Thanks for the support!
There was so much information, it couldn't all be packed into one post. You're absolutely right, turtles did start burrowing for protection, but it wasn't fear of predators that drove them underground. Fossil evidence suggests that these turtles live in an incredibly hot and dry climate (hence why they didn't take to the water earlier) and they began burrowing to escape the intense midday heat. It's astounding to think the whole concept of the shell originally stemmed from a turtle just trying to get out of the sun for a bit!
WOW, no kidding? It makes sense about the temperatures too. I read somewhere, albeit a long time ago so my memory may not be good, that animals in the desert would dig what I call a "butt hole" (LOL) only a few inches deep as it's approximately 10 degrees cooler and gives them a respite from the midday heat.