Cotylorhynchus, a genus of late Permian synapsids. It's herbivorous and it's huge gut was possibly necessary to digest the nutrient-poor plant life it fed on.
I don't know where this particular fella fits in the history, but turtles evolved from similar quadruped reptiles. Basically, their ribs evolved flat and wide so that their chests could support them better while burrowing tunnels. That turned into the turtle plastron.
You know what, I feel like the shell part of the turtle is best represented by the "turt" part of the world turtle. It just looks the way "turt" sounds.
So a turtle without it's shell I think should start to be called a "le" (pronounced like in the world turtle), which represents very well, as a sound, a "turt"-less turtle
I can picture it now. An Ice Age movie where Sid, Manny, and Diego are hanging around a bar and this chonky boy shows up out of nowhere and makes an outrageous claim. Queue "It's Always Sunny in Philadelphia" music.