this is
>correct, that would mean that the monotremes share that common
>ancestor, making that common ancestor almost certainly
>egg-laying. It is possible that the marsupials and placentals shared
>a slightly more recent common ancestor that was live-bearing,
>however (though there would not have been much time available for
This could further the idea of the transition from egg-laying to
live-birth through an intermediate step of an ovoviviperous stage (egg
kept internal). From this stage, two distinct populations used very
different strategies to solve the problem of complete vivipory (true
live birth). Hense the differences between placentals and marsupials.
>It must be pointed out, however, that the views of _Steropodon_'s
>authors are far from established, so the monotremes may not have
>been tribosphenic anyway. So, are there any mammal experts out there
>who can clarrify the current consensus of opinion here?
Ultimately, there would have to be an egg-laying ancestor *somewhere*
in the fossil record. IMHO, the transition from egg-laying to
live-birth would've happened pretty fast (the selection pressure to
complete the transition would be strong). Whether that species was a
monotreme, or a descendant of a monotreme is open to debate.
This will probably be my last post for the semester (as my university
e-mail account expires at the end of the week). I will probably be
back next fall (we'll see how my graduate school schedule looks), but
it will be with a different address. Until then, have a great summer,
and may fortune smile on y'all.
"Run away!!! Run Away!!!"