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:
- Monophyletic within the temnospondyli
- Monophyletic within lepospondyli
- 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.
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.
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