Lissamphibia

The lissamphibians (Lissamphibia), also known as modern amphibians, are a clade of tetrapods that, as the common name indicates, includes all extant amphibians, which are represented by more than 7,420 species. The group is made up of the clades Gymnophiona (caecilians), Caudata (salamanders), Salientia (frogs), and the extinct group Albanerpetontidae. They share numerous characteristics, including the presence of pedicel teeth in which the base and crown are separated by an area of fibrous tissue, reduced ribs and generally welded to the vertebral transverse processes, loss of the jugal bone, presence in the middle ear of a sensory element to capture low-frequency vibrations (papilla amphibiorum), vascularized skin as an adaptation to cutaneous respiration, fatty bodies associated with the gonads, etc.

Evolution and systematics

The origin of the group is still uncertain, and current hypotheses can be divided into three main categories. In the first, Lissamphibia is considered a monophyletic group derived from the temnospondyles, in which case the sister group may be the genus Doleserpeton , Doleserpeton and Amphibamus, Branchiosauridae or a subgroup of the latter. Although it is possible that these similarities of certain modern amphibians (Batrachia) with species of the Dissorophoidea group, such as Doleserpeton, correspond to simplesiomorphic conditions of tetrapods. On the other hand, the second hypothesis also establishes Lissamphibia as a monophyletic group, but derived from lepospondyls, while the third hypothesis suggests a polyphyletic (diphyletic and in some studies triphyletic) character of lissamphibians, with frogs and salamanders originating from temnospondyles, while caecilians (and sometimes salamanders) derive They belonged to lepospondyls.

This last hypothesis was reinforced by the discovery of the species Eocaecilia micropodia, a caecilian from the Lower Jurassic that was associated with the genus Rhynchonkos (Lower Permian) of the group Microsauria. However, this view of a relationship between Eocaecilia and Microsauria has been rejected by some researchers, who argue that the apomorphies attributed to that relationship are homoplastic, reflecting convergent adaptations to a lifestyle fossorial. On the other hand, a monophyletic origin of modern amphibians with respect to current lineages of amniotes is strongly supported by molecular analyses. Therefore, if lepospondyls are a group closer to amniotes, the The hypothesis of a polyphyletic origin can be indirectly rejected since it would require a closer relationship of the caecilians with the amniotes than with the Batrachia clade.

Kumar & Hedges (1998) estimated the origin of lissamphibians at 360 Ma, while Zhang et al. (2005) determined it at 337 Ma. These findings were questioned due to contradiction with the fossil record. of the tetrapods of the Paleozoic and because they would favor the hypothesis of polyphyletic origin. The records of lissamphibian fossils are scarce (the oldest being those of Triadobatrachus and Czatkobatrachus) and are found in much younger geological ages than these results suggest. Recent molecular studies have reduced this discrepancy with the fossil record, rescuing some analyzes a date between 322 and 274 Ma respectively, while others estimate the origin of the lissamphibians at 294 Ma and that of the Batrachia clade at 264 Ma, being this last date close to the records of Triadobatrachus (250 Ma) and Czatkobatrachus. These estimates are generally controversial, with a difference of between 87 and 103 Ma between the oldest and youngest estimates respectively, although as noted, recent studies have tended to favor a more recent divergence.

Cladogram based on Marjanovic & Laurin (2007).