Turtle shells are basically exoskeletons. Everyone knows that. In fact, I wonder if there is a skin analog on the outside of the shell?
Anyway, you see parts of turtle shells everywhere, the roughly rectangular “rib” and “backbone” pieces laying separated, with the knit-lines visible.
…you can clearly see that the “ribs” pin-fit into a circumferential ring, creating a classic tensile ring/compressive web steel-tired-wagon-wheel structure. Obviously, a turtle shell is properly engineered.
We never find turtle heads on the beach. Dunno why.