25 Amphibians Leading a Life of Slime

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David Cannatella

David M. Hillis

430

Amphibians have generally not been regarded with much

favor. An often-cited paraphrasing of Linnaeus’s Systema

Naturae suggests that they are such loathsome, slimy creatures

that the Creator saw fit not to make many of them. In

fact, the number of living amphibians, about 5300, exceeds

that of our own inclusive lineage, Mammalia (Glaw and

Kцhler 1998). The rate of discovery of new species exceeds

that of any other vertebrate group. Since the publication of

Amphibian Species of the World (Frost 1985), the number of

recognized amphibians has increased by 36%. More than 100

undescribed frog species have been reported in Sri Lanka

(Meegaskumbura et al. 2002). Yet, the decline and extinction

of amphibian populations are visible signals of environmental

degradation (Hanken 1999).

Amphibians are named for their two-phased life history:

larva and adult. Typically, the larva is aquatic and metamorphoses

into a terrestrial adult. In a loose, descriptive sense,

amphibians bridge the gap between fishes, which are fully

aquatic, and amniotes, which have completely escaped a

watery environment and have abandoned metamorphosis.

However, amphibians are not in any sense trapped in an

evolutionary cul-de-sac, because they exhibit a far greater

diversity of life history modes than do amniotes.

Each type of living amphibian—frog, salamander, and

caecilian—is highly distinctive. Frogs are squat, four-legged

creatures with generally large mouths and eyes and elongate

hind limbs used for jumping. There is no tail (the meaning

of Anura), because the caudal vertebrae have coalesced into

a bony strut. About 90% of the living amphibian species are

frogs; they rely mostly on visual and auditory cues. Salamanders

are more typical-looking tetrapods, all with a tail

(hence, Caudata) and most with four legs. Some are elongate

and have reduced the limbs and girdles; these are usually

completely aquatic or fossorial species. In general, they

rely more on olfactory cues. Living caecilians are all limbless

and elongate. Grooved rings encircle the body, evoking the

image of an earthworm; most caecilians are fossorial, but

some are aquatic. All have reduced eyes, although the root

caecus—Latin for “blind”—is a misnomer. Near the eye or

the nostril is a unique protrusible tentacle used for olfaction.

The tail is essentially absent.

Modern Amphibians

By modern amphibians, we mean the lineage minimally circumscribed

by living taxa; this is known as the crown clade

Amphibia. In the language of phylogenetic taxonomy (discussed

below), Amphibia are a node-based name defined as

the most recent common ancestor of frogs, salamanders,

caecilians, and all the descendants of that ancestor (Cannatella

and Hillis 1993). Frost (1985) and Duellman (1993)

summarized the species of amphibians. Up-to-date Internet

resources include Frost (2002) and D. B. Wake (2003). The

Amphibians 431

distribution of modern amphibians is treated in Duellman

(1999). Aspects of modern amphibian biology can be found

in two recent textbooks (Pough et al. 2001, Zug et al. 2001)

and in a treatise (Laurent 1986). The most comprehensive

treatment is that of Duellman and Trueb (1986).

Modern amphibians are at times called lissamphibians to

distinguish them from the Paleozoic forms referred to as “amphibians.”

Modern amphibians include frogs, salamanders,

and caecilians, and their Mesozoic [245–65 million years ago

(Mya)] and Cenozoic (65 Mya to present) extinct relatives (including

albanerpetontids), all of which are readily identifiable

as belonging to this group. In contrast, their Paleozoic relatives

include the traditional groups termed the Labyrinthodontia

and Lepospondyli. Labyrinthodonts, including the

earliest four-legged vertebrates, ranged from the Upper Devonian

(375 Mya) through the Permian (290 Mya), with numbers

declining into the Triassic and one small lineage persisting

into the Cretaceous. Lepospondyls range from the Lower Carboniferous

(240 Mya) to the base of the Upper Permian (250

Mya). Labyrinthodonts are a paraphyletic group and also gave

rise to amniotes. Lepospondyls are a heterogeneous group but

have a characteristic vertebral morphology (Carroll et al. 1999);

their monophyly is unclear.

Several features set modern amphibians apart from other

vertebrates. Some of these characters support monophyly of

the group compared with both fossil and living taxa. The significance

of other characters, such as soft tissue features (Trueb

and Cloutier 1991a), is less certain because they cannot be

assessed in extinct forms. But these characters do support

amphibian monophyly relative to amniotes and fishes.

Most adult amphibians have teeth that are pedicellate and

bicuspid, or modified from this condition. Pedicellate teeth

have a zone of reduced mineralization between the crown and

the base (pedicel). In fossils the crowns are often broken off,

leaving a cylindrical base with an open top. Pedicellate teeth

are also found in a few temnospondyl labyrinthodonts believed

to be closely related to modern amphibians (Bolt 1969).

Living amphibians also share the absence or reduction

of several skull bones. On the dorsal skull, the jugals, postorbitals,

postparietals, supratemporals, intertemporals, and

tabulars are absent. On the palate, the pterygoid, ectopterygoid,

and palatines are reduced or absent so as to produce a

large space, the interpterygoid vacuity, below the eye sockets

(Reiss 1996). The reduction/loss of many skull bones in

modern amphibians is a result of pedomorphosis (Alberch

et al. 1979). Pedomorphosis is a pattern derived from a

change the timing of development; specifically, a species

becomes sexually mature (adult) at an earlier stage of development

than its immediate ancestor. As a result, the adult of

amphibians resembles the juvenile (or larval) stage of Paleozoic

relatives. A secondary result of pedomorphosis is miniaturization

(Hanken 1985); because living amphibians mature

at an earlier age, they are typically much smaller than the Paleozoic

forms (Bolt 1977, Schoch 1995).

Amphibians employ a buccal force-pump mechanism for

breathing (Brainerd et al. 1993, Gans et al. 1969). Air is

forced back into the lungs by positive pressure from the

mouth cavity. In contrast, amniotes use aspiration to fill the

lungs, in which the rib cage and/or diaphragm creates negative

pressure in the thorax. Amphibians have distinctive short

ribs that do not form a complete rib cage as in amniotes, so

aspiration is not possible.

In addition to the stapes-basilar papilla sensory system

of tetrapods, living amphibians have a second acoustic pathway,

the opercular-amphibian papilla system. This system

is more sensitive to lower frequency vibrations than is the

stapes-basilar papilla pathway. In frogs and salamanders, the

operculum (a bone of the posterior aspect of the braincase,

not in any way similar to the homonymous bone of fishes) is

also connected to the shoulder girdle by way of a modified

levator scapulae muscle, the opercularis. This muscle transmits

vibrations from the ground through the forelimb and

shoulder girdle to the inner ear.

The skin is a significant respiratory organ; it is supplied

by cutaneous branches of the ductus arteriosus (the presence

of these is not clear in caecilians). The skin has a stratum

corneum (outer layer) like that of other tetrapods, although

it is thinner than that of amniotes. However, living amphibians

retain the primitive feature of mucous glands and granular

glands. Granular glands secrete poisons of varying toxicity,

some lethal. Mucous glands keep the skin moist, which allows

the dissipation of heat, as well as the loss of water

through the skin. Many caecilians have dermal scales, similar

to those of teleost fishes, embedded in the skin.

The Name “Amphibia”

In the Systema Naturae of Carolus Linnaeus, the Amphibia

were one of six major groups of animals, the others being

mammals, birds, fish, insects, and mollusks. The group included

not only frogs, salamanders, and caecilians but also

reptiles and some fish that lacked dermal scales. Later, as early

fossil tetrapods were uncovered, these were also relegated to

“Amphibia” because of their presumed ancestral position to

other tetrapods. In 1866 the great German biologist Ernst

Haeckel divided Amphibia into Lissamphibia (salamanders

and frogs) and Phractamphibia (caecilians and fossil labyrinthodonts;

Haeckel 1866). “Liss-” refers to the naked skin

of frogs and salamanders, and “phract-” means helmet, in

reference to the armor of dermal skull bones and scales found

in early tetrapods and, in a reduced form, in caecilians. Gadow

(1901) transferred the caecilians from Phractamphibia to

Lissamphibia.

For most of the 20th century, the name Amphibia was used

for tetrapods that were not reptiles, birds, or mammals. Thus,

the earliest tetrapods (labyrinthodonts from the Devonian)

were included in Amphibia, as were the Lepospondyli. This

432 The Relationships of Animals: Deuterostomes

rendition of Amphibia appeared in most comparative anatomy

and paleontology texts, largely because of the influence of the

paleontologist Alfred Romer. Modern amphibians were believed

to be polyphyletic and derived from different “amphibian”

lineages; frogs from Labyrinthodontia, and salamanders

and caecilians from Lepospondyli. Parsons and Williams adduced

evidence for the monophyly of modern amphibians and

resurrected Gadow’s Lissamphibia for living amphibians (Parsons

and Williams 1962, 1963). However, the term Lissamphibia

is used mainly among specialists to distinguish the

modern groups from extinct Paleozoic forms. Most biologists

and most textbooks refer to frogs, salamanders, and caecilians

simply as amphibians.

Use of Amphibia in the Romerian sense of a paraphyletic

taxon has been largely abandoned and the name has been

redefined as a monophyletic group in two contrasting ways

(fig. 25.1). First, the name Amphibia is applied to the node

that is the last (most recent) ancestor of living frogs, salamanders,

and caecilians (de Queiroz and Gauthier 1992).

