Final answer:
Amphibians are not nested within the reptiles as they have a different mode of reproduction and development, lacking the amniotic eggs characteristic of reptiles.
Step-by-step explanation:
The lineage that is not nested within the reptiles is amphibians. Amphibians, which include groups like salamanders, frogs, and caecilians, differ from reptiles in several key aspects.
One fundamental difference is the type of egg each class produces. Amphibians are characterized by aquatic, gill-breathing larval stages that undergo metamorphosis into land-adapted adults. In contrast, reptiles produce amniotic eggs equipped with extra-embryonic membranes that allow for a terrestrial life from the earliest stages of development. Reptiles include four living clades, namely Crocodilia (comprising crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles), which are all part of the Amniota clade together with birds and mammals. However, amphibians do not share this amniotic adaptation and are, therefore, not reptiles.