Final answer:
Both extant reptiles and mammals typically possess keratinized skin, which is a shared characteristic that adapts them for life on land by reducing water loss from their skin.
Step-by-step explanation:
The characteristic that most extant reptiles and most extant mammals share is keratinized skin. Reptiles are well-adapted to living on dry land with tough keratin scales, which reduce water loss. Similarly, mammals possess various adaptations in their skin, including keratin, that help to protect and insulate their bodies. Reptiles lay calcareous or leathery eggs enclosed in shells on land, while mammals have a complex placenta to nourish their embryos. Both reptiles and mammals are also endothermic, meaning they can regulate their body temperature internally.
While mammals are endothermic, regulating their body temperature internally, reptiles are ectothermic, relying on external sources to regulate their body temperature. Lastly, except for a few, such as the egg-laying platypus and echidna, most mammals give birth to live young, whereas reptiles typically lay shelled eggs on land, with some exhibiting ovoviviparity or viviparity.