seventee n The Early Tetrapods and Modern Amphibians

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Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
17
c h a p t e r • • • • • •
s e v e n t e e n
The Early Tetrapods
and Modern Amphibians
Vertebrate Landfall
The chorus of frogs beside a pond on a spring evening heralds one of
nature’s dramatic events.Masses of frog eggs soon hatch into limbless,
gill-breathing,fishlike tadpole larvae.Warmed by the late spring sun,
they feed and grow.Then,almost imperceptibly,a remarkable transfor-
mation takes place.Hindlegs appear and gradually lengthen.The tail
shortens.The larval teeth are lost,and the gills are replaced by lungs.
Eyelids develop.The forelegs emerge.In a matter of weeks the aquatic
tadpole has completed its metamorphosis to an adult frog.
The evolutionary transition from water to land occurred not
in weeks but over millions of years.A lengthy series of alterations
cumulatively fitted the vertebrate body plan for life on land.The ori-
gin of land vertebrates is no less a remarkable feat for this fact—a
feat that incidentally would have a poor chance of succeeding today
because well-established competitors make it impossible for a poorly
adapted transitional form to gain a foothold.
Amphibians are the only living vertebrates that have a transi-
tion from water to land in both their ontogeny and phylogeny.Even
after some 350 million years of evolution,few amphibians are com-
pletely land adapted;most are quasiterrestrial,hovering between
aquatic and land environments.This double life is expressed in their
name.Even the amphibians that are best adapted for a terrestrial
existence cannot stray far from moist conditions.Many,however,
have developed ways to keep their eggs out of open water where
the larvae would be exposed to enemies.
A frog tadpole undergoing
metamorphosis.
325
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
326 chapter seventeen
Adaptation for life on land is a major theme of the Early Evolution
remaining vertebrate groups treated in this and the fol-
of Terrestrial Vertebrates
lowing chapters.These animals form a monophyletic
unit known as tetrapods. The amphibians and the amniotes
(including reptiles, birds, and mammals) represent the two Devonian Origin of Tetrapods
major extant branches of tetrapod phylogeny.In this chapter,
The Devonian period,beginning some 400 million years ago,
we review what is known about the origins of terrestrial verte-
was a time of mild temperatures and alternating droughts and
brates and discuss the amphibian branch in detail.We discuss
floods.During this period some primarily aquatic vertebrates
the major amniote groups in Chapters 18 through 20.
evolved two features that would be important for permitting
the subsequent evolution for life on land:lungs and limbs.
Movement onto Land The Devonian freshwater environment was unstable.
During dry periods,many pools and streams evaporated,water
The movement from water to land is perhaps the most dra- became foul,and dissolved oxygen disappeared.Only those
matic event in animal evolution,because it involves the inva- fishes able to acquire atmospheric oxygen survived such con-
sion of a habitat that in many respects is more hazardous for ditions.Gills were unsuitable because in air the filaments col-
life.Life originated in water.Animals are mostly water in com- lapsed, dried, and quickly lost their function. Virtually all
position,and all cellular activities occur in water.Nevertheless, freshwater fishes surviving this period,including lobe-finned
organisms eventually invaded land,carrying their watery com- fishes and lungfishes (p.314),had a kind of lung that devel-
position with them.Vascular plants,pulmonate snails,and tra- oped as an outgrowth of the pharynx.It was relatively simple
cheate arthropods made the transition much earlier than to enhance the efficiency of the air-filled cavity by improving
vertebrates,and winged insects were diversifying at approxi- its vascularity with a rich capillary network,and by supplying
mately the same time that the earliest terrestrial vertebrates it with arterial blood from the last (sixth) pair of aortic arches.
evolved.Although the invasion of land required modification Oxygenated blood returned directly to the heart by a pul-
of almost every system in the vertebrate body,aquatic and ter- monary vein to form a complete pulmonary circuit.Thus the
restrial vertebrates retain many basic structural and functional double circulationcharacteristic of all tetrapods originated:
similarities.We see a transition between aquatic and terrestrial a systemic circulation serving the body and a pulmonary cir-
vertebrates most clearly today in the many living amphibians culation supplying the lungs.
that make this transition during their own life histories. Vertebrate limbs also arose during the Devonian period.