Amphibia includes this ancestor and all its descendants,

which are the modern forms, including albanerpetontids.

Second, Amphibia is defined as the stem or branch that contains

living frogs, caecilians, salamanders, and all other taxa

more closely related to these than to amniotes (e.g.,

Gauthier et al. 1989, Laurin 1998a). In other words, the

stem-based name Amphibia includes all taxa along the stem

leading to modern amphibians; this includes either the

temnospondyls, the lepospondyls, or both, depending on

which phylogeny one accepts. Under a stem-based definition,

the content of Amphibia, in terms of fossil taxa, may

change dramatically. Laurin (1998a) proposed such changes

based on his application of principles of priority and synonymy

to phylogenetic taxonomy. He argued that the definition

of Amphibia as a stem-based name by Gauthier et al.

(1989) must be accorded priority over the node-based definition

of Amphibia of de Queiroz and Gauthier (1992). One

result of accepting the stem-based definition is that the

content of Amphibia under Laurin’s phylogeny (Laurin and

Reisz 1997) is very different compared with the content

under other definitions of Amphibia.

Node- and stem-based names have their respective advantages

in communicating taxonomy. However, a stembased

definition of Amphibia, a name in general parlance,

has an undesirable effect, because generalizations about the

biology of modern amphibians can be wrongly extended to

extinct temnospondyls and/or lepospondyls (de Queiroz and

Gauthier 1992). These groups bear little resemblance to the

living forms, and their biology was presumably very different.

Under a stem-based definition of Amphibia, the common

statement “all amphibians have mucous glands” would

be interpreted to mean that lepospondyls had mucous

glands, an inference for which there is no evidence. In contrast,

under the node-based definition of Amphibia, one can

reasonably infer that extinct frogs, salamanders, and caecilians

have mucous glands, but the inference does not extend

inappropriately to extinct temnospondyls and lepospondyls.

Although some neontologists and most paleontologists appreciate

the semantic distinction between Amphibia and

Lissamphibia, most biologists use Amphibia to mean frogs,

salamanders, and caecilians.

Amphibians and the Origin of Tetrapods

The exact relationships of modern amphibians to extinct

Paleozoic forms is not clear. Heatwole and Carroll (2000)

provided a summary of the phylogeny of various fossil

groups. The favored family of hypotheses (fig. 25.2A,B) posits

that the group of frogs, salamanders, and caecilians is

monophyletic and that this clade is nested within dissorophoid

temnospondyls (Bolt 1977, 1991, Milner 1988,

1993, Trueb and Cloutier 1991a). (Temnospondyls are

labyrinthodonts that include Edopoidea, Trimerorhachoidea,

Eryopoidea, Stereospondyli, and Dissorophoidea.)

The most thorough and data-rich analysis, in terms of characters

and taxa (Ruta et al. 2003; fig. 25.2B), also reached

this conclusion.

A recent variant of the monophyly hypothesis (fig. 25.2C)

is that modern amphibians are nested within the lepospondyls

(e.g., Anderson 2001), particularly within the Microsauria

(Laurin 1998a, 1998b, Laurin et al. 2000a, 2000b, Laurin and

Reisz 1997; but see Coates et al. 2000, Ruta et al. 2003).

Because temnospondyls are distantly related to amphibians

under this second hypothesis, the derived similarities between

them and dissorophoid temnospondyls are interpreted

as convergent.

A very different hypothesis claims polyphyly of the modern

groups (fig. 25.2D), with caecilians derived from goniorhynchid

microsaurs (Carroll 2000b, Carroll and Currie

1975), and salamanders and frogs from temnospondyls. The

polyphyly hypothesis gained some strength with the discovery

of the fossil Eocaecilia (see below), which possessed characters

seemingly intermediate between goniorhynchid

Figure 25.1. Node-based (boldface) and stem-based definitions

of Amphibia.

Amniota

Gymnophiona

Temnospondyls or

Lepospondyls?

Caudata

Anura

Triadobatrachus

Karaurus

Eocaecilia

Amphibia (node-name)

Urodela Salientia

Extinct

Extant

Apoda

Amphibia (if used as a stem-name)

Amphibians 433

microsaurs and living caecilians (Carroll 2000a)—this interpretation

remains controversial.

Interrelationships of Modern Amphibians

Two general alternative hypotheses have been considered

for relationships among the groups of modern amphibians.

One tree, based primarily, but not exclusively, on nonmolecular

data, allies frogs and salamanders, with caecilians

as the odd group out (fig. 25.2A,B). The name Batrachia,

formerly synonymous with Amphibia, has been applied to

this clade. In the second hypothesis, the earliest analyses

of DNA sequence data slightly favored salamanders and

caecilians, a group named Procera, as closest relatives (Feller

and Hedges 1998, Hedges and Maxson 1993, Hedges et al.

1990), as in figure 25.2D. However, Zardoya and Meyer

(2001) analyzed complete mitochondrial sequences of one

species each of a frog, salamander, and caecilian and found

the frog and salamander to be sister groups. Although their

level of taxon sampling was shallow, the results suggest significant

uses for character-rich data sets such as mitochondrial

genomes.

A fourth group of amphibians is Albanerpetontidae, known

only from fossils from the Jurassic to the Miocene (Milner

2000); the name Allocaudata has been used infrequently for

these, because it is redundant with Albanerpetontidae. This

group closely resembles salamanders in skull shape and in the

primitive tetrapod features of a generalized body shape, four

limbs and a tail. Albanerpetontids lack most of the same dorsal

skull bones as do living amphibians but do not have pedicellate

teeth. They have been considered to be nested within

salamanders, or the sister group of Batrachia (McGowan and

Evans 1995); the most recent and extensive analysis (Gardner

2001) placed them in the latter position. Ruta et al. (2003;

fig. 25.2B) placed them in a basal polytomy with the modern

forms.

Both nucleotide sequence data and “soft” anatomy ally

frogs, salamanders, and caecilians as a clade relative to living

amniotes and fishes. Because fossils do not so easily yield

information about nucleotides or soft tissue characters, these

data sets provide no direct evidence for the monophyly of

Figure 25.2. (A–D) Alternative relationships among modern amphibians (caecilians, frogs, and

salamanders) and Paleozoic groups (temnospondyls, microsaurs, and lepospondyls).

Caecilians

Salamanders

Frogs

B. Ruta et al. (2003)

Temnospondyls

Albanerpetontidae

Anthracosaurs

Amniota

Diadectomorphs

Lepospondyls

Temnospondyls

Frogs

Microsaurs

Lepospondyls

?

D. Carroll (2000)

Caecilians

Salamanders

Temnospondyls

A. Trueb and Cloutier (1991)

Caecilians

Salamanders

Frogs

Caecilians

Salamanders

Frogs

C. Laurin and Reisz (1997)

Temnospondyls

Anthracosaurs

Microsaurs

Lepospondyls

Amniota

Diadectomorphs

434 The Relationships of Animals: Deuterostomes

Amphibia with respect to Paleozoic tetrapods (Trueb and

Cloutier 1991a, 1991b).

Caecilians

The node-based name for modern caecilians is Gymnophiona,

meaning “naked snake.” Caecilians include 165 extant species,

restricted to tropical America, Africa, and Asia. They are

grouped into five or six families (fig. 25.3, table 25.1).

Because of their habits, caecilians are rarely seen in the

wild. A dedicated herpetologist might find them by digging,

and occasionally individuals are found on the surface of the

ground after a heavy tropical rains Most caecilians are 0.3–

0.5 m long, although one species is as large as 1.5 m and one

as small as 0.1 m. All caecilians are elongate, but some are

more elongate than others; the number of vertebrae ranges

from 86 to 285. Caecilians are almost unique among amphibians

(two species of frogs are the exception) in having a male

intromittent organ, the phallodeum, and internal fertilization

occurs during copulation.

Living caecilians have reduced eyes with small orbits, and

scolecomorphids and some caeciliids have eyes covered by

the skull bones. Compared with other amphibians, the skulls

of caecilians are highly ossified and many bones are fused.

The resulting wedge-shaped cranium is used for digging and

compacting the soil. Most caecilians are oviparous with freeliving

larvae. Viviparous species occur in a few families; in

some of these the embryos derive nutrition from the lining

of the oviduct, so far as is known. They have a species-specific

“fetal dentition” that apparently is used to help ingest the

nutritive secretions. Most caecilians are fossorial, but the

Typhlonectidae are aquatic and most have laterally compressed

bodies, especially posteriorly, and a slight dorsal “fin,”

presumably for swimming.

Fossil caecilian vertebrae are known from the Upper

Cretaceous, Tertiary, and Quaternary of Africa north of the

Sahara and Mexico to Bolivia and Brazil (summarized in

Wake et al. 1999). Although living caecilians are limbless, and

nearly or completely tailless, the earliest putative caecilian had

legs and a tail! Eocaecilia micropodia from the Jurassic has a

somewhat elongate body and small but well-developed

limbs (Carroll 2000a, Jenkins and Walsh 1993, Wake

1998). Eocaecilia has pedicellate teeth and a groove in the

edge of the eye socket is interpreted to be for passage of the

tentacle; thus Eocaecilia is inferred to have a feature otherwise

unique to living caecilians. The evidence suggests it is

the sister group of all other caecilians. The stem-based name

for the clade containing Eocaecilia + Gymnophiona is Apoda

(Cannatella and Hillis 1993).

