I noticed that marine iguanas were mentioned. If you look at film
footage of these creatures swimming you'll notice that they tuck
their appendages against their sides and use body and tail
undulations to move underwater. This is also true of South
American green iguanas. ( I have empirical knowledge of these
animals as I have taken a few out of heir enclosures to let them
swim. )
As for the marine iguanas, It isn't until they reach a rock with a
healthy growth of underwater vegetation that they hold on to the
substrate with their claws. Because of the surf these animals must
hold on with "all fours." In calmer areas I imagine holding on with
hind limbs may facilitate foraging as they could move their anterior
end in arcs to reach food without repositioning.
I've noticed that no one has mentioned sources of vegetation
growing underwater (Or for that matter submerged land based
vegetation) nor have they mentioned tail marks in the preserved
sediment.
Perhaps I'm mentioning something that is not plausible due to my
ignorance of the specific subject matter.
Sorry.
I'll continue to monitor your discussion, if you don't mind, as I have
learned quite a bit from doing so.
Kevin Koenig