Waterproof outer surface of reptiles and other amniotes.
Keratinized is an adaptation to living, and particularly within environments in which is substantially less than 100%. This way precious can be retained within the body rather than being subject to excess .
This contrasts, for example, with mucous membranes, where both and water evaporation can occur (versus keratinized skin where gas exchange instead does not readily occur).
The waterproofing of keratinized skin, perhaps not surprisingly, is associated with the protein, , which is also the primary constituent of , , , , , , , and . This protein accumulates in cells which then die, leaving behind a strong, flexible, and waterproof layer. Keratinized skin is seen predominantly in reptiles, birds, and mammals, i.e., aminiotes, but also particularly well terrestrially adapted amphibians, such as certain .
Keratin is also the major constituent of the baleen of baleen whales, which they employ to filter krill. So too is keratin thought to have made up much the outer features of . Contrast , however, which consist instead of . ( ☺)