Amphibians evolved from lobe-finned fishes and primitive tetrapods about 340 million years ago. Early tetrapods lacked amphibian characteristics and are classified as basal tetrapods. The earliest amphibians were temnospondyls, which grew in size and diversity during the Carboniferous Period. Lissamphibians, the group that includes all modern amphibians, emerged about 9 million years ago.
Amphibians are a class of animals that includes modern-day frogs, toads, salamanders, newts, and caecilians. They first evolved from lobe-finned fishes and primitive tetrapods about 340 million years ago. Sometimes this date is erroneously given as 400 or 380 million years ago, but no fossils have been found from these periods.
About 380 million years ago, during the Devonian Period, some fish began to evolve legs and toes. 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 minds on the matter. This is why the origin of this class is sometimes incorrectly cited as 380 million years ago.
Some early tetrapods include Tiktaalik, among the first with a weight-bearing wrist structure, and Acanthostega, which had eight toes on each foot. These early species would have been mostly aquatic and used their limbs to navigate through swamps rather than undertaking long journeys on land.
Between 380 and 360 million years ago there is a period called “Romer’s gap”, in which barely any tetrapod fossils were found, casting a cloud of mystery over the evolution of the first amphibians from the first basal tetrapods. Prior to the gap, no fossils are found, and the earliest 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 earliest amphibians were temnospondyls, long-headed animals with a sprawling gait and distinctive appearance. These were the first true land tetrapods, and they would have gobbled up fools by consuming insects that lacked specialized adaptations to defend themselves against large vertebrate predators. Early temnospondyls were the size of a large fish, ranging from about 1.6 to 5 feet (0.5 to 1.5 meters) in length. The former had stumpy feet and probably couldn’t move very fast.
During the Carboniferous Period, temnospondyls grew in size and diversity to occupy many of the predatory and herbivorous niches that terrestrial animals exploit today. At the end of the Permian, some reached 30 meters in length and resembled crocodiles. This animal, Prionosuchus, is the largest known amphibian. In the Carboniferous, the temnospondyls were joined by the diverse but less numerous lepospondyls. Lissamphibians, the group that includes all modern amphibians and their common ancestors, emerged about 9 million years ago.
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