02 Sept. 2010. Amphibians.ppt. 2. Class Amphibians. Old group of tetrapod (4-footed) vertebrates, older than reptiles.Origin in mid-Paleozoic. 02 Sept. 2010. Amphibians.ppt. 3. Class Amphibians. Less well adapted to terrestrial environmentSmooth skin, no scales (no keratin ?)External fertilizationdepend on water for mating; sperm and eggs released together into waterNo embryonic membranes,Egg must stay moist usually in water, rarely in moist soil.