Figure 1

The three competing theories for the evolutionary position of turtles among amniote vertebrates. (a) Morphology-1 (turtles early): the classic morphological hypothesis, with turtles branching early. (b) Morphology-2 (turtles with lepidosaurs): another morphological hypothesis, which groups turtles with lizards and their relatives. (c) Molecules (turtles with archosaurs): the molecular hypothesis, which groups turtles with birds and crocodilians.