Submitted by fil- t3_1193rdl in askscience
derioderio t1_j9p4tzj wrote
Reply to comment by Jason-_B in How did the placenta evolve? by fil-
Do we have any fossil examples of any intermediates between egg-laying mammals and placental mammals?
djublonskopf t1_j9u5sbk wrote
Yes. Marsupial fossils would represent mammals that gave live birth, but aren’t placental mammals either.
Multituberculates are a lineage of mammal that is completely extinct today, but they were closer to therians (placentals+marsupials) than monotremes. Some of them had incredibly narrow pelvic openings too, hinting that they may also have been live-birthers (of tiny, underdeveloped offspring like the marsupials.)
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