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Meta- vs. eutherian K/T extinctions
Well, the bolide scenario seems pretty parsimonious to me, and so
catastrophic (especially for land vertebrates) that it is indeed often
easier to explain what survived rather than what didn't (the exact opposite
of how one would approach bacterial extinction).
One big factor in why eutherians as a group did better than the
metatherians is probably that they had more clades that lived underground.
Who is going to do better during the catastrophe (especially in North
(1) a ground squirrel (which may have stored food or been able to
sustain itself on roots and tubers);
or (2) a dumb-struck koala-like metatherian sitting in or under a tree and
breathing in smoke. A little simplistic, but you get the idea. And not
surprisingly, the only monotreme survivors are excellent diggers and readily
retreat underground for protection. Generalist scavengers like crocs (who
could retreat into the water) also survived. A bolide catastrophe (and its
aftermath) makes perfect sense to me.
P.S. I understand about marsupials being a subclade (specifically a crown
clade) of metatherians. But aren't eutherians and placentals the same
group? Even McKenna made Placentalia as synonym of Eutheria. Isn't
Eutheria (unlike Metatheria) a crown group?
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