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.

Is solid always best? Cranial performance in solid and fenestrated caecilian skulls

“…Although we did not gather data that directly bears on the question of caecilian ancestry [e.g. temnospondyl versus lepospondyl origins (see Anderson, 2008)], we have shown that with respect to the current utility of the skull during burrowing, a stegokrotaphic skull does not result in improved performance over the zygokrotaphic skull. This suggests that if stegokrotaphy is the derived condition within caecilians, it did not evolve primarily as an adaptation to burrowing.…”

Salamander limb development: Integrating genes, morphology, and fossils

“…6). This is of particular interest when considered in the framework of the controversial origin(s) and relationships of modern amphibians for which three scenarios are being discussed in the literature: the polyphyly hypothesis proposes a sister taxon relationship between salamanders and branchiosaurids, and frogs and amphibamids, respectively, while caecilians are considered most closely related to microsaurs within lepospondyls (e.g., Carroll, 2007;Anderson et al, 2008;Anderson, 2008;). In the most widely accepted temnospondyl hypothesis, modern amphibians form a monophyletic clade (''Lissamphibia'') nested within temnospondyls and most closely related to amphibamids or branchiosaurids (e.g., Bolt, 1991;Milner, 1993;Ruta et al, 2003;Schoch and Milner, 2004;Ruta and Coates, 2007;Sigurdsen and Bolt, 2009).…”

New specimen of <i>Cacops woehri</i> indicates differences in the ontogenetic trajectories among cacopine dissorophids

“…Dissorophoidea is a large clade that was very successful during the Paleozoic and continues to be so today, as most authors consider some or all three groups of modern amphibians to be part of Dissorophoidea (Anderson, 2008;Anderson et al, 2008b;Milner, 1988Milner, , 1993Schoch and Milner, 2004;Sigurdsen and Green, 2011), but see, for example, Marjanović and Laurin (2013) for a different view. Recent phylogenetic analyses have recovered two distinct subclades within fossil dissorophoids, one comprising the Olsoniformes (dissorophids and trematopids) and one comprising the Xerodromes (amphibamids and branchiosaurids) (Anderson et al, 2008b;Fröbisch and Schoch, 2009;Schoch, 2012;Schoch and Milner, 2014).…”