This post was updated February 8, 2017 with new scores for several basal tetrapod taxa and new taxa.
A new paper by Marjanovic and Laurin 2013 reviews the various hypotheses regarding the enigmatic origin of modern amphibians: frogs, salamanders and caecilians, together considered (by some, but not all paleontologists) members of a monophyletic clade, the Lissamphibia.
From their abstract (condensed)
“The origins of the extant amphibians (frogs, salamanders, caecilians) remain controversial after over a century of debate. Three groups of hypotheses persist in the current literature: the “temnospondyl hypothesis”… the “lepospondyl hypothesis”, and the “polyphyly hypothesis” according to which the frogs and the salamanders are temnospondyls while the caecilians are lepospondyls. We present a review of recent publications and theses in this field, several of which show more support for the Lepospondyl hypothesis.”
They conclude, “Still, a consensus will not be reached soon, despite the increasing range of data and types of analysis that are used.”
Also included at ReptileEvolution.com are Rana (the extant bull frog) and its ancestors, Triadobatrachus (a Triassic pre-frog) and Gerobatrachus (an early Permian pre-pre-frog and pre-salamander.) A sister to Gerobatrachus is Doleserpeton, also from the Early Permian.
An ancestral caeceilian, Eocaecelia, is also featured at ReptileEvolution.com.
All three find an ancestor in sisters to Utegenia (Fig. 2) then Seymouria and Eldeceeon. 32 is not a complete listing, nor a full gamut listing, by any means. Still, these nestings all appear to be good sister taxa without any “strange bedfellows” that just don’t seem to fit. In other words, the large reptile tree found a plausible solution with Utegenia near to the origin of modern amphibians plus two clades of microsaurs, each with 6 or 7 taxa.
Utegenia (Fig. 2) does not play a big part in the Marjanovic and Laurin (2013) study. Neither does Gephyrostegus. Marjanovic and Laurin (2013) separate ‘Amniota’ from Diadectomorpha, Solenodonsaurus and Westlothiana, but these are all amniotes in the large reptile tree.
Celtedens (Fig. 3) is one of the few complete skeletons attributed to a clade of near salamanders known as the Albanerpetontidae. First time I’ve heard of them was in the Marjanovic and Laurin (2013) paper.
Frogs and salamanders lack a jugal. The maxilla connects directly to the quadratojugal, if it connects to anything at all.
The lysorophian lissamphibian, Brachydectes, likewise has no jugal, but this snake-like amphibian nested closer to caecelians (in the large reptile tree) and apparently has lost its jugal by convergence.
Utegenia retains a jugal that separated the maxilla and quadratojugal, but it is also ancestral to microsaurs, which have a similar cheek pattern. Amphibamus retains a similar jugal. Cacops, however, reduces the jugal and strongly connects the maxilla and quadratojugal. Doleserpeton is more similar to modern amphibians in having a much smaller jugal. Gerobatrachus likewise has a gracile jugal and a lateral temporal fenestra. Celtedens (Fig. 3), an albanerpetontid close to salamanders, also has a jugal.
Marjanovic and Laurin (2013) report, “Be that as it may, palatal tusks are absent throughout the amniote-diadectomorph-lepospondyl clade (the tetrapod crown‑group according to the LH), with only three reversals (all of them among “microsaurs”)”. The large reptile tree found tusks absent in reptiles and lissamphibia + microsauria, but present in other tested non-amniotes, including Cacops.
According to Wikpedia , “Lepospondyli are a group of small but diverse Carboniferous to early Permian tetrapods. Six different groups are known, the Acherontiscidae, Adelospondyli, Aïstopoda, Lysorophia, Microsauria and Nectridea, and between them they include newt-like, eel- or snake-like, and lizard-like forms, along with species that do not fit any current category.” Here (fig.1) Adelospondyli and Lysorophia are related to Caecelians. Nectridea nest within the Microsauria and these are sisters to the ancestors of frogs and salamanders, Gerobatrachus and Doleserpeton. The other clades have not been tested.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Marjanovic D and Laurin M 2013. The origin(s) of extant amphibians: a review with emphasis on the “lepospondyl hypothesis.” GEODIVERSITAS • 2013 • 35 (1) online here
Caecelians by Darren Naish