A monophyletic group is one which forms a clade: group of animals which comprises an ancestor and all of its descendants. For example, birds are monophyletic. Reptiles (such as crocodiles, lizards, snakes, and turtles) are not monophyletic, because the group does not include birds, which are also descendants of the most recent common ancestor of crocodiles,lizards, snakes, and turtles. See also paraphyletic.