Which characteristic is common to all the modern representatives of all major reptilian lineages (turtles, lepidosaurs, crocodilians, and birds) ?

Presence of notochord is the characteristic is common to all the modern representatives of all major reptilian lineages. As Chordates, all vertebrates have a similar anatomy and morphology with the same qualifying characteristics; a notochord, a dorsal hollow nerve cord, pharyngeal slits among other features. A notochord is flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine.