A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders

Abstract

The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record. Current discussions focus on three competing hypotheses: a monophyletic origin within either Temnospondyli or Lepospondyli, or a polyphyletic origin with frogs and salamanders arising among temnospondyls and caecilians among the lepospondyls. Recent molecular analyses are also controversial, with estimations for the batrachian (frog-salamander) divergence significantly older than the palaeontological evidence supports. Here we report the discovery of an amphibamid temnospondyl from the Early Permian of Texas that bridges the gap between other Palaeozoic amphibians and the earliest known salientians and caudatans from the Mesozoic. The presence of a mosaic of salientian and caudatan characters in this small fossil makes it a key taxon close to the batrachian (frog and salamander) divergence. Phylogenetic analysis suggests that the batrachian divergence occurred in the Middle Permian, rather than the late Carboniferous as recently estimated using molecular clocks, but the divergence with caecilians corresponds to the deep split between temnospondyls and lepospondyls, which is congruent with the molecular estimates.

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).…”

“…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). Amphibamids and branchiosaurids are represented by very small, miniaturized taxa which share a large number of characters with modern amphibians.…”

The largest specimen ofApateonand the life history pathway of neoteny in the Paleozoic temnospondyl family Branchiosauridae

“…In a closely related group of small dissorophoid amphibians, the terrestrial Amphibamidae, carpals and tarsals are frequently ossified (e.g. Daly 1994;Schoch & Rubidge 2005;Anderson et al 2008) and it seems likely that the mesopodial region of metamorphosed branchiosaurids ossified as well. In comparison, the low degree of ossification and the lack of distinct processes and pronounced muscle attachment areas in the limb elements of specimen GPIM-N-9u contrasts strongly with the condition found in the metamorphs of A. gracilis.…”