Dr Adequate writes:
Obviously the pouch didn't just turn around. A plausible intermediate form would involve both an anterior and a posterior flap. Then whichever one of them was basal got smaller, eventually disappearing altogether, while the other one got bigger and took over its function. (This supposes that the difference between the pouches has been explained right and that I'm visualizing it right.)
I think that perhaps this is the problem that Percy initially envisioned. What kind of intermediate steps are available for such a shift (shall we say) from a pouch that opens in the front to one that opens in the rear? I can only come up with either an opening at the side that moves backward, or two openings, one that the front and one at the rear.
Or is the whole notion of the necessity of a shift or rotation really a question that only a creationist would ask?
Of course, it could be that the common ancestor had neither a definitive front or rear pouch, and then two subsequent marsupial lines split off, each developing its own arrangement. But I have absolutely no knowledge of marsupial cladistics to know if there's any basis for this supposition.
The difference between me and a creationist being that I admit up front that this is just a supposition and not meant to be a statement of fact.
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