Amphibians

Amphibians compose a taxon of vertebrate animals that are characterized by having legs (unless lost) and glandular skin, and by lacking the features of the other tetrapod groups: amniote eggs ("reptiles," mammals, and birds), hair (mammals), feathers (birds), or epidermal scales ("reptiles" and birds). Many of the fossil groups referred to the "Amphibia" in this sense are evolutionarily more closely related to "reptiles" and mammals than they are to living amphibians, so use of the term to include these fossil taxa promotes poor communication. In a more restricted sense, all living amphibians are generally considered to be members of the group Lissamphibia, which is considered by most systematists to be the closest living taxon to the Amniota ("reptiles," birds, and mammals). Nevertheless, the reality of the Lissamphibia remains an open question, on occasion resulting in heated discussion, with some paleontologists suggesting that salamanders and caecilians are only distantly related to frogs and other tetrapods, although the preponderance of the anatomical and molecular evidence suggested so far does support Lissamphibian monophyly. The current wisdom is that Lissamphibians were derived from dissorophoid labyrinthodonts about 280 million years ago. The Lissamphibia are composed of the three living groups, each at least 200 million years old: frogs (Anura, 4,765 species); salamanders (Urodela, 495 species); and caecilians (Gymnophiona, 161 species). Amphibians are found worldwide in temperate and tropical communities, with the exception of extremely dry areas, or most oceanic islands.

Amphibians show the greatest diversity in reproductive modes of any vertebrate group. Primitively, Lissamphibians lay eggs in water, have external fertilization, and exhibit an aquatic larval stage and a terrestrial adult (reproductive) stage. This has been highly modified through evolutionary history in many groups, and now most species of salamanders and caecilians lack aquatic larvae; most of these have internal fertilization and direct development in terrestrial eggs. (Direct development is development in which an animal after birth or emergence from an egg differs from the adult in only comparatively minor details, with no larval stage or metamorphosis.) Internal fertilization is accomplished through the use of a penis in caecilians and a male spermatophore in most salamanders, which the female salamander picks up with her cloacal lips during courtship. Similarly, a large number of frogs exhibit direct development, although the primitive reproductive mode of external fertilization, aquatic eggs, and aquatic larvae, is still the most common. The morphological diversity of amphibians is also enormous, from the legless burrowing or aquatic caecilians to tail-less hopping frogs, to the superficially mainstream but internally highly derived salamanders.

One of the most surprising things about living amphibians is that about 36 percent of all species have been named in the last sixteen years, with the rate of discovery of new species increasing each year. This growth in our knowledge is not driven by growth in the number of scientists who study this kind of diversity, but by new tools (for example, protein electrophoresis and DNA sequencing) that allow rapid discovery of species and by the fact that the extensive human modification of the planet has allowed some areas previously inaccessible to be studied with ease. Concomitant with the rapid discovery of amphibians is the realization that much of this diversity may well be at risk, with rather large numbers of species becoming very rare or extinct (for example, the gastric-brooding frog, Rheobatrachus silus, and the golden toad, Bufo periglenes) in the last few years.

Within the Lissamphibia (generally ranked as a subclass by those preoccupied by such things) the taxonomy can be characterized by the following:

Order Gymnophiona, the caecilians. The bizarre caecilians form a group of 161 recognized species of legless, eel-like amphibians, usually cast into five families, found in the wet tropics of the Americas, Africa, India, and southeastern Asia, with outliers of clear biogeographic interest in the Seychelles. They range in size from about 7 cm to 1.5 m, depending on the species. They are also seemingly generalists, eating any animal sufficiently smaller than themselves, especially earthworms, termites, and orthopterans. The number of recognized species has been stable since at least 1985, although because they are generally difficult to collect and external morphological variation is quite limited, the true number of species is likely much higher than currently appreciated. The phylogenetic relationships of the group are not well known, although careful work on this topic suggests the relationships shown in Figure 1. Caecilians are all legless burrowers in mud or leaf litter, and some are aquatic; primitively, they have aquatic larvae, but most species have direct development from eggs laid on land; some have become live-bearing. The oldest fossil caecilian is from the lower Jurassic.

Figure 1 Caecilian Relationships

Ichthyophiidae Uraeotyphlidae Scolecomorphidae "Caeciliidae" Typhlonectidae

Ichthyophiidae. The semiaquatic ichthyophiids (thirty-seven species in two genera) are found in the Philippines and from southern India to southern China, Thailand, and the Malayan Archipelago. They are oviparous and have free-swimming larvae.

Uraeotyphlidae. Very little is known about this taxon (four species in one genus), which is found in southern India. Because it is likely the closest relative of the Ichthyophiidae, one expects the species to be semiaquatic with aquatic larvae.

"Caeciliidae." The Caeciliidae are a poorly resolved and paraphyletic group (with respect to at least the Typhlonectidae and possibly with respect to the Scolecomorphidae as well) of 101 species found in the wet tropics of the Americas, Africa, Seychelles, and southern and eastern India. The morphological and ecological diversity in this group is, not surprisingly, large, with variation encompassing such things as bright orange body color, species whose eyes move with facial tentacles, and giant species measuring more than a meter in length. Some species are oviparous, with direct development within the egg, and other species are viviparous.

