Short answers, pulled from the story.
Amphibians evolved from lobe-finned fish during the Devonian period around 370 million years ago. Ichthyostega represents one of the earliest examples of this transition with four sturdy limbs and a neck.
Reptiles displaced amphibians from terrestrial environments after evolving the amniotic egg which prevented embryos from drying out. This innovation allowed reptiles to reproduce on land while most amphibians still needed water to lay shell-less eggs.
As of the 31st of March 2019 there were exactly 8,000 known amphibian species with nearly 90% being frogs. The smallest vertebrate is Paedophryne amauensis from New Guinea measuring just 7.7 millimetres.
Amphibian skin serves as a secondary respiratory interface allowing gas exchange through cutaneous respiration. Their highly vascularized skin must remain moist for oxygen to diffuse at a sufficient rate.
Fejervarya raja populations inhabit brackish water while Anderson's salamander occurs in brackish or salt water lakes. Most amphibians remain restricted to moist habitats and are not found in the sea except for one or two frog species living in mangrove swamps.