60 million years ago the shape may have made Puentemys mushaisaensis hard to swallow. At the time Titanoboa was a major player and the round shell on adult turtles may have not only helped them warm quickly in the sun by increasing surface area but also exceeded Titanoboa's ability to swallow. http://www.sciencedaily.com/releases/2012/…

Having kept or worked around ophidians for most of my life, I would also hazard a guess that the round shape may also have confounded them. Snakes instinctively look for the tapered head (or sometimes back end) of a prey item. They do not attempt to swallow from the middle as this can lead to injury for the snake. I can easily imagine a Titanoboa working it's mouth ceaselessly around this critter's shell until it abandoned it. In that case even an immature and therefore possibly consumable turtle might live to frustrate another snake another day.