Beyond the obvious difference in water content,there Although fish fins at first appear very different from the jointed
are several important physical differences that animals must limbs of tetrapods,an examination of the bony elements of the
accommodate when moving from water to land.These include paired fins of the lobe-finned fishes shows that they broadly
(1) oxygen content, (2) density, (3) temperature regulation, resemble the equivalent limbs of amphibians.In Eusthenop-
and (4) habitat diversity. Oxygen is at least 20 times more teron, a Devonian lobe-fin,we can recognize an upper arm
abundant in air and it diffuses much more rapidly through air bone (humerus) and two forearm bones (radius and ulna) as
than through water. Consequently, terrestrial animals can well as other elements that we can homologize with the wrist
obtain oxygen far more easily than aquatic ones once they pos- bones of tetrapods (figure 17.1).Eusthenopteroncould walk—
sess the appropriate adaptations,such as lungs.Air,however, more accurately flop—along the bottom mud of pools with its
has approximately 1000 times less buoyant density than water fins,since backward and forward movement of the fins was lim-
and is approximately 50 times less viscous.It therefore pro- ited to about 20 to 25 degrees.Acanthostega,one of the earli-
vides relatively little support against gravity,requiring terres- est known Devonian tetrapods,had well-formed tetrapod legs
trial animals to develop strong limbs and to remodel their with clearly formed digits on both fore- and hindlimbs,but the
skeleton to achieve adequate structural support.Air fluctuates limbs were too weakly constructed to enable the animal to
in temperature more readily than water does,and terrestrial hoist its body off the surface for proper walking on land.
environments therefore experience harsh and unpredictable Ichthyostega,however,with its fully developed shoulder girdle,
cycles of freezing,thawing,drying,and flooding.Terrestrial ani- bulky limb bones,well-developed muscles,and other adapta-
mals require behavioral and physiological strategies to protect tions for terrestrial life,must have been able to pull itself onto
themselves from thermal extremes;one such important strat- land,although it probably did not walk very well.Thus,the
egy is homeothermy (regulated constant body temperature) of tetrapods evolved their legs underwater and only then,for rea-
birds and mammals. sons unknown,began to pull themselves onto land.
Despite its hazards,the terrestrial environment offers a Evidence points to lobe-finned fishes as the closest rela-
great variety of habitats including coniferous,temperate,and tives of tetrapods;in cladistic terms they contain the sister
tropical forests,grasslands,deserts,mountains,oceanic islands, group of tetrapods (figures 17.2 and 17.3). Both the lobe-
and polar regions.Provision of safe shelter for protection of vul- finned fishes and early tetrapods such as Acanthostega and
nerable eggs and young may be accomplished much more read- Ichthyostegashared several characteristics of skull,teeth,and
ily in many of these terrestrial habitats than in aquatic ones. pectoral girdle.Ichthyostega(Gr.ichthys,fish,+ stege¯,roof,or
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
Eusthenopteron Acanthostega
Cleithrum
Humerus
Skull
Ulna
Ulnare
Clavicle
Radius
Intermedium Humerus
Dermal fin rays
Radius Ulna
Phalanges Carpals
Ichthyostega
Limnoscelis
Pelvis
Femur
Tibia Fibula
Fibulare
Tarsals
Phalanges
figure 17.1
Humerus
Evolution of the tetrapod leg.The legs of tetrapods evolved from fins of Paleozoic
fishes.Eusthenopteron,a late Devonian lobe-finned fish had paired muscular fins
supported by bony elements that foreshadowed the bones of tetrapod limbs.The
anterior fin contained an upper arm bone (humerus),two forearm bones (radius Radius Ulna
and ulna),and smaller elements homologous to the wrist bones of tetrapods.As
typical of fishes,the pectoral girdle,consisting of the cleithrum,clavicle,and other Phalanges Carpals
bones,was firmly attached to the skull.In Acanthostega,one of the earliest known
Devonian tetrapods (appearing about 360 million years BP),dermal fin rays of the
anterior appendage were replaced by eight fully evolved fingers.Acanthostegawas probably exclusively aquatic because its limbs were too weak for
travel on land.Ichthyostega,a contemporary of Acanthostega,had fully formed tetrapod limbs and must have been able to walk on land.The hindlimb
bore seven toes (the number of forelimb digits is unknown).Limnoscelis,an anthracosaur amphibian of the Carboniferous (about 300 million years BP)
had five digits on both fore- and hindlimbs,the basic pentadactyl model that became the tetrapod standard.
