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Lissamphibia is the only surviving subclass of amphibians, with three orders: frogs/toads, salamanders, and caecilans. The earliest amphibians were basal temnospondyls, evolving from lobe-finned fishes. The origin and relationship between groups of Lissamphibia is unclear, with possible ancestors proposed from temnospondyls or lepospondyls. The origins of tetrapods are also mysterious due to “Romer’s Gap,” but new fossil sites have been found.
Lissamphibia is the only surviving subclass of amphibians: the other two, Lepospondyli (lepospondyls) and Labyrinthodontia (labyrinthodonts) became extinct in the Permian (290 million years ago) and Cretaceous (120 million years ago) respectively. Labyrinthodonts were already in decline due to the Permian-Triassic (251 million years ago) and especially Late Triassic (210 million years ago) extinctions, around the same time that dinosaurs became dominant. Lissamphibia consists of three orders: frogs/toads, salamanders, and limbless caecilans.
The earliest known members of the subclass Amphibia are basal temnospondyls dated to 340 million years ago, in the early Carboniferous. These fish-like tetrapods had sprawling positions that put them close to the ground and were relatively large, 3-5 feet long, like the lobe-finned fishes from which they evolved. They expended so much energy walking that they would often be exhausted and in need of rest. Luckily for them, since carnivorous tetrapods hadn’t evolved yet, they could rest in peace for a while.
The earliest known member of Lissamphibia, the ‘ancient Hotton frog’, also known as the ‘frogamander’, is dated to the early Permian, 290 million years ago. As the name indicates, it has a combination of salamander and frog-like characteristics. However, even though this is known, scientists are relatively confused about the details of the origin and relationship between groups of Lissamphibia.
Every possible ancestor has been proposed: that Lissamphibia is a clade within temnospondyls (an abundant type of labyrinthodont), a clade within lepospondyls, or that caecylans originate within lepospondyls and that frogs and salamanders originate in temnospondyls. Due to the large number of features shared among all members of Lissamphibia, most scientists recognize that they are a natural clade, making the latter possibility the least likely. Some scientists tend to assume that Lissamphibia evolved from temnospondyls, because they were more diverse and numerous during the period in question (the Carboniferous) than leposondyls. However, lepospondyls were considerably smaller, so it may have made sense for a clade of small amphibians to evolve from another group of small amphibians, rather than from a group that is significantly larger on average.
Due to a period in the fossil record when few tetrapod fossils were found, known as “Romer’s Gap” (360-340 mya), the origins of tetrapods are somewhat mysterious. Recently, however, new fossil sites have been found with specimens from this elusive period. We have much to learn about this foggy period in ancient evolutionary history.
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