The reptiles called turtles (Order Testudines) share ancient ancestors with the crocodiles. Evolution of the turtles took a different track, though, leading to their unique feature: their shells. Early turtle ancestors first developed the bony plate under their bodies (plastron) for protection, then later developed the shell on top (carapace). In addition, toothed snouts evolved into beaks.
Today, 320 species of turtle relatives can be found living in freshwater lakes, ponds and rivers, or living entirely on land (the tortoises). Some others, the sea turtles, live in marine environments and can be found far out to sea. The limbs of the sea turtles further evolved into fins. Whether terrestrial, freshwater, or marine, all of these lay their eggs on land.