Re: Mammal Reproduction

[Robert.J.Meyerson@uwrf.edu (Rob Meyerson)]

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 development).

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.

Rob

***
"Run away!!!  Run Away!!!"
        -MPATHG