(Sources:R.L.Carroll,Vertebrate Paleontology and Evolution,1988,W.H.Freeman & Co.,NY;M.I.Coates,and J.A.Clack,Nature,347:66–69,1990;J.L.Edwards,American
Zoologist,29:235–254,1989;E.Jarvik,Scientific Monthly,1955:141–154,March 1955;and C.N.Zimmer,Discover,16(6):118–127,1995.)
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
328 chapter seventeen
e
r
Sarcopterygii Choanata Tetrapoda Neotetrapoda Temnospondyli Lissamphibia Diverse†tetrapodstegans†CaudataApodaAnuraAmniotagroups Diversetemnospondyl†groups Modifications of skullModifications of thebones (tentative)skull and teeth Four digits on forelimb Modifications of the braincase,notochord, and bony fin supports Presence of digits in forelimbs andhindlimbs, definitive ankle and wristjoints, well-developed pectoral andpelvic skeletons, strengthened andPresence ofventrally directed ribs, numerousinternal naresskull modifications(choanae) figure 17.2Tentative cladogram of the Tetrapoda with emphasis on descent of the amphibians.Especially controversial athe relationships of major tetrapod groups (Amniota,Temnospondyli and diverse early tetrapod groups) andoutgroups (coelacanth,lungfish,rhipidistian fishes).All aspects of this cladogram are controversial,however,including relationships of the Lissamphibia.The relationships shown for the three groups of Lissamphibia arebased on recent molecular evidence.Extinct groups are marked with a dagger symbol (†). (Source:From E.W.Gaffney in Bulletin of the Carnegie Museum of Natural History,13:92–105,1979.)
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Icht ed fin
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Diversegfishes"rhipidistian"†groups Characteristicsof jaw, skull Three-lobedtail; ossifiedswim bladder;double jawarticulation Strength †Extinct groups
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L
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a
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Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
The Early Tetrapods and Modern Amphibians 329
covering, in reference to the roof of the skull which was terrestrial as adults, developed specialized hindlimbs with
shaped like that of a fish) represents an early offshoot of tetra- webbed feet better suited for swimming than for movement
pod phylogeny that possessed several adaptations,in addition on land.All amphibians use their porous skin as a primary or
to jointed limbs,that equipped it for life on land (figure 17.1). accessory breathing organ.This specialization was encouraged
These include a stronger backbone and associated muscles to by swampy environments of the Carboniferous period but pre-
support the body in air, new muscles to elevate the head, sented serious desiccation problems for life on land.
strengthened shoulder and hip girdles,a protective rib cage,a
more advanced ear structure for detecting airborne sounds,a
foreshortening of the skull,and a lengthening of the snout that The Modern Amphibians
improved olfactory powers for detecting dilute airborne odors.
Yet Ichthyostegastill resembled aquatic forms in retaining a tail The three living amphibian orders comprise more than 4200
complete with fin rays and in having opercular (gill) bones. species.Most share general adaptations for life on land,includ-
ing skeletal strengthening and a shifting of special sense pri-
orities from the ancestral lateral-line system to the senses of
The bones of Ichthyostega,the most thoroughly studied of smell and hearing.Both the olfactory epithelium and the ear
all early tetrapods,were first discovered on an east Green- are redesigned to improve sensitivities to airborne odors and
land mountainside in 1897 by Swedish scientists looking for sounds.
three explorers lost two years earlier during an ill-fated Nonetheless, most amphibians meet problems of inde-
attempt to reach the North Pole by hot-air balloon.Later pendent life on land only halfway.In the ancestral life history of
expeditions by Gunnar Säve-Söderberg uncovered skulls of amphibians,eggs are aquatic and hatch to produce an aquatic
Ichthyostegabut Säve-Söderberg died,at age 38,before he larval form that uses gills for breathing.A metamorphosis fol-
was able to make a thorough study of the skulls.After lows in which gills are lost and lungs, which are present
Swedish paleontologists returned to the Greenland site throughout larval life,are then activated for respiration.Many
where they found the remainder of Ichthyostega’sskeleton, amphibians retain this general pattern but there are some
Erik Jarvik,one of Säve-Söderberg’s assistants,assumed the important exceptions.Some salamanders lack a complete meta-
task of examining the skeleton in detail.This study became morphosis and retain a permanently aquatic,larval morphology
his life’s work,resulting in the detailed description of throughout life. Others live entirely on land and lack the
Ichthyostega available to us today. aquatic larval phase completely.Both of these are evolutionarily
derived conditions.Some frogs also have acquired a strictly ter-
restrial existence by eliminating the aquatic larval stage.