Gymnophiona are the least understood of all vertebrate

lineages, given its size. Caecilians are restricted to tropical regions

of America, Africa (excluding Madagascar), the Seychelles

Islands, and much of Southeast Asia. In general, phylogenetic

relationships among caecilian families have not generated as

much controversy as have those among salamanders or frogs,

but little work has been done and sampling of species is poor.

Taylor (1968) presented a monographic revision of the systematics

of caecilians that stimulated work for the next 30

years, including considerable molecular and morphological

research. Lescure et al. (1986) presented a radically different

classification of caecilians based on sparse new data.

Nussbaum and Wilkinson (1989) reviewed this unorthodox

classification in a larger context; they argued for maintaining

the current generic and familial relationships pending

further research.

Hedges et al. (1993) analyzed sequence data for the 12S

and 16S ribosomal RNA (rRNA) genes for 13 species in 10

genera; and M. Wilkinson et al. (2002) examined relationships

among Indian species. Although molecular data have added

substantially to caecilian phylogenetics, new morphological

characters have contributed as well. Wake (1993, 1994) found

that neuroanatomical characters in isolation are not a robust

character base, but are useful within a larger morphological

set; Wilkinson (1997) confirmed the “eccentricity” of the neuroanatomical

set. The description of a bizarre new typhlonectid

used 141 morphological characters and resulted in a new

analysis of Typhlonectidae (Wilkinson and Nussbaum 1999).

Similarly, phylogenetic analysis of Uraeotyphlidae has made

use of new anatomical features (Wilkinson and Nussbaum

1996). Only recently has the osteology of the entire group been

surveyed (M. H. Wake 2003).

Rhinatrematidae are almost universally considered to be

the sister taxon of other living gymnophiones (fig. 25.3) based

on both morphological and molecular data (Hedges et al. 1993,

Nussbaum 1977). These caecilians retain a very short tail behind

the cloaca, as do the Ichthyophiidae, in contrast to other

Figure 25.3. A generally accepted phylogenetic hypothesis of

relationships among caecilians. “Caeciliidae” indicates a group

that is paraphyletic with respect to Scolecomorphidae and

Typhlonectidae. The dagger indicates extinction.

Eocaecilia†

Rhinatrematidae

Gymnophiona

Stegokrotaphia

Uraeotyphlidae

Ichthyophiidae

Scolecomorphidae

"Caeciliidae"

Typhlonectidae

Amphibians 435

Table 25.1

Geographical Distribution of the Major Extant Groups of Amphibia.

Taxon Distribution

Gymnophiona

Rhinatrematidae Northern South America

Ichthyophiidae India, Sri Lanka, Southeast Asia

Uraeotyphlidae South India

Scolecomorphidae Africa

“Caeciliidae” Mexico, Central and South America, Africa, Seychelles, India, Southeast Asia

Typhlonectidae South America

Caudata

Hynobiidae Continental Asia to Japan

Sirenidae Eastern United States and adjacent Mexico

Cryptobranchidae China, Japan, eastern United States

Ambystomatidae North America

Rhyacotritonidae Northwest United States

Dicamptodontidae Western United States and adjacent Canada

Salamandridae Eastern and western North America, Europe and adjacent western Asia, northwest Africa,

eastern Asia

Proteidae Eastern United States and Canada, Adriatic coast of Europe

Amphiumidae Southeast United States

Plethodontidae North and Central America, northern South America, Italy and adjacent France, Sardinia

Anura

Ascaphus Northwest United States and adjacent Canada

Leiopelma New Zealand

Bombinatoridae Europe and eastern Asia, Borneo and nearby Philippine Islands

Discoglossidae Europe, northern Africa

Pipidae South America and adjacent Panama, sub-Saharan Africa

Rhinophrynidae Central America, Mexico, and south Texas

Pelobatidae North America, Europe, western Asia

Pelodytidae Western Europe, western Asia

Megophryidae Southern Asia to Southeast Asia

Heleophryne Southern Africa

Myobatrachinae Australia, New Guinea

Limnodynastinae Australia, New Guinea

“Leptodactylidae” South America, Central America, Mexico, southern United States

Bufonidae All continents (including Southeast Asia) except Australia and Antarctica

Centrolenidae Mexico, Central and South America

Dendrobatidae Northern South America, Southeast Brazil, Central America

Sooglossidae Seychelles

Hylidae The Americas, Europe and adjacent Asia, northern Africa, eastern Asia, Japan, New Guinea,

Australia

Pseudidae South America

Rhinoderma Southern South America

Allophryne Northern South America

Brachycephalidae Atlantic forests of southeastern Brazil

Microhylidae Southern United States, Mexico, Central America, South America, sub-Saharan Africa,

Madagascar, southern Asia, Southeast Asia, New Guinea, northeastern Australia

“Ranidae” (including Mantellinae) All continents (northern South America only northeastern Australia only)

Arthroleptidae Sub-Saharan Africa

Hyperoliidae Sub-Saharan Africa, Madagascar, Seychelles

Hemisus Sub-Saharan Africa

Rhacophoridae Sub-Saharan Africa, Madagascar, southern Asia, Southeast Asia, Japan

family groups. Ichthyophiidae are a group of semi-fossorial

forms from southern and Southeast Asia. Uraeotyphlidae,

generally considered the sister taxon of Ichthyophiidae, are also

from southern Asia; these are tailless.

Most taxonomic uncertainty resides in the geographically

and biologically diverse taxon “Caeciliidae,” which is probably

paraphyletic with respect to Scolecomorphidae and

Typhlonectidae. Caeciliids occur pantropically, and include

a great diversity of taxa—including the smallest and largest

species—and many reproductive modes, such as egg-layers

with free-living larvae, direct developers, and viviparous

forms, and several kinds of maternal care.

436 The Relationships of Animals: Deuterostomes

Scolecomorphidae are an African group with some bizarre

features; in some taxa the eye is completely covered by

a layer of bone, and in at least one species the eye can be

protruded beyond the skull because of its attachment to the

base of the tentacle (O’Reilly et al. 1996). The last group of

caecilians, Typhlonectidae, is semi-aquatic to aquatic with

attendant modifications, such as slight lateral compression

of the posterior part of the body. Hedges et al. (1993) found

the one species of Typhlonectidae analyzed to be nested

among neotropical caeciliids. Accordingly, they synonymized

the Typhlonectidae within Caeciliidae. Wilkinson and Nussbaum

(1996, 1999) rejected that conclusion because of poor

taxon sampling, preferring to wait until the relationships of

the Caeciliidae, sensu lato, were fully explored.

Salamanders

The node-based name for living salamanders is Caudata. The

502 species of living salamanders are arranged into 10 families

(fig. 25.4, table 25.1). Historically, salamanders are a

primarily Holarctic group of the north temperate regions; one

clade, the Bolitoglossini, has diversified in the Neotropics.

The largest salamanders are the Cryptobranchidae; adult

Andrias can reach 1.5m in total length. The smallest are

Thorius (Plethodontidae), which may have an adult length

as small as 30 mm.

Several salamanders are elongate and have reduced limbs.

Some are larger, aquatic, neotenic forms, such as Sirenidae,

Proteidae, and Amphiumidae. Fully aquatic salamanders

typically retain gill slits, and some have external gills resembling

crimson tufts of feathers. Elongate terrestrial salamanders

typically have reduced limbs and digits, and occupy

a semifossorial niche in leaf litter or burrows. At another

extreme are arboreal forms with palmate hands and feet and

reduced digits resulting from heterochrony.

Most of the major groups of salamanders have internal

fertilization accomplished by way of a spermatophore, typically

a mushroom-shaped mass of spermatozoa and mucous

secretions. The male deposits a spermatophore either in water

or on land, depending on the group. The female retrieves

it with her cloaca during courtship. The sperm may be retained

live in a cloacal pocket, the spermatheca, for months or even

years. Fertilized eggs are deposited and develop either directly,

in which case a small salamander hatches, or indirectly, in

which a larval salamander emerges, and later metamorphoses.

Relationships among Salamanders

Karaurus sharovi, the oldest salamander, is a fully articulated

Middle Jurassic fossil from Kazakhstan. The stem-based name

for the clade of Karaurus + Caudata is Urodela (“with a tail”),

so Karaurus is a urodele but not part of Caudata. Although

the fossil Karaurus firmly established salamanders in the Jurassic,

the fossil record of salamanders has not contributed

to resolution of relationships among extant taxa until recently.

However, crown-group salamanders belonging to the

Cryptobranchidae are now known from the Middle Jurassic

(Gao and Shubin 2003). Also, Gao and Shubin’s (2001)

analysis of Jurassic urodeles (fig. 25.4) placed these at the

base of the extant salamander tree with Hynobiidae and

Cryptobranchidae (Cryptobranchoidea). The Sirenidae

formed a clade with two other neotenic taxa (Proteidae and

Amphiumidae). In contrast, Duellman and Trueb (1986)

placed Sirenidae as the sister of all other salamanders, followed

by Cryptobranchoidea as sister to remaining salamanders.

