How did Testudines get their limbs and scapulae under their ribcages?

Question

Turtles appear suddenly in the fossil record, but I can't remember from my herp class 8 years ago if we had discussed any theories or hypotheses as to why they are this way morphologically. I really can't remember what happens embryologically, either, and whether that would give a clue as to how it happens.

Answer 1

Isn't the whole theory that prenatal development demonstrates evolution pretty much debunked?

Proto-turtles probably started with heavy skin/bony nodule protection on the back like many other reptiles and dinosaurs. The rib cage probably drove the changes- broader, flatter, wider ribs. you see this in some of the earlier fossils. At the same time, the breastbone was getting wider and becomeing more of an anchor structure.

As the ribs became more and more of a rigid box, the limbs changed attachment points and were functionally pushed out of the way.

Early turtles had large gaps in front and back openings- armored skin and articulating the limbs to cover the face must have been part for the protection at that point. It is also likely that sheer size and using water as a protection helped as well.

Once we have the general structure down, and the adaptations are in place for the neck, limbs, and breathing, the stage was set for the shell to really begin to develop- thicker, heavier, boxier, more encompassing, etc.

Early turtles and tortoises seem to have developed in the tropics and shallow seas, rather free from weather concerns that more 'modern' reptiles fight. Slow metabolisms mean that they can thrive in high population densities, low resources situations, etc.

It is a nasty cycle, though- slow metabolism- more predation- heavier shell and defenses- slower metabolism- more predation...