Even the most terrestrial amphibians remain dependent
Carboniferous Radiation of Tetrapods
on very moist if not aquatic environments.Their skin is thin,
and it requires moisture for protection against desiccation in
The capricious Devonian period was followed by the Car-
air. An intact frog loses water nearly as rapidly as a skinless
boniferous period,characterized by a warm,wet climate dur-
frog.Amphibians also require moderately cool environments.
ing which mosses and large ferns grew in profusion on a
Being ectothermic,their body temperature is determined by
swampy landscape.Tetrapods radiated quickly in this environ-
and varies with the environment, greatly restricting where
ment to produce a great variety of forms,feeding on the abun-
they can live.Cool and wet environments are especially impor-
dance of insects, insect larvae, and aquatic invertebrates
tant for reproduction.Eggs are not well protected from desic-
available.Evolutionary relationships of early tetrapod groups
cation,and they must be shed directly into the water or onto
are still controversial.We present a tentative cladogram (figure
moist terrestrial surfaces. Completely terrestrial amphibians
17.2),which almost certainly will undergo future revision as
may lay eggs under logs or rocks,in the moist forest floor,in
new data are collected.Several extinct lineages plus the Lis-
flooded tree holes,in pockets on the mother’s back,or in folds
samphibia, which contains the modern amphibians, are
of the body wall.One species of Australian frog even broods its
placed in a group termed the temnospondyls (see figures
young in its vocal pouch (see figure 17.16,p.337).
17.2 and 17.3).This group is distinguished by having generally
We now highlight special characteristics of the three
only four digits on the forelimb rather than the five character-
major groups of amphibians.We will expand coverage of gen-
istic of most tetrapods.
eral amphibian features when discussing groups in which par-
The lissamphibians diversified during the Carboniferous
ticular features have been studied most extensively.For most
to produce ancestors of the three major groups of amphibians
features,this group is the frogs.
alive today,frogs(Anura or Salientia),salamanders(Caudata
or Urodela), and caecilians (Apoda or Gymnophiona).
Amphibians improved their adaptations for living in water dur- Caecilians: Order Gymnophiona (Apoda)
ing this period. Their bodies became flatter for moving
through shallow water. Early salamanders developed weak The order Gymnophiona (jim’no-fy’o-na) (Gr.gymnos,naked,
limbs and their tail became better developed as a swimming + ophineos, of a snake) contains approximately 160 species
organ.Even anurans (frogs and toads),which are now largely of elongate,limbless,burrowing creatures commonly called
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
330 chapter seventeen
See Chapters 18 through
20 for details
Frogs and
Anthracosauria toads
Early tetrapod
groups related to amniotes Amniota
Salamanders
Ichthyostega
Lissamphibians
Caecilians
Lungfishes
Diverse
Diverse rhipidistian temnospondyl groups
Sarcopterygian groups
ancestor
Coelacanth
Devonian Carboniferous Permian
PALEOZOIC MESOZOIC CENOZOIC
figure 17.3
Early tetrapod evolution and the descent of amphibians.The tetrapods share most recent common ancestry with the extinct Devonian rhipidistian
fishes (of livinggroups,the tetrapods are most closely related to the lungfishes).The amphibians share most recent common ancestry with the diverse
temnospondyls of the Carboniferous and Permian periods of the Paleozoic,and Triassic period of the Mesozoic.
anus.The eyes are small,and most species are totally blind as
adults.Their food consists mostly of worms and small inverte-
brates,which they find underground.Fertilization is internal,
and the male is provided with a protrusible copulatory organ.
The eggs are usually deposited in moist ground near water.In
some species the eggs are carefully guarded in folds of the
body during their development.Viviparity also is common in
some caecilians,with the embryos obtaining nourishment by
eating the wall of the oviduct.