A salamander (Oedipina grandis) on a leaf in the 3,000-foot high Talamanca Range, Costa Rica (Michael and Patricia Fogden/Corbis)

Scolecomorphidae. The scolecomorphids are a group (six species in two genera) of live-bearing burrowing caecilians found in tropical West and East Africa.

Typhlonectidae. The typhlonectids are a group (thirteen species in five genera) of secondarily aquatic caecilians found in the Amazon, Orinoco, Magdalena, and La Plata drainages of South America, and also in places on the coast of Venezuela. On the basis of phylogenetic analysis, this group is believed to have been derived from within the Caeciliidae.

Order Urodela, the salamanders. The salamanders are an easily recognizable group of tailed and four-legged amphibians of 495 species, generally placed in ten families, found in the temperate areas of Eurasia and North America, south into the tropics of northern

South America, and into the northern parts of tropical Asia. Species range in habitus from obligately aquatic giants (Andrias) of 1.5 m to "standard" salamanders having aquatic larvae and terrestrial adults (for example, most tiger salamanders) to tiny lungless salamanders that have direct development (for example, Tho-rius). The relationships of the nominal families are becoming clearer, with the most evi-dentiarily supported arrangement shown in Figure 2. All families of salamanders are mono-phyletic and of considerable antiquity. The oldest salamanders are from the Upper Jurassic, with evidence suggesting that at least the major extant salamander families were all present in the Cretaceous.

Sirenidae. The obligate aquatic sirens (two genera, four species), with external gills in adults, are found in the southeastern United States and extreme northeastern Mexico. Unique among salamanders, they lack hind limbs but retain well-developed forelimbs. Reproduction is not well understood in this group, but they apparently have external fertilization of eggs laid in submerged vegetation.

Cryptobranchidae. The giant salamanders (two genera, three species) are found in eastern China, southern Japan, and the eastern United States. They get enormous (Andrias japonicus is up to 1.4 m) and are all obligate aquatic with internal gills, external fertilization, and aquatic larvae.

Hynobiidae. The Asiatic salamanders are a generalized group of eight genera and thirty-nine species found east of the Urals and south of the Arctic Circle to Iran, as well as in southern China and Japan. All have external fertilization and aquatic eggs, and larvae and adults of most species are terrestrial.

Rhyacotritonidae. The Olympic salamanders (one genus, four species) are found only in the Pacific Northwest of the United States and Canada. They have reduced lungs, associated with their high gradient stream habitat, aquatic larvae, internal fertilization with a male spermatophore, and semiaquatic adults that are generally found along the margins of water.

Plethodontidae. The lungless salamanders (25 genera and 342 species) predominantly have direct development within terrestrial eggs, although one tribe (Hemidactyliini of the largest subfamily, Plethodontinae) and the subfamily Desmognathinae have aquatic larvae and, except for a few examples, terrestrial adults. The Desmognathinae (two genera, eighteen species) are restricted to the eastern United States and adjacent Canada, but the Plethodontinae extend from the U.S.-Canada border region to Amazonia with large numbers

Figure 2

Salamander Relationships

Sirenidae Cryptobranchidae Hynobiidae Rhyacotritonidae Plethodontidae Amphiumidae Proteidae Salamandridae Dicamptodontidae Ambystomatidae of species and morphological diversity. Within the Plethodontinae, three tribes are recognized, of which evidence of monophyly is strong only for the Bolitoglossini. The Hemi-dactyliini represent the species that retains the primitive condition of aquatic larvae and is restricted to the eastern United States and adjacent Canada. The Plethodontini contain the speciose genus Plethodon, as well as Aneides, both in the eastern and western United States, as well as Ensatina, found only in the far western United States and adjacent Mexico. The Bolitoglossini is a major radiation found from California south to Amazonia, with a surprising outlier in that some of the species of Hydromantes (otherwise in the Sierras of California) are found in southern France, Sardinia, Corsica, and adjacent Italy.

Amphiumidae. The Amphiumas (one genus, three species) are eel-like, obligately aquatic species with diminutive limbs, unpleasant dispositions, and a large size (as much as 1 m). Reproduction is via a male sper-matophore, and the adults retain internal gills. The taxon is restricted to the southern coastal plain of the eastern United States.

Figure 2

Proteidae. The mudpuppies (two genera, six species) are aquatic salamanders found in the eastern United States and adjacent Canada, as well as one genus, one species on the Adriatic coast of northeastern Italy south to Montenegro. All species have internal fertilization (through use of a male spermatophore) and aquatic larvae, with external gills retained into adulthood.

Salamandridae. The newts and fire salamanders (fifteen genera, fifty-nine species) occur in eastern and far western North American from southern Canada to northern Mexico, but with the bulk of their diversity in Eurasia (central Siberia west to Norway, Britain, and northwestern Africa, as well as southern China and adjacent Indochina). The salamandrids are mostly aquatic as adults, although most species have adult morphologies. Reproduction is via internal fertilization and aquatic larvae, although a few species have developed viviparity.

