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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
How is this a fact -- for one thing, it depends on the calibration --, and
how does it fit the fact that we find the basalmost and very plesiomorphic
meta- and eutherians in the Barremian?
is in no way
contradictory to the fact that crown group placentals have a more
recent divurgence time (possibly even Cenozoic),
Of course not, but most molecular divergence date estimates _for the origin
of the crown-group_ are deep in the Cretaceous as well (often at the end of
the Early Cretaceous), though this again depends on the calibration.
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".
But synapomorphies of two clades have to evolve, by definition, before these
two clades split from each other. The lack of Cretaceous fossils with
placental autapomorphies ( = synapomorphies of Supraprimates +
Laurasiatheria-Afrotheria-Xenarthra under the topology of the new paper), in
the presence of plenty of Cretaceous mammals that lack these features, is
strongly suggestive, probably even statistically significant. Likewise, the
lack of Jurassic fossils with therian autapomorphies ( = synapomorphies of
Metatheria + Eutheria) in the presence of plenty of Jurassic mammals that
lack these features tells us something.
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
To toot my own horn, a big paper on (among other things) the strong
dependence of molecular divergence date estimates on the choice and
treatment of calibration points is coming out in the current issue of