I’m a big fan of what we charitably call ‘non-standard hypotheses’ in evolutionary biology, and the good news is that you are too. Non-standard hypotheses of several sorts – pertaining to various distantly related tetrapod groups – have been discussed here over the years (see the list below) and some definitely need revisiting.

This is why we're here. Amphibians. This is a captive Mountain chicken Leptodactylus fallax I photographed at Chester Zoo, UK. Credit: Darren Naish

But there’s one cluster of non-standard hypotheses that haven’t ever been covered here, namely, those that pertain to living amphibians (aka lissamphibians). I’m busy with amphibians at the moment since I’m currently completing the large chapter on them that will appear in my giant textbook (on which go here). And it isn’t lost on me that an enormous number of things worth saying about amphibian evolution just aren’t covered online at all. It’s almost as if nobody cares. Well, we can do better than that. To business…

Did frogs evolve more than once? The case of Triadobatrachus. Frogs and toads – the anurans – are among the weirdest and most distinctive of all tetrapods, an idea I hopefully conveyed in the recent article on their incredible skeletal anatomy. There is nothing quite like an anuran, and for this reason you might think that their monophyly (the idea that all species, ancient and modern, descend from the same single ancestor) has never really been doubted. Wrong. Sort of.

Rage & Roček's(1989) diagram depicting casts of the holotype specimen of the Early Triassic stem-frog Triadobatrachus: the original is preserved in a concretion that was split in half, meaning that both a dorsal half and ventral half are preserved... though both are moulds with no actual bone present. Triadobatrachus is small, less than 10 cm long. Credit: Rage & Roček 1989

A few very ancient relatives, ancestors or near-ancestors of anurans are known from the fossil record of the Mesozoic, and by far the most familiar is Triadobatrachus from the Early Triassic of Madagascar. Triadobatrachus shares a list of features with anurans (a fused frontoparietal in the skull, elongated shaft-like iliac bones in the pelvis and a shortened vertebral column: we looked at all of these in the previous article) and consequently has generally been considered a near-relative of the group, current convention being to recognise the clade Salientia for Triadobatrachus, anurans, and any additional descendants of their most recent common ancestor.

Rage & Roček's (1989) skeletal reconstruction of Triadobatrachus. Note that the hands and feet are mostly missing. Credit: Rage & Roček 1989

But the weirdness of Triadobatrachus relative to anurans has led to the occasional suggestion that the similarity might be convergent: that Triadobatrachus might be some kind of anuran-mimicking…. other thing (Hecht 1962). This idea was considered favourably by Sanchiz (1998) in what remains the standard work on the anuran fossil record. Given how weird the salientian body plan is, the idea that it might have evolved more than once appears radical (from the tetrapod perspective; yeah yeah invertebrates…). But this idea isn’t considered all that favourably today: Triadobatrachus may not be a true frog, but it exhibits a more froggy degree of frogishness than any non-frogs. Hey, the word frog sounds really weird. Say it out loud, slowly, right now, and you’ll see what I mean.

At left, Triadobatrachusas reconstructed by Rage & Roček (1989). At right, a speculative life reconstruction. Some thought went into the details you see here (full discussion at my patreon). Note that I avoided the urge to make this animal look incredibly skinny (as you might think based on its skeleton), and also that it most likely had four fingers, not five. Credit: Rage & Roček 1989, Darren Naish

Did frogs evolve more than once? The case of tailed frogs and New Zealand frogs. Even more radical is the idea that two living frog groups – typically considered ancient, archaic members of Anura – might, similarly, be convergent mimics of true anurans, and not proper frogs at all. The groups concerned are the North American tailed frogs (or ascaphids) and the New Zealand frogs (or leiopelmatids) (the two are included in the same ‘family’ by some authors, and the same higher clade – Leiopelmatoidea – by others); the idea that they might have evolved separately from true anurans was proposed by Griffiths (1963) on the basis of information from the vertebral column.

Are New Zealand frogs and tailed frogs (both shown at right) of distinct ancestry from all other frogs? No, but the idea was out there for a while. Credit: Darren Naish

Griffiths’ idea was actually quite subtle, and involved ascaphids and leiopelmatids evolving from ‘proanurans’; that is, from animals that were also ancestral to anurans proper. In that case, the proposal concerns what’s known as parallelism rather than convergence (parallelism is where similar organisms have evolved from closely related ancestors; convergence is where those ancestors are distantly related. There is, of course, some subjectivity in determining which of the two has occurred in some cases). Today this idea has mostly been forgotten. Morphological and molecular data shows that ascaphids and leiopelmatids are indeed card-carrying members of Anura (Frost et al. 2006, Pyron & Wiens 2011).

More of this sort of thing in the next article. For previous Tet Zoo articles on‘non-standard hypotheses’, see...

And for previous Tet Zoo articles on frog evolution, see…

Refs - -

Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., De Sá, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green, D. M. & Wheeler, W. C. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297, 1-370.

Griffiths, I. 1963. The phylogeny of the Salientia. Biological Reviews 38, 241-292.

Hecht, M. K. 1962. A reevaluation of the early history of the frogs. Part I. Systematic Zoology 11, 39-44.

Pyron, R. A Wiens, J. J. 2011 A large-scale phylogeny of Amphibia including over 2,800 species, and a revised classification of extant frogs, salamanders, and caecilians. Molecular Phylogenetics and Evolution 61, 543-583.

Rage, J.-C. & Roček, Z. 1989. Redescription of Triadobatrachus massinoti (Piveteau, 1936) an anuran amphibian from the Early Triassic. Palaeontographica Abteilung A 206, 1-16.

Sanchiz, B. 1998. Salientia. Handbuch der Paleäoherpetologie. Dr. F. Pfeil, Munich.