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.

Well, both latitude and intensity of light affect temperature because, the higher the light intensity, the higher the temperature and since latitude is the measure of how close you are to the equator, then the closer to the equator you are, the higher the temperature is.