Amphibians are a class of animal that includes modern-day frogs, toads, salamanders, newts, and caecilians. They first evolved from lobe-finned fish and primitive tetrapods about 340 million years ago. Sometimes this date is incorrectly given as 400 or 380 million years ago, but fossils have not been unearthed from these periods.
About 380 million years ago, during the Devonian period, some fish began to evolve legs and digits. These early "tetrapodomorphs" lacked the characteristics that define amphibians, so they are classified as basal tetrapods. Decades ago, they were classified as amphibians, though taxonomists have changed their view on the matter. This is why the origin of this class is sometimes incorrectly cited as 380 million years ago.
Some of the earliest tetrapods include Tiktaalik, among the earliest with a weight-bearing wrist structure, and Acanthostega, which had eight digits on each foot. These early species would have been mostly aquatic, and used their limbs to navigate through swamps rather than taking extensive journeys over the land.
Between 380 and 360 million years ago is a period called "Romer's gap," in which barely any tetrapod fossils have been found, casting a cloud of mystery on the evolution of the first amphibians from the early basal tetrapods. Prior to the gap, no fossils are found, and the first known amphibian fossil appears shortly after the gap. After the gap, the world was in the Carboniferous period, where sea levels were high and the coasts were covered with flooded forests and swamps.
The first amphibians were temnospondyls, long-headed animals with a sprawling gait and distinctive look. These were the first truly terrestrial tetrapods, and would have eaten themselves silly by consuming insects that lacked specialized adaptations for defending against large vertebrate predators. The early temnospondyls were the size of large fish, ranging from about 1.6 to 5 feet (0.5 to 1.5 meters) in length. The earliest ones had stubby feet, and probably couldn't move very fast.
Throughout the Carboniferous period, temnospondyls grew in size and diversity until they occupied many of the predatory and herbivorous niches that terrestrial animals exploit today. By the late Permian, some even grew to 30 feet (9 m) in length, and resembled crocodiles. This animal, Prionosuchus is the largest amphibian known. In the Carboniferous, temnospondyls were joined by the diverse but less numerous lepospondyls. Lissamphibians, the group that includes all modern amphibians and their common ancestors, emerged about 300 million years ago.