Q. Which is the vertebrate class that is considered the first entirely terrestrial?

Totally independent from the aquatic habitat, The first entirely terrrestrial vertebrate class, is the class Reptilia.

Q. Compared to amphibians what is an example of evolutionary novelty present in beings of the class Reptilia against the loss of water through the skin?

The reptile skin is impermeable and keratinized to water while the amphibian skin is permeable. The skin impermeability made impossible the cutaneous gas exchange performed by amphibians and respiration became dependent on internal organs like lungs and airways.