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Re: Mammal Reproduction
Lee J. McLean writes;
>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.
Sure, similar traits do show up in vastly unrelated species (i.e.,
wings in insects, birds, and bats). However, if similar traits show
up in groups that are known to be closely related, then the
probability of that trait belonging to the most recent common ancestor
(I forget the term that the cladists use for this) is very high. By
that same token, the probability of the trait evolving seperately is
rather low.
Taking this concept to mammallian reproduction, it can be said that
since live birth is common to both groups, it had to have been present
when the two groups diverged. To clarify things a little, I don't
claim that placental reproduction is superior to marsupial
reproduction. On the contrary, I suspect that the common ancestor of
both groups may have had a far less derived form of reproduction
(perhaps they formed the egg, but kept it within the body; common in
snakes and lizards), where the marsupials simply found their way to
improve on the idea, while placentals found another.
Ultimately, this is the continuing question between homolgies and
analogies. IOW, are similarities between animal groups due to a
common heritage, or is it due to a common environmental pressure.
>>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.
Touche'.
>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.
The only way we could conclusively prove that mammals laid eggs, is
for us to find a nest. Since mammal eggs would be extremely fragile,
due to their small size, the chances of finding one would be slim at
best. I know that negative evidence is the worst kind; but coupled
with phylogenic studies, it would seem to help the idea of
placental/marsupial live birth (I can't comment on other mammal
groups)
>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.)
Multis did survive. We find them up into the early Tertiary.
Rob
***
"And now, a Scotsman on a horse."
-MPFC