Amphibians and reptiles: The earliest known amphibians, such as Acanthostega and Ichthyostega, lived about 363 million years ago at the end of the Devonian period (409–363 million years ago). Their limbs may have evolved from the muscular fins of lungfishlike creatures. These fish can use their fins to push themselves along the bottom of lakes and some can breathe at the water’s surface. While amphibians can exist on land, they are dependent on a wet environment because their skin does
not retain moisture and most species must return to the water to lay their eggs. Evolving from amphibians, reptiles first appeared during the Carboniferous period (363–290 million years ago): Westlothiana, a possible early reptile, lived on land 338 million years ago. The development of the amniotic egg, with an embryo enclosed in its own wet environment (the amnion) and protected by a waterproof shell, freed reptiles from the amphibian’s dependence on a wet habitat. A scaly skin
protected the reptile from desiccation on land and enabled it to exploit ways of life closed to its amphibian ancestors. Reptiles include the dinosaurs, which came to dominate life on land during the Mesozoic era (245–65 million years ago).
