Evolutionary Biology

pp 389-417

Developmental Evidence for Amphibian Origins

  • James HankenAffiliated withDepartment of Environmental, Population, and Organismic Biology, University of Colorado

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The origin of Recent amphibians remains one of the foremost unresolved puzzles in vertebrate paleobiology. One fundamental aspect concerns the phylogenetic relationships among the three extant orders, Anura (frogs), Caudata (salamanders), and Gymnophiona (caecilians). Several contradictory theories have been argued (Table I) [reviewed by Carroll and Holmes (1980); Gaffney (1979); Jarvik (1980); Løvtrup (1977); Saint-Au-bain (1981)]. At one extreme is the view that Recent amphibians are each other’s closest relatives among extant tetrapods, specifically that they comprise a single, monophyletic group, Lissamphibia, whose members shared a common amphibian ancestor relatively recently (e.g., Parsons and Williams, 1963). At the other extreme are views that deny monophyly of the Lissamphibia, and hold instead that modern amphibians represent at least two independent derivations of tetrapods from bony fishes (e.g., Jarvik, 1980). All views in this category entail a polyphyletic origin of amphibians, but they differ from each other in details, such as whether caecilians are more closely related to frogs or to salamanders (if indeed they consider caecilians at all), the particular fish lineage directly ancestral to each order, and phylogenetic relationships among the orders and the various amniote classes. (In this chapter I refer to all theories that entail a multiple origin of amphibians by the term polyphyly, including those that entail a dual origin, or diphyly.) Between these extremes are views that emphasize the distinctiveness of the modern orders but stop short of arguing for amphibian, and thus tetrapod, polyphyly (e.g., Carroll and Holmes, 1980).