A possible explanation for this discordance is that

salamanders are notorious for the amount of homoplasy in

pedomorphic features (Wake 1991). Of course, this alone

does not explain incongruence in nuclear and mitochondrial

rRNA data (mt-rRNA; see below).

Larson and Wilson (1989) and Larson (1991) presented a

tree (fig. 25.4) based on nuclear-encoded rRNA, which differed

dramatically in placing Plethodontidae and Amphiumidae at

the base of the tree. Larson and Dimmick (1993) combined

these molecular data with morphological data from Duellman

and Trueb (1986). The resulting tree effectively rerooted the

Larson (1991) tree to place Sirenidae and Cryptobranchoidea

at its base. Analyses of 12S and 16S mitochondrial DNA

(mtDNA; Hay et al. 1995, Hedges and Maxson 1993) also

placed Sirenidae at the base (fig. 25.4), but with different

relationships among other taxa.

Figure 25.4. Alternative relationships among the families of

salamanders.

Sinerpeton

Laccotriton

Cryptobranchidae

Hynobiidae

Amphiumidae

Sirenidae

Proteidae

Dicamptodontidae

Rhyacotritonidae

Ambystomatidae

Salamandridae

Plethodontidae

Sirenidae

Cryptobranchidae

Plethodontidae

Amphiumidae

Ambystomatidae

Rhyacotritonidae

Salamandridae

Dicamptodontidae

Proteidae

Hay et al. (1995)

mtDNA

Gao and Shubin (2001)

Fossils and morphology

Larson (1991)

rRNA

Amphiumidae

Plethodontidae

Rhyacotritonidae

Sirenidae

Cryptobranchidae

Hynobiidae

Proteidae

Salamandridae

Dicamptodontidae

Ambystomatidae

Larson and Dimmick (1993)

rRNA and morphology

Sirenidae

Cryptobranchidae

Hynobiidae

Rhyacotritonidae

Proteidae

Salamandridae

Dicamptodontidae

Ambystomatidae

Amphiumidae

Plethodontidae

Amphibians 437

Compared with caecilians and frogs, the placement of

family-level groups of salamanders remains in an extreme

state of flux, with very different topologies resulting from

different data sets (sequences, morphology, and fossils) and

combinations of those data sets. In contrast, there is almost

no disagreement about the content of the Linnaean families.

Ten families of living salamanders are generally recognized;

all clearly are monophyletic. Four are species-rich and extensively

sampled using molecular techniques. Substantial

progress has been made in generating phylogenetic hypotheses

at the species level, in contrast to frogs and caecilians.

In several families nearly all species have been examined.

Sirenidae include two genera of non-metamorphosing,

elongate neotenic forms that retain external gills as adults.

In contrast to most elongate salamanders, the front limbs are

present and robustly developed, whereas the hind limbs and

pelvic girdle is absent. Cryptobranchoidea are a clade generally

acknowledged to be among the most plesiomorphic

of living salamanders. The included families are Cryptobranchidae

and Hynobiidae. Cryptobranchidae include the

largest salamanders; adult Andrias may reach 1.5 m in length.

Recently described Jurassic cryptobranchid fossils (Gao and

Shubin 2003) represent the oldest crown-group salamanders,

i.e., members of Caudata. All Hynobiidae but 2 of the 42

species have been studied using mtDNA (A. Larson and R.

Macey, unpubl. obs.).

The Dicamptodontidae and Rhyacotritonidae each include

one living genus. Dicamptodon and Rhyacotriton have

been considered closely related and were united in the

Dicamptodontidae, but recent analyses (Good and Wake

1992, Larson and Dimmick 1993) place them as separate but

adjacent lineages.

Amphiumidae include only Amphiuma. This elongate

neotenic form lacks external gills and has limbs reduced to

spindly projections with remnants of the digits. Proteidae

include species both in North America and Europe. Necturus,

the beloved mudpuppy of comparative anatomy labs, is a

large pedomorphic salamander with large fluffy external gills.

Proteus, a very elongate and aquatic cave-dweller in SE Europe,

also retains external gills.

Ambystomatidae include 30 extant species of Ambystoma.

Nearly all have using mtDNA sequences and allozymes

(Shaffer 1984a, 1984b, Shaffer et al. 1991). Some species are

facultatively neotenic and retain the ability to metamorphose;

others are obligately trapped in the larval morphology, spending

their entire lives in lakes. Most have a larval period that

is always followed by metamorphosis to the adult condition.

Many species of Salamandridae are aposematic (having

a bright warning coloration) and have highly effective cutaneous

poison glands to deter predators. At least two species

are viviparous, a rare occurrence among salamanders.

Salamandridae are also diverse in morphology and life history,

although not as speciose as Plethodontidae (see below).

Relationships among salamandrids have been examined using

morphological data (Цzeti and Wake 1969, Wake and

Цzeti 1969), although not with current phylogenetic algorithms.

All 62 species of this widely distributed family have

been studied using molecular markers (Titus and Larson

1995, D. Weisrock and A. Larson, unpubl. obs.).

Plethodontidae are by far the largest family taxon, with

27 genera and 348 species (from a total of 502 species of

salamanders). Plethodontids are lungless and use primarily

cutaneous respiration. The release of the hyoid musculoskeleton

from the constraints of buccal force-pump breathing

has apparently permitted the diversification of mechanisms

prey capture by tongue protrusion. In addition to being the

most diverse in morphology and life history—there are highly

arboreal, aquatic, terrestrial, saxicolous, and fossorial forms—

this is the only clade with a neotropical radiation. Four major

groups of Plethodontidae are recognized: Desmognathinae,

Plethodontini, Bolitoglossini, and Hemidactyliini; work has

concentrated on relationships within each clade, and relationships

among the four are not clear.

All species of Desmognathinae have been studied with

mtDNA (Titus and Larson 1995, 1996). Detailed studies of

many species of Plethodontini have been published by

Mahoney (2001), and studies of all species are in progress

(M. Mahoney, D. Weisrock, and D. Wake et al., unpubl. obs.).

The Bolitoglossini have been sampled broadly. About 40%

of all salamanders are in the mainly Middle American clade

Bolitoglossa, and all genera and about 80% of its species have

some sequence data (Garcнa-Parнs et al. 2000a, 2000b, Garcнa-

Parнs and Wake 2000, Parra-Olea et al. 1999, 2001, Parra-

Olea and Wake 2001). Data from three mtDNA genes have

been collected for almost all tropical species in the lab of

D. Wake (pers. comm.). Jackman et al. (1997) examined relationships

of bolitoglossines based on a combination of morphological

and molecular data sets. Work is also underway

on the mostly aquatic plethodontids, the Hemidactyliini,

using ribosomal mtDNA and recombination activating protein

1 (RAG-1) (P. Chippindale and J. Wiens, unpubl. obs.).

Frogs

Living frogs include about 4837 species arranged in 25–30

families (fig. 25.4, table 25.1). The earliest forms considered

as proper frogs are Notobatrachus and Vieraella from the Middle

Jurassic of Argentina. Prosalirus vitis from Lower Jurassic of

Arizona (Jenkins and Shubin 1998, Shubin and Jenkins

1995) is fragmentary, but clearly a frog. All of these have

skeletal features that indicate that the distinctive saltatory

locomotion of frogs had evolved by this time.

The sister group of frogs proper is Triadobatrachus massinoti,

known from a single fossil from the Lower Triassic of

Madagascar. It has been called a proanuran and retains many

plesiomorphic features, such as 14 presacral vertebrae (living

frogs have nine or fewer) and lack of fusion of the radius

and ulna and also of the tibia and fibula (living frogs have

fused elements, the radioulna and tibiofibula; Rage and Rocek

438 The Relationships of Animals: Deuterostomes

1989, Rocek and Rage 2000). The clade containing Triadobatrachus

and all frogs is named Salientia.

Frogs have a dazzling array of evolutionary novelties associated

with reproduction. Their diverse vocal signals of the

males are used for mate advertisement and territorial displays.

Parental care is highly developed in many lineages, including

brooding of developing larvae on a bare back, in pouches

on the back of females, in the vocal sacs of males, and in the

stomach of females. Some females in some unrelated lineages

of Hylidae and Dendrobatidae raise their tadpoles in the

watery confines of a bromeliad axil and supply their own

unfertilized eggs as food. Whereas amniotes escaped from

the watery environment once with their evolution of the

amniote egg, frogs have done so many times; direct development,

with terrestrial eggs in which the tadpole stage is

bypassed in favor of development to a froglet, has evolved at

least 20 times.

Although some frogs have escaped an aquatic existence,

many have embraced it, taking the biphasic life to an extreme.

In contrast to caecilian and salamander larvae, frog tadpoles

are highly morphological specialized to exploit their transitory

and often unpredictable larval niche. The tadpole is

mostly a feeding apparatus in the head and locomotor mechanism

in the tail. The feeding apparatus is a highly efficient

pump that filters miniscule organic particles from the water.

Tadpoles do not reproduce; there are no neotenic forms.

They live their lives eating until it is time to make a quick

and awkward metamorphosis to a froglet.

Anura and Salientia

The names of higher frog taxa are used here following Ford

and Cannatella (1993). Their general rationale was (1) to

recognize only monophyletic groups except when it was not

feasible to reduce the nonmonophyletic group to smaller

clades (as in the case of “Leptodactylidae” and “Ranidae”),

(2) to identify nonmonophyletic groups as such, and (3) to

avoid the use of family names that are redundant with the

single included genus.

