A PAPER PUBLISHED recently in Nature details the discovery of a common ancestor of salamanders and frogs, Gerobatrachus hottoni, by Anderson and coworkers. This creature had a salamander-like build, but has a broadened skull like frogs. A variety of traits were studied to determine this organism’s relationships, such as the teeth, various skull bones, presacral spine, and otic notch. Its position in the early Permian places the frog/salamander divergence in the Middle Permian, about 270-260 million years ago. Of course this find is interesting, but I was perhaps more interested by the phylogenetic tree that they composed including this new species.
There is some debate about whether Lissamphibia, the group containing all extant amphibians, is monophyletic or not. The temnospondyls and lepospondyls are two major amphibian groups that diverged about 328-335 million years ago. Some paleontologists consider lissamphibians to have evolved from temnospondyls, while others take the opposite tack and consider them as having evolved from lepospondyls. Still others think that there is a split in Lissamphibia, with some members of the group descended from temnospondyls while others descended from lepospondyls.
This is the picture that Anderson and coworkers present. They suggest that the caecilians, a group of unusual legless amphibians with a tendency towards fossoriality, originated within the lepospondyls, while the other lissamphibians (including Gerobatrachus) evolved from temospondyls. Their phylogenetic tree is shown here (Reprinted by permission from Macmillan Publishers Ltd: Nature 453, 515-518, copyright 2008).
They place Batrachia (frogs + salamanders) in Temnospondyli as a sister group to Gerobatrachus. The caecilians, represented by Eocaecilia, appear in Lepospondyli. While this tree is the most parsimonious of the trees constructed (trees for Lissamphibia monophyletic in Temnospondyli and Lepospondyli were also constructed) the bootstrap values are lower than desirable.
The relationships of the amphibians are controversial and will probably remain so for some time. If it is ultimately determined that this phylogenetic tree is correct, there is the question of how to keep Lissamphibia monophyletic. Traditionally Lissamphibia has been limited to extant amphibians, and expanding Lissamphibia to include caecilians would bring in many extinct temnospondyls and lepospondyls. The situation might be more easily dealt with by expelling caecilians from Lissamphibia, or moving entirely away from Lissamphibia as a clade. Perhaps “lissamphibian”, like “reptile”, will remain as a holdover of Linnaean taxonomy.
Anderson, J.S., Reisz, R.R., Scott, D., Frabisch, N.B., Sumida, S.S. (2008). A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature, 453(7194), 515-518. DOI: 10.1038/nature06865