Really interesting stuff.
I was hoping somebody could explain how the incremental changes you discuss give a "survival of the fittest" advantage along the way. I can see how digging holes faster would be advantageous if you were digging them while under attack. But, whether it takes 1 hour or 2 hours to dig a burrow doesn't seem to explain why evolution would head in that direction - since evolution has no look-ahead engineering capability, every incremental change must pay for itself.
What was it about sacrificing mobility today that would select for digging or defense or anything else in the future?
What did all the disadvantaged unfit generations of turtles do to survive while waiting for evolution to "develop a new method of breathing called "buccal pumping" to overcome this" restriction on ability to breath? How many simultaneous modifications did it take to create this new method of breathing before it became functional and what selected for each of those intermediate modifications that added no advantage until they were all complete?
Thank you and good questions. They didn't just sacrifice their mobility off the bat for the digging benefits down the road. They were already a fossorial species, and probably modestly adapted for that life-style. Their ribs began shifting and widening to serve as forelimb-anchors because that fit well with that particular lifestyle. The trade off was that it did make them slower, however since they spent the large amount of their time underground, speed was not the most pressing issue (it wasn't until turtles took to the water that we believe they started to compensate for their slow speed by developing a protective shell). So yes, they did sacrifice their speed, but it was to increase qualities that made their immediate life easier.
Buccal pumping probably evolved alongside the ribs. We still don't know exactly how that method of respiration arose (it's still up for debate) but it probably developed as the ribs were expanding and making normal respiration more difficult. I think it will be a while before we have an answer to that question.