Ford and Cannatella (1993) defined Anura as the ancestor

of living frogs and all its descendants. The use of living

taxa as reference points or anchors for the definition follows

the rationale of de Queiroz and Gauthier (1990, 1992), who

convincingly argued that this stabilizes a definition. In contrast,

the incompleteness of fossil taxa and the discovery of

new fossils renders definitions based on extinct reference taxa

less stable.

The taxonomy of frogs illustrates this issue of taxonomic

practice. The Jurassic fossil Notobatrachus was considered by

Estes and Reig (1973) to be closely related to, and in the same

family as, the living taxa Ascaphus and Leiopelma. Thus, Notobatrachus

would be included in Anura according to Ford and

Cannatella’s definition. In contrast, the analyses by Cannatella

(1985) and Bбez and Basso (1996) placed Notobatrachus as

the sister group to the clade containing Ascaphus, Leiopelma,

and other living frogs. The latter placement means that Notobatrachus

is not part of Anura, because Anura is defined as

the last common ancestor of living frogs and all its descendants.

Some herpetologists or paleontologists may be rankled

by the proposition that the very froglike Notobatrachus is not

part of Anura. But this concern is based on a typological

notion that the definition of a taxon name is tied to a combination

of characters, rather than to a branch of the Tree of

Life.

We can ask, Are there characters that make a frog a frog?

The eidos of a frog requires a big head, long legs, no tail, and

a short vertebral column. But how short? Most living frogs

have eight presacral vertebrae. Notobatrachus and two of the

most “primitive” frogs, Ascaphus and Leiopelma, have nine.

Another Jurassic fossil, Vieraella herbsti, has 10 vertebrae

(Bбez and Basso 1996). All of the aforementioned look like

proper “frogs.” The Triassic fossil Triadobatrachus has 14

vertebrae (Rage and Rocek 1989). It has several unambiguous

synapomorphies that place it as the sister group of frogs.

It is considered froglike, but not quite a frog. In summary, it

seems the consensus of published work is that 10 or fewer

presacral vertebrae make a frog a frog.

When fossil X with 11 presacral vertebrae is discovered,

will the boundary of “frogness” move one node lower in the

tree, so as to include fossil X? This question highlights the

problem: when a taxon name is defined by a diagnostic character,

each new fossil with an intermediate condition will

stretch the definition of the name (Rowe and Gauthier 1992).

However, it is less likely that the discovery of a new living

frog species will stretch our concept of frogness. Therefore,

attaching the taxon name Anura to a node circumscribed by

living taxa will yield a more stable definition. Because Anura

is defined as the ancestor of living frogs and all its descendants,

the discovery of a new fossil just below this node, no

matter how froglike, will not require a change in the meaning

of Anura. And, we can still argue about which characters

make a frog a frog.

“Salientia” is the stem-based name for the taxon including

Anura and taxa (all fossils) more closely related to Anura than

to other living amphibians. Salientia include Triadobatrachus,

Vieraella, Notobatrachus (Bбez and Basso 1996), Czatkobatrachus

(Evans and Borsuk-Bialynicka 1998), and Prosalirus

(Shubin and Jenkins 1995). Because the name is tied to a stem,

the discovery of new fossils on this stem will not destabilize

the name. The use of Salientia for Triadobatrachus plus all other

frogs is widespread and not controversial.

Our understanding of frog phylogeny rests primarily on

morphological data (Griffiths 1963, Inger 1967, Kluge and

Farris 1969, Lynch 1973, Noble 1922, Trueb 1973), summarized

by Duellman and Trueb (1986) and Ford and

Cannatella (1993; fig. 25.5). In general, morphological characters

resolved the plesiomorphic basal branches known as

archaeobatrachians (Cannatella 1985, Duellman and Trueb

1986, Haas 1997). The family-level relationships within NeoAmphibians

439

batrachia, a large clade with more than 95% of frog species,

are mostly unresolved (Ford and Cannatella 1993) by morphological

data, although Ranoidea is strongly supported.

Most remaining neobatrachians are known as hyloids or

bufonoids, but no morphological evidence for their monophyly

has been proposed (with the possible exception of

sperm morphology; Lee and Jamieson 1992). Ranoidea is

primarily Old World; hyloids are mostly New World.

The distinctive and diverse morphology of tadpoles has

been a source of characters to elucidate frog phylogeny. At

one time it was thought that the larval morphology of the

pipoid frogs argued for their position as the most primitive

(early-branching in this context), but highly specialized,

group (Starrett 1968, 1973). However, other interpretations

(Cannatella 1999, Haas 1997, Sokol 1975, 1977) indicate

that although pipoids are highly specialized, the discoglossoids

are the earliest-branching frog lineages (see below).

However, the most comprehensive analysis of larval morphology

(Haas 2003) found Ascaphus to be the most basal

frog and pipoids to be the next adjacent clade (fig. 25.6),

rather than other discoglossoids. Maglia et al. (2001) reported

Pipoidea to be the sister taxon of all other frogs, a

hypothesis reminiscent of Starrett (1968, 1973).

The fossil record of frogs was thoroughly reviewed by

Sanchiz (1998). Bбez and Basso (1996) included Jurassic fossils

in a phylogenetic analysis of early frogs. Gao and Wang

(2001) analyzed data for a combined treatment of fossil and

living archaeobatrachians and pre-archaeobatrachians, but

they reached very different conclusions than did Ford and

Cannatella (1993); a full analysis of this is beyond the scope

of this chapter.

A range of morphological phylogenetic studies treats

relationships within particular family-level groups: Pelobatoidea

(Maglia 1998); Hyperoliidae (Drewes 1984); Rhacophoridae

and Hyperoliidae (Liem 1970); Myobatrachidae

sensu lato, including Myobatrachinae and Limnodynastinae

(Heyer and Liem 1976); Leptodactylidae (Heyer 1975);

Hylinae (da Silva 1998); Microhylidae (Wu 1994); Hemiphractinae

(Mendelson et al. 2000); and Pipidae (Cannatella

and Trueb 1988a).

Sequences from both nuclear and mt-rRNA genes provided

new data (Emerson et al. 2000, Graybeal 1997, Hay

et al. 1995, Hedges and Maxson 1993, Hedges et al. 1990,

Hillis et al. 1993, Ruvinsky and Maxson 1996, Vences et al.

2000). Several alternative hypotheses emerged from these

works, including (1) monophyly of “Archaeobatrachia,” (2)

weak monophyly of the bufonoids (= Hyloidea), (3) dendrobatids

excluded from Ranoidea, and (4) extensive paraphyly

of the large families Hylidae and Leptodactylidae.

The “Basal” Frogs—Discoglossoids

A group of plesiomorphic lineages includes Ascaphus, Leiopelma,

Bombinatoridae, and Discoglossidae (Ford and Cannatella

1993); this group has been called discoglossoids and

is paraphyletic with respect to other frogs, the Pipanura.

Figure 25.5. Alternative phylogenies of frogs. The tree on the left is labeled with taxon names

(see text).

Ascaphus

Leiopelma

Bombinatoridae

Discoglossidae

Pelobatoidea

Mesobatrachia

Pipoidea

Pipanura

Discoglossanura

Bombinanura

Anura

Rhinophrynus

Pipidae

Pelobatidae

Pelodytidae

Sooglossidae

Myobatrachinae

Heleophryne

Limnodynastinae

"Leptodactylidae"

Bufonidae

Rhinoderma

Centrolenidae

Pseudidae

Hylinae

Hemiphractinae

Phyllomedusinae

Pelodryadinae

Dendrobatidae

Microhylidae

Hyperoliidae

Hemisus

Artholeptidae*

"Ranidae"

Rhacophoridae

Allophryne

Brachycephalidae

Bufonoidea

(Hyloidea)

Ascaphus

Leiopelma

Bombinatoridae

Discoglossidae

Rhinophrynus

Pipidae

Pelobatidae

Pelodytidae

Sooglossidae

Heleophryne

Leptodactylidae

Pseudidae

Bufonidae

Rhinoderma

Centrolenidae

Hyla

Dendrobatidae

Microhylidae

Hyperoliidae

Mantella

Rana

Limnodynastinae

Hylidae

"Ranidae"

"Myobatrachidae" Bufonoidea

Ranoidea

Neobatrachia

Ford and Cannatella, 1993 Hay et al., 1995

Archaeobatrachia

Ranoidea

Neobatrachia

Archaeobatrachians

Megophryidae

440 The Relationships of Animals: Deuterostomes

Ascaphus and Leiopelma are plesiomorphic, now-narrowly

distributed relicts of a once more widely distributed Mesozoic

frog fauna (Green and Cannatella 1993). The family

name Ascaphidae is redundant with Ascaphus. The family

name Leiopelmatidae is redundant with the single genus

Leiopelma. Formerly, The name Leiopelmatidae (sensu lato)

has also been used to include Ascaphus and Leiopelma, a group

that is probably paraphyletic.

“Bombinanura” is the node-based name for the last common

ancestor of Bombinatoridae + Discoglossanura; Bombinatoridae

is the node name for the ancestor of Bombina and

Barbourula and all of its descendants (Ford and Cannatella

1993); this node is well supported (Cannatella 1985; but see

Haas 2003).

