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Mammal reproduction



Lee J. McLean writes;

> it would still have been _much_
>easier to eat the eggs of smaller animals, including lizards, snakes
>and in all probability other mammals (there is no evidence that any
>were live bearers at this time). So these should have been the sorts
>of animals that one would expect to have become extinct _first_ in
>such a scenario. And yet it is they who survived and thrived!

Ummm...

I hate to spoil your rant here, but there is some suggestive evidence
 that does allow for mammalian live birth by the Late-K.  It is very
 well established that placentals and marsupials have a common ancestor
 in the Cretaceous (known from dentition comparison studies).  Looking
 back from today, the dentitions between these two groups gets more and
 more similar; this trend continues till the Mid-Cretaceous, where the
 dentitions become so similar, that it's hard to tell one group from
 another.  Since live birth is a trait of both groups (although
 placentals lost the need for the pouch) then it can be concluded that
 live birth was in place by this time.  How far back this trend goes,
 and how pervasive it was among mammal groups, is open to debate.

For the therapsid experts out there: Is there any skeletal evidence
 for live birth (i.e. hip shape/size) in the more advanced therapsids?
 This could shed a little light on the subject.

Rob

***
"No, 'OOOH' in surprise and alarm."
                -MPATHG