Focal Review: The Origin(s) of Modern Amphibians

Abstract

The recent description of the stem batrachian Gerobatrachus has changed the terms of the ongoing debate on the origin of extant amphibians (Lissamphibia: frogs, salamanders, and the limbless caecilians). This important fossil, through a shared mosaic of unique derived salientian and urodele characters, links frogs and salamanders with an archaic group of fossil amphibians known as amphibamid temnospondyls. The present paper reviews the impact of this fossil on morphological and molecular phylogenies, and divergence timing estimates based on molecular models and the fossil record. In morphology, most recent efforts have focused on better characterizing the anatomy and relationships of amphibamid temnospondyls. Progress has also been made with the complete description of the earliest caecilian Eocaecilia; however, the question of caecilian origins remains unresolved at present. The large scale phylogenetic analyses all agree on the overall tetrapod tree phylogenetic structure, and the largest analyses agree that the origin of at least frogs and salamanders among fossils from family Amphibamidae. Conversely, all molecular based analyses find a monophyletic Lissamphibia, and a Batrachia terminal dichotomy, which raises questions over either the validity of morphological analyses that support lissamphibian polyphyly or about the possibility of long branch attraction given the short internal divergences and long subsequent branches. Paradoxically, the estimated date of the lissamphibian divergence best matches the fossil record if timed to the split between lepospondyls and temnospondyls. Future research should focus on development and fine details of cranial anatomy of fossil and extant amphibians to produce new evidence and clarity into the question of lissamphibian, and especially caecilian, origins.