The names Discoglossoidei (Sokol 1977) and Discoglossoidea

(e.g., Duellman 1975, Lynch 1971) were used for the

group containing Ascaphus, Leiopelma, Bombina, Barbourula,

Alytes, and Discoglossus. The Discoglossoidei of Sokol (1977)

and Duellman and Trueb (1986) were a clade; however, other

morphological analyses strongly reject this conclusion. As an

informal term, the name discoglossoids is a useful catchall

for plesiomorphic anurans that are not part of Pipanura. One

general primitive feature of this group is the rather rounded,

disklike tongue; hence the name. Alytes and Discoglossus are

included in the Discoglossidae, although the two are fairly

divergent and evidence of monophyly is not overwhelming.

Some evidence indicates that Discoglossidae are more closely

related to other frogs than to Bombinatoridae, Ascaphus, or

Leiopelma (Ford and Cannatella 1993).

Pipanura

Pipanura consists of Pipoidea, Pelobatoidea, and Neobatrachia,

that is, living frogs minus discoglossoids. Specifically,

it is the node name for the last ancestor of Mesobatrachia +

Neobatrachia, and all of its descendants (Ford and Cannatella

1993). Pipoidea and Pelobatoidea are regarded as intermediate

lineages between discoglossoids and Neobatrachia.

Mesobatrachia is the node name applied to the last ancestor

of Pelobatoidea + Pipoidea. Support for this clade is not

strong (Cannatella 1985). Pelobatoids and pipoids are represented

by a large number of Cretaceous and Tertiary fossils

(Rocek 2000, Sanchiz 1998).

The node name Pelobatoidea was defined by Ford and

Cannatella (1993) as the (last) common ancestor of living

Megophryidae, Pelobatidae, and Pelodytes, and all its descendants.

The content of Pelobatoidea is not controversial.

Historically, Pelobatidae has included Megophryidae

as a subfamily (e.g., Duellman and Trueb 1986), although

recent summaries recognize Megophryidae (e.g., Zug et al.

2001). This follows Ford and Cannatella (1993), who defined

Pelobatidae as the node name for the last common

ancestor of Pelobates, Scaphiopus, and Spea, and all its descendants.

This definition was based on a sister-group

relation between the European (Pelobates) and American

spadefoots (Scaphiopus + Spea), which were united by

synapomorphies related to their habitus as fossorial species

(Cannatella 1985, Maglia 1998).

In contrast, Garcнa-Parнs et al. (2003) reexamined relationships

among all pelobatoids using mtDNA and found

Scaphiopus + Spea to be the sister group of other pelobatoids

(Pelobates, Pelodytidae, and Megophryidae). Because Scaphiopus

+ Spea, which they termed Scaphiopodidae, were no

longer related to Pelobates, they inferred the fossorial habitus

of the two groups to be convergent. The taxonomic implication

of this finding is that Pelobatidae as defined by Ford

and Cannatella (1993) applies to the same node as

Pelobatoidea. One solution would be to redefine Pelobatidae

as a stem name so as to include the fossil taxa that are thought

to be closely related, such as Macropelobates. But the issue

remains unresolved.

The node name Megophryidae was used by Ford and

Cannatella (1993) for the group of taxa referred to as megophryines,

previously been considered to be a subfamily (Megophryinae)

of Pelobatidae. Although preliminary work exists

(Lathrop 1997), relationships among the Megophryidae have

not been assessed in detail; however, the content is uncon-

Figure 25.6. A phylogeny of frogs based mostly on larval

morphology, simplified from Haas (2003: fig. 3).

Bufonidae

Leptodactylidae

Neobatrachia

Ranoidea

Ranoidea

Hylinae: Hylidae

Hemiphractinae:

Hylidae

Pelodryadinae:

Hylidae

Discoglossidae

(sensu lato)

Pipoidea

Pelobatoidea

Myobatrachidae

Ceratophryinae:

Leptodactylidae

Pseudidae

Dendrobatidae

Phyllomedusinae:

Hylidae

Centrolenidae

Ascaphus

Rhinophrynus

Xenopus

Pipa

Alytes

Discoglossus

Bombina

Spea

Heleophryne

Pelodytes

Leptobrachium

Pelobates

Megophrys

Limnodynastes

Cochranella

Lepidobatrachus

Ceratophrys

Nyctimystes

Litoria

Litoria

Phyllomedusinae

Hyla

Scinax

Aplastodiscus

Smilisca

Phrynohyas

Osteocephalus

Pseudis

Ranidae

Ranidae

Gastrotheca

Rhacophoridae

Hemisus

Hyperoliidae

Microhylidae

Microhylidae

Microhylidae

Odontophrynus

Leptodactylus

Physalaemus

Pleurodema

Crossodactylus

Hylodes

Dendrobatidae

Bufonidae

Bufonidae

Bufonidae

Amphibians 441

troversial. In contrast to most of the family-level names,

Pelodytidae was defined as a stem name by Ford and Cannatella

(1993) because its use as a node name for the clade

of living taxa would make it redundant with Pelodytes. Also,

use of a stem name retains the several taxa of fossil

pelodytids within Pelodytidae, a placement that is well supported

(Henrici 1994).

Pipoidea was implicitly defined as the node name for the

most recent common ancestor of Pipidae and Rhinophrynidae,

and all its descendants. By this definition, the fossil family

Palaeobatrachidae are included within Pipoidea, as has generally

been the case (but see Spinar 1972). Relationships among

pipoids have been examined by Cannatella and Trueb (Bбez

1981, Bбez and Trueb 1997, Cannatella and de Sб 1993, Cannatella

and Trueb 1988a, 1988b, de Sб and Hillis 1990)

As pointed out by Ford and Cannatella (1993), the phylogenetic

definition of the name Pipidae excluded several

fossils previously and currently included in Pipidae (Bбez

1996). The stem name Pipimorpha was proposed to accommodate

these. Because the name applies to those taxa that

are more closely related to (living) Pipidae than to Rhinophrynidae,

it is a useful descriptor for the increasingly specialized

taxa on the stem leading to the Pipidae. Bбez and

Trueb (1997) defined Pipidae slightly differently; their tree

is unresolved at the crucial point. The single species of highly

fossorial frog Rhinophrynus dorsalis is regarded to be the sister

group of Pipidae, among living forms. Like Pelodytidae,

the name Rhinophrynidae was defined as a stem name by

Ford and Cannatella (1993).

Neobatrachia

Neobatrachia consist of the “advanced” frogs and includes

95% of living species. Except for the Late Tertiary, they are

not well represented in the fossil record. Neobatrachia is well

supported by both morphological and molecular data (Ford

and Cannatella 1993, Ruvinsky and Maxson 1996, but see

Haas 2003). Two groups of Neobatrachia have been generally

recognized: Bufonoidea (Hyloidea has priority; see below)

for arciferal neobatrachians, and Ranoidea for the firmisternal

neobatrachians. These correspond roughly to the classic

Procoela and Diplasiocoela of Nicholls (1916) and Noble

(1922), respectively. Hyloidea are primarily a New World

clade, and Ranoidea an Old World group, although the

hyloids have significant radiations in the Australopapuan

region as do Ranidae and Microhylidae in the New World.

Hyloidea (formerly Bufonoidea) include Bufonidae,

Hylidae, “Leptodactylidae,” Centrolenidae, Pseudidae, Brachycephalidae,

Rhinoderma, and Allophryne. Ford and Cannatella

(1993) noted that Hyloidea and Bufonoidea apply to

a nonmonophyletic group, that is, neobatrachians that were

not ranoids. Ranoidea (see below) consist of ranids (including

arthroleptids and mantellines), hyperoliids, rhacophorids,

Hemisus, and microhylids. Some authors have placed

microhylids in the superfamily Microhyloidea to reflect the

distinctiveness of the microhylid larva (e.g., Starrett 1973).

But agreement is universal that microhylids are more closely

related to ranoids than to hyloids.

Lynch (1973) considered Pelobatoidea an explicitly paraphyletic

group transitional between “archaic frogs” and the

“advanced frogs.” He included here Pelobatidae, Pelodytidae,

Heleophrynidae, the myobatrachids, and Sooglossidae. His

dendrogram (Lynch 1973: fig. 3-6) showed Bufonoidea and

Ranoidea as independently derived from the paraphyletic

Pelobatoidea. Duellman (1975) used Reig’s (1958) Neobatrachia

to include Lynch’s Bufonoidea and Ranoidea. Subsequent

morphological and molecular analyses have supported

monophyly of Neobatrachia (Cannatella 1985, Hay et al. 1995,

Ruvinsky and Maxson 1996). However, supposed basal neobatrachians

such as myobatrachids, sooglossids, and Heleophryne

are of uncertain position.

Until recently, Limnodynastinae and Myobatrachinae

were included as subfamilies of “Myobatrachidae” (e.g., Heyer

and Liem 1976). Ford and Cannatella (1993) could find no

synapomorphies for “Myobatrachidae.” However, Lee and

Jamieson (1992) provided some characters from spermatozoan

ultrastructure that support myobatrachid monophyly.

Some textbooks (Zug et al. 2001) have recognized each group

as a distinct family [which was not Ford and Cannatella’s

(1993) intention]. Ruvinsky and Maxson (1996) placed Myobatrachinae,

Limnodynastinae, and Heleophryne (Heleophrynidae)

in a clade of at the base of Hyloidea. Some recent

phylogenies have placed Sooglossidae as the sister group of

all other Hyloidea (Ruvinsky and Maxson 1996), sister group

to Ranoidea (Emerson et al. 2000), basal to both (Hay et al.

