Re: Mammal Reproduction

[ljmac@magna.com.au (Lee J. McLean)]

Rob Meyerson wrote:

>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.

Live-bearing has been independantly evolved by many completely
phylogenetically seperate groups, so I don't see the common ancestry
of these two groups as being a particularly strong indicator of this,
especially considering the differences in their 'tactics'. (I do not
believe that the placental birth strategy is simply a more derived
version of the marsupial one.) There may, however, be details of their
birth strategies which point to a common ancestry that I am unaware
of. If so, I would like to know about them.

>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.

I think I can safely rule out _this_ possibility, for one very simple
reason. Monotremes, which every taxonomist that I know of classify as
mammals, lay eggs.

I am not, by any means, ruling out the _possibility_ that the mammals
of this time _were_ live-bearing, but simply pointing out that this
cannot be safely assumed. I am not aware of any evidence that layed
eggs either. But the monotremes suggest, to me at least, that
egg-laying is perhaps a little more likely than live-bearing,
especially if one accepts them as being tribosphenic (what is the
latest consensus on the phylogentic position of the monotremes?). I
could very easily be wrong, however.

For John's speculation to be tenable, however, he must be able to
demonstrate that they _were_ live-bearing, which I don't believe he
can do at present. (And he still would not be able to explain why the
monotremes, and probably also the multituberculates, survived.)