Most modern amphibians are small, but giant amphibians were once dominant predators on Earth. They began to decline when reptiles evolved, and became almost extinct 251 million years ago. Only the Lissamphibia subclass of amphibians still exist, and there is debate about their evolution. Giant amphibians were part of the Labyrinthodontia group, with the most common body plan being a large lizard with tentacled legs and an oversized head and jaws. The largest amphibian ever to live was Prionosuchus, up to 9 m (30 ft) long, resembling a crocodile.
Today, most amphibians, such as frogs, toads, salamanders, and newts, are relatively small, usually under 18 cm (7 in) in size. The largest living amphibian, the Chinese giant salamander, is significantly larger and can reach 1.83 m (6 ft). However, this is very atypical for modern amphibians.
Long ago, before sophisticated amniotes (animals with eggs that can be laid outside water) such as proto-crocodilian reptiles evolved, amphibians were the dominant tetrapods on Earth and included top predators. This period began when amphibians first evolved, in the late Devonian/early Carboniferous, about 360 million years ago. Giant amphibians had their heyday in the Carboniferous and began to experience a decline in the Early Permian, when bigger and better reptiles, such as pelycosaurs, evolved. They became almost completely extinct at the Permian-Triassic extinction event 251 million years ago, but survived in small pockets in what is now Australia and China until 120 million years ago.
Throughout the history of life on earth, there have been three subclasses of amphibians: Lissamphibia, which includes all modern amphibians, lepospondyls, an ancient Paleozoic group of relatively small amphibians, and labyrinthodonts, which includes all giant amphibians, and many others. Of these, only the Lissamphibia are still alive today. There is much debate as to whether modern amphibians evolved from one of these ancient groups or independently acquired their modern characteristics.
Some giant amphibians have been called “killer newts” due to their superficial resemblance to modern newts. The group to which all giant amphibians belonged, Labyrinthodontia, is so named because the dentin and enamel wrapping on their teeth resembles a labyrinth in appearance. The most numerous labyrinthodonts were called temnospondyls, which diversified to fill many empty terrestrial niches. The most common body plan for these animals was as a very large lizard with tentacled legs and an oversized head and jaws. Some had long, slender bodies and others had short, stocky bodies.
Most of the giant amphibians were between 2 and 4 meters in length, although some, such as Prionosuchus, were up to 9 m (30 ft) long, the largest amphibian ever to live, and would have been one of the largest living complex animals at the time. This giant amphibian showed a great deal of convergence with the crocodile, which it superficially resembled.
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