Basic thoughts on amphibian body shapes

(writing in process)
  
Amphibians have three different shapes associated with three different orders: serpentine in Gymnophiona, lizard-like (and sometimes serpentine) in Urodela, and frog-like in Anura.
 
And the three orders of amphibians, with their extremely different body shapes, have different degrees of success in the world: frogs are extremely widespread, salamanders are largely restricted to the Northern Hemisphere, and caecilians are patchily and inconsistently distributed on three continents, mainly in the tropics.
 
What this means is that the serpentine amphibians, the caecilians, are so uncommon that even an ardent biologist might be forgiven for not even knowing that they exist.

Many people who stumble on caecilians actually take them for large earthworms, because they bear no resemblance to other amphibians, and are somewhat lost among the many convergently serpentine forms such as skinks, glass lizards, legless lizards, snakes, eels, and various serpentine salamanders. The serpentine salamanders are not widespread or common enough to alter the fact that serpentine amphibians are the exception rather than the rule.
 
Essentially, amphibians have a morphological spectrum of which the extremes are the leaping, long-legged typical frog on one side and the legless, serpentine caecilian on the other side. Both body forms are suited to both terrestrial and aquatic locomotion: frogs can both leap and swim using the same hindlegs, and caecilians can both slither on land and swim in the water (and burrow as well, as can the shorter-legged types of frogs).
 
Given this polarity and given the repeated evolution of the serpentine form in so many lineages of animals, in contrast with the fact that the anuran body form is restricted to frogs, my question is simply this: why has it been the frogs that are the common and widespread amphibians, rather than serpentine amphibians?
  
Thought-experiment:
Imagine that, everywhere that frogs occur today, there are instead serpentine amphibians, with tiny or no legs. And the frogs are restricted to the out-of-the-way places where caecilians today live. In this ‘parallel universe’, when anyone says ‘amphibian’, what springs to mind is a snake- or worm-like animal because that is the most successful and predominant form of amphibian in most places.
 
I can easily imagine such a world, because

• both the frog form and the serpentine form are about equally ‘bizarre’ or extreme as modifications of the generalised tetrapod body-plan seen in a typical salamander, and

• we are already used to encountering other wormlike or serpentine animals everywhere, e.g. earthworms, snakes, small-legged lizards, etc.
   
  So that is the basic question: why is your common-or-garden amphibian not a caecilian?
   
  For example, the only amphibian found in southern African afrotemperate forest (e.g. near Knysna), away from water, is a dumpy little frog (Breviceps), about the most aberrant of frogs worldwide, that burrows and eats earthworms. Why would a caecilian not be even better-designed for that niche?
   
  One realm where no serpentine amphibian seems to venture is in the trees, where tree frogs are common. Frogs include many climbers, whereas no serpentine salamander or caecilian seems able to climb. However, it is hard to know how to interpret this. Is it really easier for a frog to climb than for a serpentine amphibian to climb? Frogs are hardly agile in the trees, are they?
   
  It is interesting that both frogs and snakes include big-mouthed forms that can swallow wide prey whole. Caecilians have small mouths, something that relatively few frogs (e.g. ant- and termite-eating Microhylidae) possess. However, again it is easy to imagine mother Nature having given caecilians an elastic, snake-like mouth, not so?
   
  One clue is that amphibians tend to run cool, and have a limited capacity bask. This means that they are generally less energetic than reptiles including snakes. Perhaps at these rather low body temperatures the serpentine body form is inherently less powerful or enduring than the frog body form?
   
  Amphibians have very sensitive skin; they need to avoid abrasion of that skin because of the microbial dangers; a worm-like shape requires a lot of contact with a surface and therefore abrasion risk during movement.

I think terrestrial caecilians depend on constructed burrows, i.e. on passages that they have excavated once for repeated use. And indeed caecilians are odd among amphibians in having dermal scales scattered under the epidermis.

Come to think it, the whole hopping locomotion, typical of frogs and toads, could be a way of minimising abrasion because it tends to get the animal over obstacles rather than through them. In the trees, even those tree frogs that do not hop can use their relatively long legs to minimise friction.

A novel concept for readers: a frog is not just a leaping animal but a friction-minimising one? And salamanders manage despite their large frictional surface, because they run so cold that they tend to be inactive compared with frogs?

(writing in progress)

Publicado el 21 de junio de 2022 por milewski