1995), or as the sister of Myobatrachidae (Duellman and

Trueb 1986) or Myobatrachinae (Ford and Cannatella 1993).

Hyloidea

Hyoidea has been used to refer to neobatrachians with an

arciferal pectoral girdle, in contrast to those with a firmisternal

girdle, the ranoids. The name has Linnaean priority over Bufonoidea

(Dubois 1986), although it has not been used often.

Ford and Cannatella found no published data to support its

monophyly. Hay et al. (1995) were the first to use character

data to support the monophyly of Hyloidea (as Bufonoidea).

This lineage included Myobatrachidae, Heleophrynidae, and

Dendrobatidae, Centrolenidae, Hylidae, Bufonidae, Rhinodermatidae,

Pseudidae, and Leptodactylidae. They also

identified the Sooglossidae as a “distinct major lineage” of

Neobatrachia apart from Hyloidea and Ranoidea. Ruvinsky and

Maxson (1996), using mostly the same data as Hay et al. (1995),

concluded that Sooglossidae was included within Hyloidea.

Darst and Cannatella (in press) identified a well-supported

clade (fig. 25.7) for which they defined the name

Hyloidea in a phylogenetic context. They excluded from the

definition taxa such as Dendrobatidae whose phylogenetic

442 The Relationships of Animals: Deuterostomes

position might make the content of this taxon unstable. Also,

they excluded certain neobatrachian groups whose placement

is more relatively basal and also less well resolved, such

as Myobatrachinae, Limnodynastinae, and Sooglossidae.

“Leptodactylidae” are a hodgepodge of hyloids that lack

distinctive apomorphies. Historically, the derived features of

the other hyloid families separated them from Leptodactylidae,

suggesting it was paraphyletic. Hylidae have cartilaginous

intercalary elements between the ultimate and

penultimate phalanges of the hands and feet; Centrolenidae

has the two elongate ankle bones (tibiale and fibulare) fused

into a single element; Pseudidae have bony intercalary elements,

in contrast to the generally cartilaginous ones found

in hylids; has a Bidder’s organ present in males; this is a

portion of embryonic gonad that retains an ovarian character.

Rhinodermatidae have rearing of larvae in the vocal sac

of the male; Brachycephalidae lack a well-developed sternum.

Phylogenetic relationships among the genera of “Leptodactylidae”

were analyzed using morphology by Heyer

(1975). Basso and Cannatella (2001) analyzed relationships

among leptodactyloid frogs from 12S and 16S mtDNA and

found “Leptodactylidae” to be polyphyletic. Darst and Cannatella

(2003) also found the same, based on a smaller sample

of leptodactylid taxa (fig. 25.7).

Pseudidae, Centrolenidae, Brachycephalidae, and Dendrobatidae

are node names whose content is not controversial.

Recent work has clarified the relationships of some of

these groups. Darst and Cannatella (in press) found Dendrobatidae

to be nested clearly within Hyloidea and were able

to reject the alternate hypothesis that dendrobatids are within

Ranoidea (Ford 1989, Ford and Cannatella 1993). Duellman

(2001) reduced Pseudidae to a subfamily. However, this

action stopped short of what would be demanded by the

Linnaean system. If Pseudidae is not acceptable as a family

within Hylidae, then Pseudinae cannot be accepted as a subfamily

within the subfamily Hylinae. Darst and Cannatella

(in press) also found Pseudidae to be nested within hylines,

specifically the sister group to Scarthyla ostinodactyla. Assuming

an adherence to Linnaean taxonomy coupled with a desire

to recognize only monophyletic groups, then there is no

basis for recognition of the group at a subfamily or even tribe

level.

Darst and Cannatella also found Brachycephalidae to be

within eleutherodactylines (“Leptodactylidae”); the taxonomic

changes necessitated by these new findings are in

progress. Allophryne ruthveni is an enigmatic hyloid (Fabrezi

and Langone 2000) that has been placed in a monotypic (and

redundant) family Allophrynidae; it is probably the sister

group of Centrolenidae (Austin et al. 2002). The two species

of Rhinoderma have been placed in Rhinodermatidae.

Were it not for the apomorphic life history of the two species,

in which the males brood the developing larvae in their

vocal sacs, Rhinoderma would be included in the “Leptodactylidae.”

Ford and Cannatella (1993) provided phylogenetic

names for these taxa.

Hylidae is the node name for the most recent common

ancestor of Hemiphractine, Phyllomedusinae, Pelodryadinae,

and Hylinae, and all of its descendants. These latter four

names have not been formally defined in a phylogenetic manner,

but the composition of each is well established. Some

workers elevated Pelodryadinae to family level (Dubois 1984,

Savage 1973). Morphology-based phylogenies of Hylinae and

Hemiphractinae exist (da Silva 1998, Mendelson et al. 2000).

According to Darst and Cannatella (in press), Hylidae is polyphyletic;

however, their sample of hemiphractines, which are

the troublesome species, was small.

Bufonidae is also a node name. Recent work (Gluesenkamp

2001, Graybeal 1997, Graybeal and Cannatella 1995) found

no basis for the subfamilies or tribes recognized by Dubois

(1984). Relationships among the higher groups of Bufonidae

are unresolved.

Ranoidea

Ford and Cannatella (1993) defined Ranoidea as the nodebased

name for the clade anchored by the last common ancestor

of hyperoliids, rhacophorids, ranids, dendrobatids, Hemisus,

arthroleptids, and microhylids. With the possible exception of

the controversial dendrobatids, the content of this group includes

the classic “firmisternal” frogs, Firmisternia. Wu (1994)

treated the Ranoidea and Microhyloidea as the two components

of Firmisternia. The resurrection of this arrangement has merit

in recognizing the two major clades of firmnisternal frogs, as

in the past where the groups were Microhyloidea and Ranoidea.

Duellman (1975), for example, recognized distinct superfamilies

Microhyloidea and Ranoidea.

Growing evidence suggests that Microhylidae (or at least

a large clade of those) is the sister group to Hyperoliidae or

Hyperoliidae + arthroleptines within the Ranoidea (Darst and

Cannatella in press, Emerson et al. 2000, Hay et al. 1995)

rather than the sister group of all other ranoids. Thus, inclusion

of Microhylidae within Ranoidea is appropriate in one

sense. However, one could argue equally that Microhyloidea

could include Microhylidae (minimally the type-genus) and

whatever else is more closely related to these than to Ranidae.

Microhyloidea and Ranoidea would be sister taxa in Firmisternia.

For example, Darst and Cannatella (in press) and

Emerson et al. (2000) each recovered two major clades of

ranoids, one including hyperoliids, arthroleptids, microhylids

(including brevicipitines), and Hemisus, and the other

containing rhacophorids, mantellines, and the remaining

“ranids.” However, Blommers-Schlцsser (1993) recognized

Microhyloidea as consisting of Microhylidae, Sooglossidae,

Dendrobatidae, and Hemisotidae. We have not followed this

unusual rearrangement pending a broader synthesis of morphological

and molecular data of ranoids. For the moment,

we continue the use of Ranoidea for all these firmisternal frogs

because of its recent common use.

Perhaps the most controversial group within Neobatrachia

has been Dendrobatidae. Hay et al. (1995) and Ruvinsky

Amphibians 443

Figure 25.7. Phylogeny of Hyloidea based on a Bayesian analysis, after Darst and Cannatella

(in press). The numbers on the branches are posterior probabilities.

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Hyloidea

Ranoidea

0.05 changes

Heleophryne purcelli

Limnodynastes salminii

Platymantis sp.

Philautus acutirostris

Rhacophorus monticola

Rana nicobariensis

Rana temporaria

Hyperolius sp.

Callulina kreffti

Hemisus marmoratum

Gastrophryne olivacea

Kaloula conjuncta

Nelsonophryne aequatorialis

Cryptobatrachus sp.

Phrynomerus bifasciatus

Eleutherodactylus cuneatus

Eleutherodactylini:

Leptodactylidae

Bufonidae

Leptodactylinae: Leptodactylidae

Centrolenidae

Dendrobatidae

Ceratophryinae: Leptodactylidae

Telmatobiinae: Leptodactylidae

Pelodryadinae: Hylidae

Phyllomedusinae: Hylidae

Hylinae: Hylidae

Brachycephalus ephippium

Hylactophryne augusti

Eleutherodactylus fitzingeri

Eleutherodactylus mexicanus

Phrynopus sp.

Eleutherodactylus w-nigrum

Eleutherodactylus duellmani

Eleutherodactylus thymelensis

Eleutherodactylus chloronotus

Eleutherodactylus sp.

Eleutherodactylus supernatis

Dendrophryniscus minutus

Osornophryne guacamayo

Didynamipus sjostedti

Schismaderma carens

Bufo steindachneri

Bufo kisoloensis

Ansonia sp.

Pedostibes hosei

Bufo retiformis

Bufo woodhousii

Bufo bufo

Bufo marinus

Bufo alvarius

Bufo valliceps

Bufo boreas

Bufo exsul

Bufo microscaphus

Lithodytes lineatus

Leptodactylus pentadactylus

Physalaemus nattereri

Physalaemus riograndensis

Hyalinobatrachium sp.

