The skeleton of a snake lacks a rib cage. In connection with what it was lost in these animals?

The skeleton of snakes is not clearly divided into sections, there are no belts of the upper and lower extremities, chest. All this atrophied in the course of evolution for the ability to lead a creeping lifestyle. In addition, the peculiarity of snake nutrition is the ability to swallow whole prey, which is several times larger than the thickness of the snake’s body. This does not require a strong chest frame, but on the contrary, the ribs must be mobile, diverge and converge, allowing food to move along the esophagus. And also the ribs are able to flatten, this helps the snake to flatten its body, for example, during defense, or to crawl into a narrow gap.