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Re: No Cretaceous placental mammals?
A salient point. Molecular divergence is more crudely
estimated, but closer to the point of actual lineage
divergence, than morphological divergence.
If well enough calibrated, yes. Is it ever well enough calibrated...? Most
examples in the literature clearly aren't.
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.
The lineages _of Placentalia_ seem not to have split that early, because we
haven't found anything with the autapomorphies of Placentalia in the
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".
Isn't there a pregnant leptictid in Messel or something? Or am I making this
up? But in any case leptictids have never been reported to have epipubes or
a narrow pelvis, AFAIK. If the Maastrichtian tooth taxon *Gypsonictops* is a
leptictid, we have placental reproduction in the Mesozoic.
E.g. the case for survivorship of maybe a dozen
Charadriiformes lineages is reasonable.
Why? There's no analysis with a large number of calibration points with
upper bounds that argues for this, right?