Category: Amphibians
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The name ‘amphibian’ refers to the two-stage life cycles of all these animals, typically involving metamorphosis from an aquatic larval stage to a terrestrial adult (with constantly wet skin). Amphibians evolved in the early Carboniferous as descendents of forms such as the famous Late Devonian "tetrapods" Ichthyostega and Acanthostega, named for their four limbs with true digits. These early tetrapods appeared in the Late Devonian around 365Ma, having evolved from lobe-finned fish similar to Panderichthys (380Ma) and the highly tetrapod-like Tiktaalik (375Ma). The earliest "true" amphibians included forms such as Eryops (Permian), "microsaurs" and limbless aistopods (Carboniferous). By the late Triassic today's amphibian groups were established, including the frogs (anurans) with their large back legs for jumping and reduced tails, salamanders (which include the newts), with four limbs and a long tail, and the unusual limbless caecilians that typically burrow and inhabit the tropics.
Amphibians are a rich source of evolutionary insights and examples of convergence. Many amphibians are highly specialised, but despite this there are still fascinating examples of ecological adaptations, whereby similar pressures of the environment strongly constrain the likely endpoints and so define reiterated ecomorphs. This is particularly obvious in frogs, as the frog body plan is generally rather invariant, mainly due to the morphological adaptations required for hopping. Given their overall morphological similarity, it is not so surprising that many convergences among frogs have, until recently, remained largely unrecognised. With the development of molecular phylogenies, however, several cases of supposed phylogenetic relationships based on morphology have turned out to be actually convergences, and convergence is now known to be rife in frogs.
Some groups of frogs have adopted radically different lifestyles, far removed from hopping, including burrowing, tree-dwelling and even gliding. A burrowing lifestyle has evolved multiple times in frogs. The overall change in morphology is rather muted among burrowing frogs, but in other amphibians there are much more spectacular examples of morphological change associated with burrowing. Notably, in the limbless caecilians we see an extraordinary adaptation to the burrowing habit, with in one case a remarkable convergence on earthworms. Limb loss associated with burrowing also occurred in the aistopods (Carboniferous amphibians) and beyond the amphibians, in groups such as skinks, anguids and amphisbaenians (all reptiles). Several frog taxa have adopted an arboreal habit, associated with specialised morphologies such as adhesive toe pads for climbing. Not only do these adhesive pads show some very striking similarities within the frogs, but more generally and equally strikingly resemble those of some insects. In terms of mode of life possibly the most remarkable departure are those frogs that glide. Gliding has evolved in a number of other groups, including both placental (e.g. flying squirrel) and marsupial (e.g. sugar gliders) mammals and, more remarkably, in snakes and even ants. Particularly extraordinary are the convergences between the frogs of Madagascar and those of Asia (mainly India) with compelling, but independent, similarities between burrowing, arboreal, rock-dwelling and torrential ecomorphs. This illustrates that, as a group, the frogs are also especially useful for the study of adaptive radiations.
Further interesting convergences occur with respect to amphibian anatomy (fangs, protrusible tongues), sensory systems (electroreception), defence (aposematism, camouflage, toxicity), mating and reproduction. Some frogs and newts display to attract a mate in defined territories called leks, and lekking behaviour can be found in many other groups including birds and ungulates. Some frogs sing like birds to attract mates, and, more remarkably, one species of frog has evolved ultrasonic communication, famously occurring in bats but also in cetaceans as well as some rodents and shrews. The birth of live young (viviparity) that can be found in several amphibian groups is strongly convergent with striking examples from many animals, notably mammals, fish and lizards. Direct development has evolved independently numerous times in all the major groups of amphibians, and involves abandonment of the characteristic aquatic tadpole stage as an adaptation to an entirely terrestrial lifestyle.
| Topic title | Teaser text | Availability |
|---|---|---|
| Ultraviolet (UV) vision in insects and vertebrates | n/a | Unavailable |
| Vibrational communication in animals | What on earth could an elephant or treehoppers have in common with a seismometer? | Available |
| Foam nests in animals | Nests crop up everywhere, but one made out of foam? Might not sound like a great idea, but it is. And no surprise, it has evolved several times... | Available |
| Lunglessness | n/a | Unavailable |
| Suction feeding in fish, amphibians, reptiles and aquatic mammals | Probably everyone is familiar with the walrus, but did you know that it generates a vacuum in its mouth to suck clams out of their shells? And this is just one example of suction feeding, the feeding mode typically used by bony fish… | Available |
| Tetrodotoxin | Not many foods served in a restaurant can kill you, but pufferfish is the exception. Tetrodotoxin, the toxin responsible for such culinary fatalities, reveals a fascinating story of convergent evolution... | Available |
| Gliding lizards, frogs and ants | Tree-dwelling (‘arboreal’) ants capable of controlled gliding do so when dislodged or threatened by predation. Gliding species include members of three disparate families: Myrmicinae, Pseudomyrmecinae and Formicinae. | Available |
| Gliding reptiles | In the reptiles, different forms of skin membrane (called ‘patagia’) and in some extinct species, primitive feathers, have evolved convergently as adaptations for gliding. | Available |
| Adhesive pads: from geckos to spiders | In terms of adhesive pads we find they have a remarkably wide distribution evolving in at least four distinct groups, including members of the reptiles, amphibians, arthropods and mammals, with tentative parallels in sea urchins. | Available |
| Terrestrialization by amphibians | In the amphibians we see the recurrent evolution of a direct life cycle, a necessary pre-requisite for the invasion of land by losing the aquatic tadpole. | Unavailable |
| Worm-like body form | Man is but a worm, but so are many other vertebrates... | Available |
| Ultrasound communication in mammals and amphibians | Amphibians are adept at vocalization, and remarkably ultrasonic communication has also evolved in the Concave-eared torrent frog, from China. | Unavailable |
| Viviparity (live birth) in animals | Viviparity is rampantly convergent, with famous examples in the reptiles, notably the lizards and snakes. | Unavailable |
| Tongues of chameleons and amphibians | Convergence in tongue function represents repeated morphological exploration within different lineages made possible by loss of an ancestral functional constraint | Available |
| Amphibian life cycles and terrestrialisation | Certain groups show a dramatic transition between a juvenile stage and the adult. This is perhaps most familiar in the frogs which in particular, are well-known for their aquatic tadpoles. | Unavailable |
| Frogs with fangs | Teeth have clearly evolved a number of times, and one of the more interesting curiosities is found in the amphibians, notably in the frogs where several groups have independently evolved fangs. | Available |
| Electroreception in fish, amphibians and monotremes | From an evolutionary point of view, electroreception is particularly intriguing as a sense modality that has been repeatedly lost and reinvented again. | Available |
| Amphibian ecomorphs: frogs and salamanders | Striking examples of convergence can be found, for example, in the cophyline frogs of Madagascar with multiple shifts to and from terrestrial and/or arboreal modes of life. | Unavailable |
| Burrowing: from worms to vertebrates | Quite a few adaptations are useful for burrowing into the soil. So it is not exactly surprising that they have evolved several times... | Available |
| Defence in frogs: toxins and camouflage | The many striking examples of convergence most famously include the case of mimicry, but the question of defence also extends to the use of toxins (and venoms), such as alkaloids, where we also find molecular convergence. | Available |