Cochranella sp.

Centrolene sp.

Centrolene sp.

Allobates femoralis

Allobates femoralis

Colostethus infraguttatus

Phyllobates bicolor

Dendrobates reticulatus

Dendrobates auratus

Lepidobatrachus sp.

Ceratophrys ornata

Ceratophrys cornuta

Telmatobius niger

Telmatobius vellardi

Alsodes monticola

Gastrotheca pseustes

Pelodryas caerulea

Nyctimystes kubori

Litoria arfakiana

Phyllomedusa tomopterna

Phyllomedusa palliata

Pachymedusa dacnicolor

Agalychnis litodryas

Agalychnis saltator

Scinax garbei

Scinax rubra

Smilisca phaeota

Pseudacris brachyphona

Osteocephalus taurinus

Trachycephalus jordani

Pseudis paradoxa

Scarthyla ostinodactyla

Hyla triangulum

Hyla pantosticta

Hyla sp.

Hyla pellucens

Hyla picturata

Hyla lanciformis

Hyla calcarata

Phrynohyas venulosa

Melanophryniscus sp.

Melanophryniscus stelzneri

Atelopus varius

Bufo biporcatus

Hemiphractinae: Hylidae

444 The Relationships of Animals: Deuterostomes

and Maxson (1996) found dendrobatids to be nested within

hyloids (bufonoids), and this was corroborated with a

broader taxon sample by Darst and Cannatella (in press) and

with mostly larval data by Haas (2003). In retrospect, Ford

and Cannatella’s (1993) inclusion of a potentially unstable

taxon (Dendrobatidae) as a specifier taxon for Ranoidea was

not wise. Accepting the new evidence for the position of

Dendrobatidae, Ranoidea as they defined it has now the same

content as Hyloidea + Ranoidea; this is a drastic departure

from its usual content. Rather than redefine Ranoidea here

(because of work in progress), for the moment we consider

statements about Ranoidea to exclude Dendrobatidae.

Microhylidae were found to be nested within Ranoidea

by Ford and Cannatella (1993), Hay et al. (1995), Ruvinsky

and Maxson (1996), and (at least close to some) Haas (2003).

Ford and Cannatella (1993) used larval evidence from Wassersug

(1984, 1989) to formally recognize Scoptanura as a

large clade within Microhylidae. This was corroborated by

Haas (2003).

Wu (1994) produced the most comprehensive survey of

microhylid osteology, examining 188 characters in 105 species

in 56 of the 64 named genera. He adopted a rankless taxonomy,

placing Microhyloidea and Ranoidea as sister groups

in the Firmisternia. His Ranoidea included Hyperoliidae, Mantellidae,

Ranidae, and Rhacophoridae, and his Microhyloidea

consisted of two families, Brevicipitidae and Microhylidae. Wu’s

Brevicipitidae was unusual in that it included a clade Hemisotinae

composed of Hemisus and Rhinophrynus. The latter has

never been placed within Neobatrachia and shares many molecular

and morphological synapomorphies with Pipidae

(Cannatella 1985, Hay et al. 1995).

Relationships of ranoid frogs (microhylids aside) are in

a kinetic state, and the taxonomy we follow is certainly arbitrary.

For years an accepted arrangement was Ranidae,

Hyperoliidae, and Rhacophoridae, the latter two families

being treefrog morphs independently derived from within

Ranidae. It was generally appreciated that the mantelline

ranids (Mantellinae or Matellidae) shared some derived features

with Rhacophoridae (e.g., Duellman and Trueb 1986).

Ford and Cannatella (1993) embellished “Ranidae” with

quotes to indicate its status as a nonmonophyletic group.

Most recent attempts to establish a classification of Ranidae

have been based on a hypothesis of phylogeny (but see Dubois

1992, Inger 1996). Phylogenetic analyses of both sequence data

and morphological characters exist for Hyperoliidae and

Rhacophoridae (Channing 1989, Drewes 1984, Liem 1970,

Richards and Moore 1996, 1998, J. Wilkinson et al. 2002).

Although Rhacophoridae have generally been thought to be

monophyletic, accumulating evidence suggests that the Malagasy

rhacophorids are not the closest relatives of the Asian

rhacophorids (J. Wilkinson et al. 2002) and may be more

closely related to other Malagasy lineages, such as mantellines.

The most comprehensive analysis of ranoids (Emerson

et al. 2000), which used mostly published molecular data and

10 morphological characters, found familiar results: the close

relationship of Microhylidae and Hyperoliidae (Hay et al.

1995); the placement of Sooglossidae outside of Ranoidea

(Hay et al. 1995); and mantelline ranids most closely related

to, or nested within, rhacophorids (Channing 1989, Ford

1989). Relationships among a small sample of Indian ranoids

were examined by Bossuyt and Milinkovitch (2001).

As had historically happened with hyloids, the recent

taxonomic tendency for ranoids has been to elevate some

loosely defined subfamily groups to family status; for example,

the recognition of Arthroleptidae by Dubois (1984).

These have usually been considered to be a subfamily of

Ranidae, and its elevation to family level was more because

of taxonomic tinkering than any new knowledge of relationships.

Ford and Cannatella (1993) considered it a metataxon.

Blommers-Schlцsser (1993) recognized a clade Ranoidea

comprising Arthroleptidae, Hyperoliidae, and Ranidae, the

last including Mantellinae and Rhacophorinae. Emerson et al.

(2000: table 1) listed subfamilies of Ranidae as Raninae,

Mantellinae, and Rhacophorinae, reportedly from Blommers-

Schlцsser (1993). Actually, Blommers-Schlцsser (1993) included

these three, plus Cacosterninae, Nyctibatrachinae,

Petropedetinae, and Indiraninae, for a total of seven subfamilies

of Ranidae. Of these, the petropedetines have been arbitrarily

elevated to familial rank by some. Hemisus, one of the

few frogs known to burrow headfirst, has usually been placed

in the redundant family Hemisotidae. It was considered to be

derived from some group of African ranids, but recent molecular

analysis suggests closer relationships to brevicipitine

microhylids (Darst and Cannatella in press; fig. 25.7), as did

a morphological analysis (Blommers-Schlцsser 1993).

Prospects for the Future

Rather than address the future of the systematics of Amphibia,

we offer some general comments are possibly applicable

to all groups. Information age technology has changed

the nature of systematics. The flood of data from molecular

systematics continue to rise as new technologies facilitate its

collection. The program solicitation for the National Science

Foundation’s Assembling the Tree of Life competition (National

Science Foundation 2003) indicated the need for “scaling

up” the level of activity of data collection. But scaling up

in nature is rarely isometric; a change in size demands a

change in shape. Put another away, we will not reach the goal

of the Tree of Life (or the Tree of Amphibia) without doing

systematics differently. We suggest that some of the core

practices of systematics pose a severe impediment to completing

the Tree of Life. Methods and theory of tree construction

have “gone to warp speed” relative to the practices of

taxonomy, nomenclature, and biodiversity studies.

Our facility at reconstructing phylogeny now exceeds our

ability to describe new species in a reasonable amount of time.

Amphibians 445

Classically trained systematists, even those with active programs

in molecular systematics, must still linger over species

descriptions. Descriptions of new species are not much

different than those published more than a century ago. Some

systematists have bemoaned the dearth of jobs for classically

trained taxonomists. But even if positions were available,

would there be systematists interested in filling them? Proposals

for automation of species descriptions have not received

rave reviews. Is the practice of taxonomy really a

different enterprise than phylogenetic analysis (Donoghue

2001)? Perhaps it is time to redefine the mode and meaning

of “describing a new species.”

We are not advocating a reductionist, barcode approach

(Blaxter 2003) in which a sequence of one gene is sole diagnosis

of a species. However, DNA sequences are a powerful

source of data for species discovery and description, and we

welcome a fusion between traditional activities of species

description and the opportunities offered by information

technology. The nature of this compromise is not clear, but

it is evident that our mandate will not succeed without consideration

of this issue.

Related to the description of new species is nomenclature,

the rules for bestowing and keeping track of names.

Although the term “Phyloinformatics” has entered the language

of systematics, it lacks a meaningful definition. We

do not attempt one here, but certainly any concept of phyloinformatics

must include storage and retrieval systems

for taxonomy and nomenclature. Like others, we suggest

that the Linnaean system needs informatics-based reengineering;

it is a square peg in the world of information

technology.

Last, the increasing difficulty of on-site biodiversity studies

must be addressed. Legitimate concerns over the loss of

natural resources and opportunities through bioprospecting

and biopiracy have grown in the same regions that harbor

the greatest proportion of biodiversity. If natural history

collections and related information are as precious as we

claim, then we must invest in the countries of origin to enable

the development of those resources on-site. The alternative,

the removal of collections to another country largely

for reasons of convenience, meets with increasing and justifiable

resistance. This investment must be genuine and durable,

so that local researchers are enabled to do long-term

research. Only this type of investment will ensure the survival

of the biodiversity that we all value.

Acknowledgments

We thank Joel Cracraft and Michael Donoghue for the opportunity

to participate in the symposium. David Wake and Marvalee

Wake were coauthors on the symposium presentation and

offered much useful criticism on this manuscript; however, the

opinions expressed herein are our own.

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