Why do turtles have shells that are part of their skeleton?

Turtle shells are unique because the top shell, or carapace, and the bottom shell, or plastron, are fused to the animal's skeleton. This means a turtle cannot simply leave its shell, because the shell is part of its body. Evolution transformed ribs and backbone into a protective structure that helps defend against predators. Fossil evidence shows shell evolution occurred over millions of years in early turtle ancestors. The shell also supports the turtle's body and influences how it breathes and moves.