Dicamptodontidae. The American giant salamanders (one genus, four species), like other advanced salamanders, have internal fertilization through use of a male sper-matophore. Most species have terrestrial adults. Found only in the Pacific Northwest of United States and Canada.

Ambystomatidae. The tiger salamanders (one genus, thirty species) are found from southern Canada to the vicinity of Mexico City, in temperate locations in semiarid to mesic environments. The mode of reproduction is typical, inasmuch as fertilization is internal via a male spermatophore, and aquatic larvae turn into terrestrial adults; a few species, however, such as the famous axolotl (Ambystoma mexicanum), retain larval morphologies as aquatic reproductive adults.

Order Anura, the frogs. Frogs (one genus, 4,765 species) represent the large majority of all species of amphibians as well as the largest diversity of life histories and morphologies. These tail-less amphibians are found essentially worldwide in temperate to tropical climates, with a few species extending out to Fiji and Tonga in the Pacific Ocean. The structural diversity among frogs is broad and well known, although the species diversity and their evolutionary history are not. At present the phy-logenetic history can be approximated by the tree shown in Figure 3. The nonranoid neo-batrachians are referred to as the "Hyloidea," a taxon for which no evidence of monophyly has been suggested, nor its monophyly refuted. The oldest indisputable frog is from the Lower Jurassic of North America, and another arguable frog relative is known from the Lower Triassic. As one can suppose from this rather problematic tree, the received wisdom regarding the phylogeny of frogs is likely to change considerably in the next few years.

Ascaphidae. Ascaphus is composed of one genus, two species of small frogs found along high gradient streams in the Pacific Northwest of the United States and adjacent Canada, where their eggs are laid and their tadpoles develop. The tailed frogs are known for their peculiar intromittent organ, from whence the common name comes as well as a number of primitive characteristics that suggest their very distant relationship to other frogs.

Leiopelmatidae. Leiopelma (one genus, four species), like Ascaphus, is a relict taxon showing a number of primitive features, with only very distant relations to other frogs. This family is found solely on New Zealand. Eggs are laid on damp terrestrial locations, and the young develop directly without a larval stage outside of the egg.

Bombinatoridae. The bombinatorids (two genera, nine species) include the firebellied toads. They are found in Europe to Turkey and western Russia, and in eastern Asia, including eastern Russia, China, Korea, and

Figure 3

Frog Relationships

Figure 3

^^eobatrachi^

^^eobatrachi^

Ascaphidae

Leiopelmatidae

Bombinatoridae

Discoglossidae

Megophryidae

Pelobatidae

Pelodytidae

Rhinophrynidae

Pipidae

Allophrynidae

Brachycephalidae

Bufonidae

Heleophrynidae

"Leptodactylidae"

"Myobatrachidae"

Sooglossidae

Rhinodermatidae

"Hylidae"

Centrolenidae

Pseudidae

Microhylidae

Petropedetidae

Hemisotidae

"Arthroleptidae"

Hyperoliidae

"Ranidae"

Rhacophoridae

Dendrobatidae

Vietnam. Another genus is found in the Philippines and northern Borneo. Reproduction is via aquatic eggs and larvae.

Discoglossidae. The discoglossids (three genera, ten species) include the famous midwife toad. They are found in western, eastern, and southern Europe and western Asia, as well as northwestern Africa. All species have aquatic eggs and larvae, but in Alytes the male carries the strings of eggs on his back and legs until they are ready to hatch.

Megophryidae. The megophryines are relatively primitive frogs of ten genera and eighty-three species, closely related to the pelobatids and pelodytids. They are found predominantly in southeastern Asian and Indonesian tropical forests and associated temperate montane habitats. All species have aquatic eggs and larvae, and in some species the tadpoles have distinctive upward-directed mouthparts that allow them to browse on the surface of the water.

Pelobatidae. The spadefoots are composed of three genera with eleven species, which are found in arid to mesic temperate North America and in Europe, western Asia, and northwestern Africa. All species have aquatic eggs and larvae and are particularly distinctive in that the rate of larval development is much higher than that of most other frogs, with some species in the western United States developing from deposited egg to froglet in less than a month.

Pelodytidae. The parsley frogs (one genus, three species) are smooth-skinned, toadlike frogs, closely related to the pelobatids, which occur in Europe and western Asia. They have typically anuran aquatic larvae.

Rhinophrynidae. The Mexican burrowing toad is a tubby, medium-size frog (one species) with a very narrow head with respect to its body; it is apparently the nearest relative of the pipids. It is found in south Texas southward to Costa Rica. Reproduction is via aquatic eggs and larvae.

Pipidae. The clawed frogs (five genera, twenty-two species) are a strictly aquatic tropical group of frogs found in tropical Central and South America and sub-Saharan Africa. Primitively, members of the group lay aquatic eggs that develop into aquatic larvae. In the South American Pipa, however, the eggs are gathered onto the back, which grows over them to produce imbedded individual cavities for each egg, from which free-living larvae or small froglets emerge, depending on the species involved.

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