On the origin of and phylogenetic relationships among living amphibians

Abstract

The phylogenetic relationships among the three orders of modern amphibians (Caudata, Gymnophiona, and Anura) have been estimated based on both morphological and molecular evidence. Most morphological and paleontological studies of living and fossil amphibians support the hypothesis that salamanders and frogs are sister lineages (the Batrachia hypothesis) and that caecilians are more distantly related. Previous interpretations of molecular data based on nuclear and mitochondrial rRNA sequences suggested that salamanders and caecilians are sister groups to the exclusion of frogs. In an attempt to resolve this apparent conflict, the complete mitochondrial genomes of a salamander (Mertensiella luschani) and a caecilian (Typhlonectes natans) were determined (16,656 and 17,005 bp, respectively) and compared with previously published sequences from a frog (Xenopus laevis) and several other groups of vertebrates. Phylogenetic analyses of the mitochondrial data supported with high bootstrap values the monophyly of living amphibians with respect to other living groups of tetrapods, and a sister group relationship of salamanders and frogs. The lack of phylogenetically informative sites in the previous rRNA data sets (because of its shorter size and higher among-site rate variation) likely explains the discrepancy between our results and those based on previous molecular data. Strong support of the Batrachia hypothesis from both molecule-and morphology-based studies provides a robust phylogenetic framework that will be helpful to comparative studies among the three living orders of amphibians and will permit better understanding of the considerably divergent vertebral, brain, and digit developmental patterns found in frogs and salamanders.iving amphibians (Lissamphibia) are highly successful tetrapods that evolved diverse body plans that differ in modes of locomotion, reproductive specializations, and life histories (1, 2). For instance, the slender body of living salamanders (Caudata) has a well developed tail and proportionally paired limbs, whereas modern caecilians (Gymnophiona) are completely limbless, and are adapted to a fossorial lifestyle, with elongated bodies, protrusible tentacles, and reduced eyes. Extant frogs (Anura) lack tails, and evolved powerful hind limbs and a shortened, stiffened vertebral column (the urostyle)-a unique adaptation for jumping. The earliest fossils currently known of salamanders (Marmorerpeton; ref. , and demonstrate that all three lineages of extant amphibians acquired their peculiar body plan early on in their evolutionary history. The diversity among amphibians coupled with the lack of shared derived characters plus a poor fossil record complicate assessment of the phylogenetic relationships among the three living orders.Early workers on amphibian systematics repeatedly rejected the monophyly of Lissamphibia by proposing independent origins of the living orders of modern amphibians (see ref. 7 for a review). However, these studies failed to distinguish between ancestral and derived c...

The evolutionary position of turtles revised

“…5A) (Cao et al 2000). The phylogenetic analyses (with MP, NJ, and ML) of a molecular data set which included complete mitochondrial protein-coding, rRNA, and tRNA genes further supported the phylogenetic position of turtles as sister group of archosaurs (crocodiles+birds) (Zardoya and Meyer 2001) (Fig. 5B).…”

“…C Turtles have diapsid affinities, and are the sister group of Archosauria (e.g. Kumazawa and Nishida 1999;Zardoya and Meyer 2001). D Turtles have diapsid affinities, and are the sister group of Lepidosauria (e.g.…”

Initial Diversification of Living Amphibians Predated the Breakup of Pangaea

“…Paradoxically, some distributional patterns and some data from the fossil record (Estes and Wake 1972;Estes and Reig 1973;Rage and Rocek 1989;Jenkins and Walsh 1993;Duellman and Trueb 1994;Evans et al 1996;Evans and Borsuk-Bialynicka 1998;Rocek 2000) point at an initial divergence of living amphibians much earlier than the Mesozoic continental fragmentation of the Pangaea supercontinent. Moreover, alternative molecular phylogenies based on complete mitochondrial genomes (Zardoya and Meyer 2001;San Mauro et al 2004) support the "Batrachia" hypothesis ( ). Anura ϩ Caudata In order to test whether lissamphibian splits were triggered by Mesozoic continental breakup events, and to distinguish among competing hypotheses, we reconstructed a robust molecular phylogeny based on the RAG1 gene, encompassing for the first time a wide taxon sampling of major lissamphibian lineages.…”

“…The Lissamphibia are widely thought to be a monophyletic group, constituted by three monophyletic orders (Anura, Caudata, and Gymnophiona) whose origin and interrelationships remain hotly debated (see Meyer and Zardoya 2003 for a recent review). The poor fossil record of some major lissamphibian groups and the fact that the three living amphibian orders possibly acquired their specialized morphology very early in their evolutionary histories (Zardoya and Meyer 2001) have left many questions unresolved regarding the origins, relationships, and historical distribution of the Lissamphibia.A recent molecular phylogeny of lissamphibians based on mitochondrial rRNA genes grouped salamanders and caecilians to the exclusion of frogs and suggested that the early evolutionary history of living amphibians was associated with the Mesozoic continental fragmentation of the supercontinent Pangaea (Feller and Hedges 1998). Paradoxically, some distributional patterns and some data from the fossil record (Estes and Wake 1972;Estes and Reig 1973;Rage and Rocek 1989;Jenkins and Walsh 1993;Duellman and Trueb 1994;Evans et al 1996;Evans and Borsuk-Bialynicka 1998;Rocek 2000) point at an initial divergence of living amphibians much earlier than the Mesozoic continental fragmentation of the Pangaea supercontinent.…”

“…The Lissamphibia are widely thought to be a monophyletic group, constituted by three monophyletic orders (Anura, Caudata, and Gymnophiona) whose origin and interrelationships remain hotly debated (see Meyer and Zardoya 2003 for a recent review). The poor fossil record of some major lissamphibian groups and the fact that the three living amphibian orders possibly acquired their specialized morphology very early in their evolutionary histories (Zardoya and Meyer 2001) have left many questions unresolved regarding the origins, relationships, and historical distribution of the Lissamphibia.…”