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Meta- vs. eutherian K/T extinctions




John,
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 America):
(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.
------Ken Kinman
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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