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.
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.
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.
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.
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.
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...
- Aquatic proto-people and the theory hypothesis of initial bipedalism
- Goodbye from the stem-haematotherm, goodbye from me
- The ‘Birds Come First’ hypothesis of dinosaur evolution
- Birds Come First – oh no they don’t!
- We flightless primates
- Amphisbaenians and the origins of mammals
- The ‘Tree-Kangaroos Come First’ hypothesis
- The Haematothermia hypothesis
And for previous Tet Zoo articles on frog evolution, see…
- In pursuit of Romanian frogs (part I: Bombina)
- In pursuit of Romanian frogs (part II: WESTERN PALAEARCTIC WATER FROGS!!)
- In pursuit of Romanian frogs (part III: brown frogs)
- The toads series comes to SciAm: because Africa has toads too
- 20-chromosome toads
- Glassfrogs: translucent skin, green bones, arm spines
- Everybody loves glassfrogs
- African tree toads, smalltongue toads, four-digit toads, red-backed toads: yes, a whole load of obscure African toads
- Parsley frogs: spadefoots without spades
- Megophrys: so much more than Megophrys nasuta
- North American spadefoot toads and their incredible fast-metamorphosing, polymorphic tadpoles
- Tadpole nests, past and present
- Gladiatorial glassfrogs, redux
- Frogs you may not have heard of: Brazil’s Cycloramphus‘button frogs’
- There is so much more to flying frogs than flying frogs
- ‘Strange bedfellow frogs’ (part I): rotund, adorable brevicipitids
- It’s the Helmeted water toad… this time, with information!
- A brief introduction to reed, sedge and lily frogs
- ‘Strange bedfellow frogs’ (part II): pig-nosed or shovel-nosed frogs, or snouts-burrowers
- Squeakers: Frogs with Claws, Frogs with "Hair"
- The Remarkably Weird Skeletons of Frogs
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.
Sanchiz, B. 1998. Salientia. Handbuch der Paleäoherpetologie. Dr. F. Pfeil, Munich.