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Re: Marsupial forelimbs... or rather hindlimbs

(I'm not certain exactly what Weisbecker et al. mean by "fully
aquatic", but full-time aquaticity in the style of sirenians and
cetaceans is one placentalian guild that has no marsupial analogs to
my knowledge.)


Yes, but then, both the whales and the sea cows evolved more or less as early as possible, and immediately spread around the world. The sea cows didn't even wait till they had lost functional hindlegs (see *Pezosiren* from Jamaica). Then those niches were occupied, and nothing else was able to evolve into them anymore.

Incidentally, it kind of seems that marsupial pouches would be
*better* for a flying guild than placentalian clinging. And there are
several lineages of gliding marsupials--it does seem odd that none of
them have developed flight. (Although, given the rarity of amniote
flight, especially amniote flight derived from skin-gliding, maybe
luck has something to do with it.)

The bats, too, apparently evolved around the Paleocene-Eocene boundary in the northern continents and immediately reached Australia. There are bat teeth in Tingamarra (dated as early Eocene last time I checked). Here, too, the marsupials may simply have come too late. Perhaps just a few hundred thousand years too late, who knows.