The palaeozoic ancestry of salamanders, frogs and caecilians

The relationships of frogs, salamanders, and caecilians (Gymnophiona) with one another and with the vast assemblage of Palaeozoic amphibians remain highly contentious phylogenetic problems. Cladistic analyses support a common ancestry of the three modern orders, but fail to achieve a consensus regarding their affinities with Palaeozoic amphibians. The most exhaustive phylogenetic analyses that have been applied to the ancestry of lissamphibians have recognized few, if any, biologically significant characters differentiating the living orders. These results can be attributed to limiting the database primarily to characters common to Palaeozoic amphibians and including few features that distinguish the modern orders. Making use of the numerous derived characters that are expressed in either the larvae or adults of extant salamanders, frogs, and caecilians provides the basis for recognizing a nested sequence of synapomorphies that support a common ancestry of salamanders and anurans with temnospondyl labyrinthodonts to the exclusion of caecilians. The larvae of Carboniferous and Permian temnospondyl labyrinthodonts provide strong evidence for their being members of the stem group of urodeles. This is based primarily on the great similarity in the sequence of ossification of the bones of the skull and appendicular skeleton, but is also supported by detailed similarities of the hyoid apparatus. Recognition of a sister-group relationship between Permo-Carboniferous branchiosaurids and crown-group salamanders makes it possible to determine the sequence of changes in the anatomy and ways of life that occurred during the origin of urodeles, and to determine their time of divergence relative to that of frogs and caecilians. The Lower Triassic salientian Triadobatrachus exhibits early stages in the evolution of the anuran skull that enable close comparison with Palaeozoic dissorophoid amphibians and point to the early evolution of anuran hearing and vocalization. The Lower Jurassic caecilian Eocaecilia shows few if any unique derived characters in common with salientians or caudates, but the anatomy of the skull and the elongate body form point to affinities with the Permian microsaur Rhynchonkos. (C) 2007 The Linnean Society of London.