Amphibian - Classification

Classification

Traditionally, the class Amphibia includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest sense (sensu lato) was divided into three subclasses, two of which are extinct:

• Subclass Labyrinthodontia† (diverse Paleozoic and early Mesozoic group)
• Subclass Lepospondyli† (small Paleozoic group, sometimes included in the Labyrinthodontia, which may actually be more closely related to amniotes than Lissamphibia)
• Subclass Lissamphibia (all modern amphibians, including frogs, toads, salamanders, newts and caecilians)
  • Superorder Salientia
    • Order Anura (frogs and toads): Jurassic to present—5,602 current species in 48 families
    • Order Caudata or Urodela (salamanders, newts): Jurassic to present—571 current species in 10 families
    • Order Gymnophiona or Apoda (caecilians): Jurassic to present—190 current species in 10 families

The actual number of species in each group depends on the taxonomic classification followed. The two most common systems are the classification adopted by the website AmphibiaWeb, University of California (Berkeley) and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database Amphibian Species of the World. The numbers of species cited above follow Frost and the total number of known amphibian species is approximately 7,000, of which nearly 90% are frogs.

With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a paraphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called Lissamphibia. The phylogeny of Paleozoic amphibians is uncertain, and Lissamphibia may possibly fall within extinct groups, like the Temnospondyli (traditionally placed in the subclass Labyrinthodontia) or the Lepospondyli, and in some analyses even in the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were formerly placed in Amphibia in Linnaean taxonomy, and included them elsewhere under cladistic taxonomy.

All modern amphibians are included in the subclass Lissamphibia, superorder Salientia, which is usually considered a clade, a group of species that have evolved from a common ancestor. The three modern orders are Anura (the frogs and toads), Caudata (or Urodela, the salamanders), and Gymnophiona (or Apoda, the caecilians). It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes. Although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is Prosalirus bitis, from the Early Jurassic Kayenta Formation of Arizona. It is anatomically very similar to modern frogs. The oldest known caecilian is Eocaecilia micropodia, also from Arizona, while the earliest salamander is Beiyanerpeton jianpingensis from the Late Jurassic of northeastern China.

Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. Practical considerations seem to favour using the former arrangement. The Lissamphibia, superorder Salientia, are traditionally divided into three orders, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of the Lissamphibia alongside the superorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, Triadobatrachus.