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.

Okay so is this photoshopped or what's going on here

Yeah this looks like a stupid degree of shrink wrapping of the neck.

Turtle shells are already on the inside. The entire surface of their shells is skin!

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.

I don't understand how they used it correctly and then didn't