Re: No Cretaceous placental mammals?

[evelyn sobielski <koreke77@yahoo.de>]

--- K and T Dykes <ktdykes@arcor.de> schrieb:

> <<ancestors of modern placental mammals were very
> small, and on total
> conjecture probably hatched their eggs internally
> like some sharks do now.>>
> 
> I'd imagine, the most recent common ancestor of
> myself and a marsupial was
> probably using a marsupial-like reproduction system,
> Rob; relatively
> early-born, poorly-developed deliveries.  The oldest
> known eutherian (=
> Placentalia plus more intimate relatives) is 125
> million year old /Eomaia/
> from Liaoning.  The pelvis has a rather narrow
> construction, and it's been
> said that this wouldn't have allowed for the arrival
> of well-developed kids.
> Cheers
> Trevor

A salient point. Molecular divergence is more crudely
estimated, but closer to the point of actual lineage
divergence, than morphological divergence. What one
gets when puzzling together a cladogram, after all, is
the pattern of *trait* divergence, not line4age
divergence. Ideally, they're close. If the uncertainty
period is 10 Ma, however, that might not be close
enough. So if you accept a placentral reproduction
system to be monophyletic (and I think there's good
reason to do this), the data points more towards it
being a Maastrichtian or later thing than being deep
in the Mesozoic - although the *lineages* might have
split that early.

Indeed, it might even be so that evolution of the
placental reporduction system was *both* faciliated by
the dinos' extinction, and one major factor as to why
they couldn't make a comeback - a trait that made for
superior better "comeback potential".

But for all that's been said, the crumbs of
Maastrichtian and early Pg birds we have seem to give
a more robust picture of Neornithes evolution than
does the fossil record of mammals. The mol-fossil
uncertainty in modern birds is far less pronounced
than in mammals; it actually comes down that in birds,
the dispute is nowadays about *which* lineages
survived, not if there were any that did, and how far
these lineages had already radiated. E.g. the case for
survivorship of maybe a dozen Charadriiformes lineages
is reasonable. But were the scolopacids already
radiated to the point where the (exclusively
Holarctic) phalaropes were distinct? Their present
distribution would suggest that either no (considering
snipes and woodcocks, which are fairly close, are
centered on the Australia-Wallacea region
evolutionarily), or they were pretty damn lucky
bastards.

(I need to look into the charadriiform data. It should
be possible to say with some confidence what the
survivor lineages were; the divergence pattern is very
well resolved thanks to the cool 2004 supertree by
Thomas et al - which by and large confirmed of the
morph data, as it should be, except in the auk-gull
group)

Eike

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