Salamanders: Order Caudata (Urodela)
figure 17.4
As its name suggests,order Caudata (L.caudatus,having a tail)
Female caecilian coiled around eggs in her burrow.
are tailed amphibians,approximately 360 species of salaman-
ders.Salamanders are found in almost all northern temperate
caecilians (figure 17.4). They occur in tropical forests of regions of the world,and they are abundant and diverse in North
South America (their principal home),Africa,and Southeast America.Salamanders are found also in tropical areas of Central
Asia.They possess a long,slender body,small scales in the skin America and northern South America.Salamanders are typically
of some,many vertebrae,long ribs,no limbs,and a terminal small;most of the common North American salamanders are less
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
The Early Tetrapods and Modern Amphibians 331
characteristics of modern amphibians
1. Skeleton mostly bony,with varying numbers of verte- 6. Respiration by lungs (absent in some salamanders),
brae;ribs present in some,absent or fused to vertebrae skin,and gills in some,either separately or in combina-
in others tion;external gills in the larval form and may persist
2. Body forms vary greatly from an elongated trunk with throughout life in some
distinct head,neck,and tail to a compact,depressed 7. Circulation with three-chambered heart,two
body with fused head and trunk and no intervening atria and one ventricle,and a double circulation
neck through the heart;skin abundantly supplied with
3. Limbs usually four (tetrapod),although some blood vessels
are legless;forelimbs of some much smaller than 8. Ectothermal
hindlimbs,in others all limbs small and inadequate; 9. Excretory system of paired mesonephric kidneys;
webbed feet often present;no true nails or claws; urea main nitrogenous waste
forelimb usually with four digitsbut sometimes 10. Ten pairs of cranial nerves
five and sometimes fewer 11. Separate sexes;fertilization mostly internal in salaman-
4. Skin smooth and moist with many glands, ders and caecilians,mostly external in frogs and toads;
some of which may be poison glands;pigment cells predominantly oviparous,some ovoviviparous or
(chromatophores) common,of considerable variety; (rarely) viviparous;metamorphosis usually present;
no scales,except concealed dermal ones in some moderately yolky eggs(mesolecithal) with
5. Mouth usually large with small teeth in upper or both jellylike membrane coverings
jaws;two nostrils open into anterior part of mouth
cavity
than 15 cm long.Some aquatic forms are considerably longer, Respiration
and the Japanese giant salamander may exceed 1.5 m in length.
At various stages of their life history,salamanders may have
Most salamanders have limbs set at right angles to the
external gills,lungs,both,or neither of these.They also share
body, with forelimbs and hindlimbs of approximately equal
the general amphibian condition of having extensive vascular
size.In some aquatic and burrowing forms,limbs are rudimen-
nets in their skin that serve the respiratory exchange of oxy-
tary or absent.
gen and carbon dioxide.Salamanders that have an aquatic lar-
Salamanders are carnivorous both as larvae and adults,
val stage hatch with gills, but lose them later if a
preying on worms,small arthropods,and small molluscs.Most
metamorphosis occurs. Several diverse lineages of salaman-
eat only things that are moving.Like all amphibians,they are
ders have evolved permanently aquatic forms that fail to
ectotherms and have a low metabolic rate.
undergo a complete metamorphosis and retain their gills and
finlike tail throughout life.Lungs,the most widespread respira-
Breeding Behavior
tory organ of terrestrial vertebrates,are present from birth in
Some salamanders are aquatic throughout their life cycle,but the salamanders that have them,and become active following
most are metamorphic,having aquatic larvae and terrestrial metamorphosis.Others,such as amphiumas,while having a
adults that live in moist places under stones and rotten logs. completely aquatic life history, nonetheless lose their gills
Eggs of most salamanders are fertilized internally,usually after before adulthood and then breathe primarily by lungs.This
the female picks up a packet of sperm (spermatophore) that requires that they periodically raise their nostrils above the
previously has been deposited by the male on a leaf or stick surface of the water to get air.
