Resolving Reptile Relationships: Molecular and Morphological Markers

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Abstract
Although the origin of tetrapods is often synonymized with the radiation of vertebrates into terrestrial habitats, most early tetrapods and many extant representatives (“amphibians”) remained partly aquatic. They possessed permeable skin and (primitively) a breeding biology requiring free water, with external fertilization and aquatic eggs hatching into gilled larvae. Many tetrapod lineages (including some living amphibians) partly circumvented this dependence on water by acquiring internal fertilization and direct development. However, only one lineage, Amniota, evolved additional adaptations permitting full terrestriality, including a waterproof epidermis and the amniotic egg (Sumida and Martin 1997). The amniotic egg is one of the most significant vertebrate innovations, consisting of a tough eggshell, outer and inner protective membranes (chorion and amnion), a yolk sac for nourishing the developing embryo, and an allantois for storage of waste products and respiration. It allows the embryo to develop terrestrially in its own private “pond,” bypassing the aquatic larval stage and hatching into a fully formed neonate. Amphibian-grade tetrapods breathe through their permeable skin, supplemented by rather inefficient buccal (throat-based) lung ventilation. The evolution of highly efficient costal (rib-based) lung ventilation has been proposed to be another critical amniote innovation, permitting them to abandon cutaneous respiration and thus waterproof their skin (Janis and Keller 2001).

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