[Skip to list of Topics for this Category →]
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.