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Re: No Cretaceous placental mammals?
Why, oh why does this keep coming up? The fact that placentals
divurged genetically from marsupials in the Jurassic is in no way
contradictory to the fact that crown group placentals have a more
recent divurgence time (possibly even Cenozoic), nor is it in
opposition to the fact that we don't see fossils with "key" placental
synapomorphies. Genetic divurgence has to occur prior to phenotypic
divurgence, and since we tend to pick "important" characters by
reference to what we can observe, we should expect genetic divurgence
to occur well before the evolution of the crown-group synapomorphies
selected via post-hoc observation of extant groups as "important".
That is, the sibling populations that were (genetically speaking) never
to mix again and give rise to groups that we would call placental and
marsupial mammals did not utilize the derived form of reproduction seen
in either group today. That evolved later, most likely several
millions of years after the speciation event that lead to the
divurgence of these linneages.
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