Which of the following is NOT a characteristic of Amphibia?

Amphibians, such as frogs, toads, and salamanders, exhibit several defining characteristics that distinguish them from other vertebrate groups. They commonly possess two pairs of limbs, which facilitate movement both in water and on land. Additionally, while many amphibians undergo a life cycle that includes an aquatic larval stage with gills, they typically develop lungs as adults to breathe atmospheric oxygen.

External fertilization is also a characteristic of many amphibians, particularly during breeding season in aquatic environments, where eggs are laid in water and fertilized externally by males.

In contrast, keratinized skin is not a characteristic of amphibians. While amphibians do have some level of moisture-retaining skin that may contain keratin, it is not extensively keratinized like that of reptiles. Amphibians generally have permeable skin that is adapted for cutaneous respiration, allowing them to absorb water and respiratory gases directly through their skin, which is critical for their survival, especially in moist environments. Therefore, the presence of keratinized skin is more typical of reptiles and serves to prevent water loss, a necessity for their more terrestrial lifestyle.