A reevaluation of the evidence supporting an unorthodox hypothesis on the origin of extant amphibians

Abstract

The origin of frogs, salamanders and caecilians is controversial. McGowan published an original hypothesis on lissamphibian origins in 2002 (McGowan, 2002, Zoological Journal of the Linnean Society, 135: 1-32), stating that Gymnophiona was nested inside the ‘microsaurian’ lepospondyls, this clade was the sister-group of a caudate-salientian-albanerpetontid clade, and both were nested inside the dissorophoid temnospondyls. We have investigated McGowan’s data matrix and disagree with the scoring of 35% of the cells. All taxa and all but two characters are affected. In some cases, we have a different interpretation about correspondence between morphology and character states, or we delimit states differently (or use information that was unknown in 2002). In others, we report probable typographic errors. When these cells and characters are revised, the most parsimonious trees – now longer by almost 64% – support one of the three commonly advocated hypotheses, namely a monophyletic Lissamphibia nested, together with its sister-group Albanerpetontidae, within the temnospondyls (next to Doleserpeton) – even though we did not add any characters or taxa to the very small data matrix. This exemplifies the impact of errors in data matrices on the results of phylogenetic analyses. Adding the lysorophian Brachydectes, however, results in the Lissamphibia-Albanerpetontidae clade becoming the sister-group of Brachydectes and settling within the lepospondyls rather than the temnospondyls, thus supporting another of the previously published three hypotheses. This latter finding does not change if the recently described Gerobatrachus is also added. Finally, when Doleserpeton is interpreted as morphologically immature (which means scoring three characters as unknown instead of known), Lissamphibia and Albanerpetontidae are again nested within the ‘microsaurian’ lepospondyls, even though Brachydectes is not included in this analysis. This, too, does not change if Gerobatrachus is added and likewise treated as morphologically immature. Bootstrap supports are rather low under all assumptions. Such lability was to be expected from the small size of the data matrix.

The Origin(s) of Modern Amphibians: A Commentary

“…Anderson favors the ''polyphyly hypothesis'' (PH), which considers the extant amphibians to be polyphyletic with respect to many Paleozoic limbed vertebrates and was most recently supported by the analysis of Anderson et al (2008). We prefer the ''lepospondyl hypothesis'' (LH-lissamphibians nested within ''lepospondyls''; most recently supported by Vallin andLaurin 2004 andLaurin 2008a). We prefer the ''lepospondyl hypothesis'' (LH-lissamphibians nested within ''lepospondyls''; most recently supported by Vallin andLaurin 2004 andLaurin 2008a).…”

Linked morphological changes during palate evolution in early tetrapods

“…This linkage generated an increase in the size of this skull element in the temnospondyls, and a concurrent decrease in the anthracosaur‐stem amniote‐lepospondyl lineage. Stem‐amniotes gave rise to mammals, modern ‘reptiles’ and birds, whereas modern amphibians may have originated in either the temnospondyls or lepospondyls (reviews: Ruta & Coates, 2007; Anderson, 2007, 2008; Marjanovic & Laurin, 2008). Such linked morphological changes, in ways not understood at present, may have been critical for the success of these groups.…”

“…Laurin, 2001; Clack, 2002; Ruta et al. 2003; Ruta & Coates, 2007; Marjanovic & Laurin, 2008). For example, whereas there is a reasonable consensus regarding the crownward end of the amniote stem group, the identity of the Palaeozoic tetrapods that gave rise to modern amphibians remains controversial (Laurin, 1998; Ruta & Coates, 2007; Anderson, 2007, 2008; Marjanovic & Laurin, 2008).…”

“…2003; Ruta & Coates, 2007; Marjanovic & Laurin, 2008). For example, whereas there is a reasonable consensus regarding the crownward end of the amniote stem group, the identity of the Palaeozoic tetrapods that gave rise to modern amphibians remains controversial (Laurin, 1998; Ruta & Coates, 2007; Anderson, 2007, 2008; Marjanovic & Laurin, 2008). Still, our understanding of the phylogenetic history of this interesting period of vertebrate evolution is becoming quite detailed, despite the fact that no species survive as evolutionary relicts that are at all closely related to the earliest tetrapods.…”

Urodelans, Ichthyostega and the origin of the tetrapod limb

“…These facts confirm that the element at the base of the first two digits in these amphibians was initially single or this is a recapitulation of the primitive condi tion. Tetrapods possessing the basale commune existed already in the Early Permian (Anderson et al, 2008, contra Marjanovic andLaurin, 2008). Thus, the Ichthyostega limb can be described as a preaxially curved axis consisting most likely of four mesomeres and preaxial and postaxial groups of lateral articulated unbranched rays (digits).…”