(figure 17.5). Aquatic species lay their eggs in clusters or The amphiumas provide an interesting contrast to many
stringy masses in water. Their eggs hatch to produce an species of the large family Plethodontidae (figures 17.5,17.6,
aquatic larva having external gills and a finlike tail.Completely and 17.8) that are entirely terrestrial but completely lack lungs.
terrestrial species deposit eggs in small, grapelike clusters Efficiency of cutaneous respiration is increased by penetration
under logs or in excavations in soft moist earth, and many of a capillary network into the epidermis or by thinning of the
species remain to guard the eggs (figure 17.6). Terrestrial epidermis over superficial dermal capillaries.Cutaneous respi-
species have direct development. They bypass the larval ration is supplemented by air pumped in and out of the mouth,
stage and hatch as miniature versions of their parents. The where respiratory gases are exchanged across the vascularized
most complex of salamander life cycles is observed in some membranes of the buccal (mouth) cavity (buccopharyngeal
American newts,whose aquatic larvae metamorphose to form breathing). Lungless plethodontids probably originated in
terrestrial juveniles that later metamorphose again to produce streams,where lungs would have been a disadvantage by pro-
secondarily aquatic,breeding adults (figure 17.7). viding too much buoyancy,and where the water is so cool and
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
332 chapter seventeen
Female Male
Spermatophore
A
figure 17.6
Female dusky salamander (Desmognathussp.)
attending eggs.Many salamanders exercise
parental care of the eggs,which includes rotating
the eggs and protecting them from fungal infec-
Base of spermatophore
beneath female's vent tions and predation by various arthropods and
B
other salamanders.
figure 17.5
Courtship and sperm transfer in the pygmy salamander,Desmognathus wrighti.After judg-
ing the female’s receptivity by the presence of her chin on his tail base,the male deposits
a spermatophore on the ground,then moves forward a few paces.A,The white mass of
the sperm atop a gelatinous base is visible at the level of the female’s forelimb.The male
moves ahead,the female following until the spermatophore is at the level of her vent.
B,The female has recovered the sperm mass in her vent,while the male arches his tail,
tilting the female upward and presumably facilitating recovery of the sperm mass.
Red eft
Gilled larva
Adults
mating
Eggs
figure 17.8
Longtail salamander Eurycea longicauda,
a common plethodontid salamander.
figure 17.7
Life history of the red-spotted newt,Notophthalmus viridescensof the family Salamandridae.
In many habitats the aquatic larva metamorphoses into a brightly colored “red eft” stage,
which remains on land from one to three years before transforming into a secondarily
aquatic adult.
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Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
The Early Tetrapods and Modern Amphibians 333
well oxygenated that cutaneous respiration alone was suffi-
cient for life.It is curious that the most completely terrestrial
group of salamanders is one that lacks lungs.
Paedomorphosis
Common mud puppy
Whereas most salamanders complete their development by A (Necturus maculosus)
metamorphosis to the adult body form,some species reach
sexual maturity while retaining their gills,aquatic lifestyle,and
other larval characteristics.This condition illustrates paedo-
morphosis(Gr.“child form”),defined as the retention in adult
descendants of features that were present only in preadult
stages of their ancestors.Some characteristics of the ancestral
adult morphology are consequently eliminated. Examples of
such nonmetamorphic,permanently-gilled species are mud pup-
Axolotl
pies of the genus Necturus(figure 17.9A),which live on bottoms
B (Ambystoma mexicanum)
of ponds and lakes;and the axolotl of Mexico (figure 17.9B).
These species never metamorphose under any conditions. figure 17.9
There are other species of salamanders that reach sexual Paedomorphosis in salamanders.A,The mud puppy Necturussp.and B,
maturity with larval morphology but,unlike permanent larvae the axolotl (Ambystoma mexicanum) are permanently gilled aquatic forms.
such as Necturus,may metamorphose to terrestrial forms under
certain environmental conditions. We find good examples in
Ambystoma tigrinumand related species from North America. most of our familiar frogs (figure 17.10A),and Hylidae,the tree
Their typical habitat consists of small ponds that can disappear frogs (figure 17.10B).True toads,belonging to family Bufonidae,
through evaporation in dry weather.When ponds evaporate the have short legs,stout bodies,and thick skins usually with promi-
aquatic form metamorphoses to a terrestrial form,losing its gills nent warts (figure 17.11).However,the term “toad”is used rather
and developing lungs.It then can travel across land in search of loosely to refer also to more or less terrestrial members of several
new sources of water in which to live and reproduce. other families.
