Zoology 10th Edition by Stephen Miller,John Harley

Exercise 6

Nonavian reptiles are characterized by having a skull with one condyle, lungs, metanephric kidneys, internal fertilization, amniotic eggs, and keratinized epidermal scales. Turtles are members of the order Testudines. They possess a bony shell and keratinized beak. Archosaurs include crocodylians, birds, and dinosaurs. Crocodiles and alligators are members of the order Crocodylia. They have unique skull characteristics, including triangular eye orbits and laterally compressed teeth. Tuataras, lizards, amphisbaenias, and snakes are lepidosaurians. Tuataras are members of the order Sphenodontia. They are primitive, lizardlike reptiles with unique tooth structure and attachment. Lizards and snakes belong to the order Squamata and are characterized by a kinetic skull. Lizards (suborder Sauria) have two pairs of legs and their upper and lower jaws unite anteriorly. Snakes (suborder Serpentes) are elongate and lack limbs. Their upper and lower jaws are loosely joined. These adaptations facilitate swallowing large prey.
How would you explain the fact that the snakes, lizards, and worm lizards, although superficially very different in body form and ecology, are members of the same order?