How the Turtle Got its Shell

You only have to look at a turtle once to realize how different they are from other vertebrates. Where did that shell come from?

The shell itself is made from broadened and flattened ribs, fused to parts of the turtle’s backbone (so that unlike in cartoons, you couldn’t pull a turtle out of its shell). The shoulder blades sit underneath this bony case, effectively lying within the turtle’s ribcage. In all other back-boned animals, whose shoulder blades sit outside their ribs (think of your own back for a start). The turtle’s torso muscles are even more bizarrely arranged.

Ed Yong looks at turtle anatomy and how this weird configuration evolved from the basic vertebrate plan. Link