The largest anuran is the West African Conraua goliath,
which is more than 30 cm long from tip of nose to anus (figure
Frogs and Toads: Order Anura (Salientia)
17.12).This giant eats animals as big as rats and ducks.The
smallest frog recorded is Phyllobates limbatus,which is only
The more than 3450 species of frogs and toads that comprise the
approximately 1 cm long.This tiny frog,which can be more
order Anura (Gr.an,without,+ oura,tail) are for most people
than covered by a dime,is found in Cuba.The largest American
the most familiar amphibians. The Anura are an old group,
frog is the bullfrog, Rana catesbeiana (see figure 17.10A),
known from the Jurassic period,150 million years ago.Frogs and
which reaches a head and body length of 20 cm.
toads occupy a great variety of habitats,despite their aquatic
mode of reproduction and water-permeable skin,which prevent
them from wandering too far from sources of water,and their
ectothermy,which bars them from polar and subarctic habitats. In addition to their importance in biomedical research and
The name of the order,Anura,refers to an obvious group charac- education,frogs have long served the epicurean frog-leg
teristic,the absence of tails in adults (although all pass through a market.Mainstay of this market are bullfrogs,which are in
tailed larval stage during development).Frogs and toads are spe- such heavy demand in Europe (especially France) and the
cialized for jumping,as suggested by the alternative order name, United States—the worldwide harvest is an estimated 200
Salientia,which means leaping. million bullfrogs (about 10,000 metric tons) annually—that
We see in the appearance and life habit of their larvae fur- its populations have fallen drastically as the result of exces-
ther distinctions between the Anura and Caudata.Eggs of most sive exploitation and the draining and pollution of wetlands.
frogs hatch into a tadpole (“polliwog”),having a long,finned tail, Most are Asian bullfrogs imported from India and Bangla-
both internal and external gills,no legs,specialized mouthparts desh,some 80 million collected each year from rice fields
for herbivorous feeding (some tadpoles and all salamander lar- in Bangladesh alone.With so many insect-eating frogs
vae are carnivorous),and a highly specialized internal anatomy. removed from the ecosystem,rice production is threatened
They look and act altogether differently from adult frogs.Meta- from uncontrolled,flourishing insect populations.In the
morphosis of a frog tadpole to an adult frog is thus a striking United States,attempts to raise bullfrogs in farms have not
transformation.The permanently gilled larval condition never been successful,mainly because bullfrogs are voracious eat-
occurs in frogs and toads as it does in salamanders. ing machines that normally will accept only living prey,such
Frogs and toads are divided into 21 families.Best-known as insects,crayfish,and other frogs.
frog families in North America are Ranidae,which contains
Hickman−Roberts−Larson: 17. The Early Tetrapods and Text © The McGraw−Hill
Animal Diversity, Third Modern Amphibians Companies, 2002
Edition
334 chapter seventeen
figure 17.10
Two common North American frogs.
A,Bullfrog,Rana catesbeiana,largest
American frog and mainstay of the
frog-leg epicurean market (family
Ranidae).B,Green tree frog Hyla
cinerea,a common inhabitant of
swamps of the southeastern United
States (family Hylidae).Note adhesive
pads on the feet.
A B
figure 17.11
American toad Bufo americanus(family Bufonidae).This principally noc-
turnal yet familiar amphibian feeds on large numbers of insect pests and
figure 17.12
on snails and earthworms.The warty skin contains numerous glands
that produce a surprisingly poisonous milky fluid,providing the toad Conraua (Gigantorana) goliath(family Ranidae) of West Africa,the world’s
excellent protection from a variety of potential predators. largest frog.This specimen weighed 3.3 kg (approximately 71/2 pounds).
est floors.The larger bullfrogs,R.catesbeiana,and green frogs,
Habitats and Distribution
R.clamitans, are nearly always found in or near permanent
Probably the most abundant frogs are the approximately 260 water or swampy regions.The leopard frogs,Rana pipiensand
species of the genus Rana (Gr.frog),found throughout the related species,are found in nearly every state and Canadian
temperate and tropical regions of the world except in New province and are the most widespread of all North American
Zealand, the oceanic islands, and southern South America. frogs.The northern leopard frog,R.pipiens,is the species most
They usually are found near water,although some,such as the commonly used in biology laboratories and for classical elec-
wood frog R.sylvatica,spend most of their time on damp for- trophysiological research.