Mammals evolved from a therapsid ancestor. This was a mammal-- like reptile. Turtles are very ancient reptiles. However, Amphibians evolved before Reptiles at the end of the Devonian period from lobe-finned fish ancestors, much like today's Coelacanth. Amphibians were only partially adapted to life on land, but had to return to the water to mate and to stay moist. Reptiles were the first true land adapted vertebrates. Mammals are considered to advanced synapsids. Synapsids have extra openings in the skull behind the eyes. This opening gave the synapsids stronger jaw muscles and jaws than earlier animals. Mammals are synapsids and evolved from mammal-like reptilian ancestors. These were pelycosaurs, cynodonts and therapsids. Amphibians are not synapsids.