Re: AAT Theory

J. Moore (
Thu, 21 Sep 95 12:44:00 -0500

Vi> Ah, sorry. I see. I was reading the wrong sense into it, ie. that you
Vi> had determined that land-based transitions of quadrupeds to bipeds would
Vi> yield the skeletal structure we see. Rather you were refering to
Vi> Elaine's comment that no skeletal evidence would be forthcoming which
Vi> could be held up as being a distinguishing feature of either an aquatic
Vi> or a terrestrial ancestor.
Vi> <== faster % Pete Vincent

And to the point that, if we think about a transition from either
quadrupedal proto-knuckle-walker or brachiator or vertical
climber, etc., to predominately bipedal animal, we would expect a
change in the pelvis but not necessarily anything more. And
that's actually what we seem to be seeing, isn't it?

On the other hand, in an aquatic change with sufficient selection
pressure to change the structure of the pelvis, we would expect to
also see, for instance, extensive limb-shortening, as we see in
aquatic mammals where selection pressure has been sufficient to
change the pelvis (such as pinnipeds, sirenia, and cetaceans).

So the AAT is forced to suggest that in this one case, the aquatic
environment somehow forced *just this one change*, one that can't be
distinguished from a land-based transition, and somehow didn't
force others, such as limb-shortening, that would also be expected.

Jim Moore (

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