The origin of Lissamphibia (frogs, salamanders, caecilians)

The origin of modern amphibians has been controversial.
A new paper by Pérez-Ben et al. 2018 seeks to clarify the issue. According to Wikpedia, “Currently, the three prevailing theories of lissamphibian origin are:

  1. Monophyletic within the temnospondyli
  2. Monophyletic within lepospondyli
  3. Diphyletic (two separate ancestries) with apodans within the lepospondyls and salamanders and frogs within the temnospondyli.”

From the Pérez-Ben et al. abstract:
Current hypotheses propose that the living amphibians (lissamphibians) originated within a clade of Paleozoic dwarfed dissorophoid temnospondyls. Morphological traits shared by these small dissorophoids have been interpreted as resulting from constraints imposed by the extreme size reduction, but these statements were based only on qualitative observations. Herein, we assess quantitatively morphological changes in the skull previously associated with miniaturization in the lissamphibian stem lineage by comparing evolutionary and ontogenetic allometries in dissorophoids. Our results show that these features are not comparable to the morphological consequences of extreme size reduction as documented in extant miniature amphibians, but instead they resemble immature conditions of larger temnospondyls. We conclude that the truncation of the ancestral ontogeny, and not constraints related to miniaturization, might have been the factor that played a major role in the morphological evolution of small dissorophoids.

The authors appear to be dividing
tiny (miniaturized) frogs from frogs in general (= immature temnospondyls). And that’s a good start.

The second hypothesis (above)
is supported and recovered in the large reptile tree (LRT (1154 taxa, subset in figure 1) in which Lissamphibians are indeed derived from dissorophids, but dissorophids are lepospondyls (yellow-green clade below), which are derived from reptilomorphs and seymouriamorphs (orange clade below), while temnospondyls are much more primitive and diphyletic (pink and blue clades below). The phylogenetic miniaturization occurred much earlier than Lissamphibia, which is a much larger clade if it is still defined by the inclusion of the more distantly related caecilians, deep within the Microsauria.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1.

Quantitive approaches
have never trumped phylogenetic approaches.

First: Recover the cladogram.
Let it tell you what happened, and when and how. The dissorophids are indeed derived from more primitive temnospondyls, but several intervening transitional clades must be accounted for. 

References
Pérez-Ben CM, Schoch RR and Báez  AM 2018. Miniaturization and morphological evolution in Paleozoic relatives of living amphibians: a quantitative approach
https://doi.org/10.1017/pab.2017.22Published online: 23 January 2018

6 thoughts on “The origin of Lissamphibia (frogs, salamanders, caecilians)

  1. A new paper by Pérez-Ben et al. 2018 seeks to clarify the issue

    Not at all, no. The authors explicitly take for granted that the lissamphibians are dissorophoid temnospondyls; then they separately investigate size reduction within Dissorophoidea and within Lissamphibia, and find that 1) even the smallest non-lissamphibian dissorophoids are not miniaturized in ways that could be distinguished from a bit of paedomorphosis, and 2) some of the features that are probably related to small size in caecilians and batrachians must have evolved independently in the two groups, because Eocaecilia retains their plesiomorphic states. These, especially the first, are very important results derived from a robust investigation; but they say nothing about where the lissamphibians come from, and that’s not the paper’s intent.

      • They take it for granted, which means 1) they say nothing new about it and 2) their actual analyses, which don’t test it, say nothing about it. I thought that was obvious?

  2. Oh. I just took a look at your tree. Lysorophus is Brachydectes – the type of the type species of L. is 2 1/2 undiagnostic vertebrae, and all the rest of the material that was referred to L. before about 1991 is now confidently referred to B.. Wellstead (1991) and Pardo & Anderson (2016) are freely accessible online, legally even.

  3. That may be so. The two specimens do nest together in the LRT. I note that Pardo and Andeerson do not employ AMNH 6172, which is distinct enough from KUVP 49541 that the two are both listed here.

    • Trust me, they’ve seen that specimen. If they don’t explicitly mention it, that’s because they didn’t find a point in doing so.

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