Download Early Evolution of Terrestrial Vertebrates

CHAPTER 25
Early Tetrapods and
Modern Amphibians
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Movement Onto Land
Adaptations
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Movement from water to land is perhaps the
most dramatic event in animal evolution
Animals are composed mostly of water
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Land represents a relatively dangerous habitat
Vascular plants, pulmonate snails, and
tracheate arthropods made transition earlier
Amphibians only living vertebrates that have a
transition from water to land in both their
ontogeny and phylogeny
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Movement Onto Land
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Quasiterrestrial
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Hover between aquatic and land environments
Physical differences that must be addressed
when moving from water to land
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Oxygen is 20 times more abundant in air
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Air is 1000 times less dense
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Provides less buoyancy than water
Limbs and skeleton must support more weight
Air fluctuates in temperature more rapidly than water
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Diffuses much more rapidly through air
Animals must adjust to these extremes
Variety of terrestrial habitats allows greater
opportunities for adaptation
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Early Evolution of Terrestrial Vertebrates
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400 million years ago, bony fishes diversified
to include many freshwater forms
Combinations of characteristics that evolved
in aquatic habitats, made it possible to
explore terrestrial habitats
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Two structures connected to pharynx
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Bony elements of paired fins
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Air-filled cavity functioned as a swim bladder
Paired internal nares functioned in chemoreception
On land, combination would be used to draw in oxygenrich air through nares into the air-filled cavity
Modified for support and movement underwater
On land, would provide same function
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Early Evolution of Terrestrial Vertebrates
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Freshwater habitats are inherently unstable
Multiple fish groups, given a combination of
structures that could be coopted for terrestrial
breathing and locomotion, evolved some
degree of terrestriality
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One transition in late Devonian period
provided the ancestral lineage of all tetrapod
vertebrates
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Ex: Mudskippers and lungfish
Evolved adaptations to air breathing
 Increased vascularization of the air-filled cavity
 Double circulation to direct deoxygenated blood
into the lungs and oxygenated blood out of the
lungs to other body tissues
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Early Evolution of Terrestrial Vertebrates
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Bony elements of fins of lobe-finned fishes
resemble homologous structures of
amphibians
 Eusthenopteron
Could paddle itself through bottom mud
 Had both lungs and “walking” fins
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Tiktaalik
Recently discovered fossil genus
 Intermediate between lobe-finned fishes and
tetrapods
 Probably used limbs to support body while
placing snout above water to breathe air in
shallow water
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Early Evolution of Terrestrial Vertebrates
 Acanthostega
 Had
clearly formed digits on both
forelimbs and hindlimbs
 Body drug on the ground
 Ichthyostega
 Bulkier
limb muscles to walk onto land,
but not with great efficiency
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Early Evolution of Terrestrial Vertebrates
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Lobe-finned fishes are the sister group to
tetrapods
Adaptations for life on land include skull,
teeth, pectoral girdle and jointed limbs
Tetrapods
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Have stronger backbone
Muscles to support the body in air
Muscles to elevate the head
Stronger shoulder and hip girdles
More protective rib cage
Modified ear structure to detect airborne sounds
Foreshortening of skull
Longer snout
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Early Evolution of Terrestrial Vertebrates
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Ichthyostega still retained aquatic features
including fin rays and opercular bones
Several extinct lineages plus the
Lissamphibia with modern amphibians
formed the temnospondyls
Temnospondyls generally had four rather
than five digits on the forelimb
During the Carboniferous, amphibians
developed additional adaptations for living in
water
Lepospondyls and anthracosaurs are, based
on skull structure, closer to amniotes than
temnospondyl
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Modern Amphibians
Diversity
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Over 60 living species are known in the three
amphibian orders
Metamorphosed adults use a redesigned
olfactory epithelium to sense airborne odors
and the ear detects sounds
Remain tied to water
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Eggs deposited in water or must be kept moist
Larvae depend on gills for respiration
Some salamanders have retained aquatic
morphology throughout life
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Others lack the larval phase
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Modern Amphibians
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Thin skin loses water rapidly
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Ectotherms
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Restricts terrestrial forms to moist habitats
Body temperature depends on environment and
restricts range
Eggs easily dessicate and must be shed into
water or kept moist
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A few brood young
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Modern Amphibians
Caecilians: Order Gymnophiona (Apoda)
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Approximately 173 living species
Elongate, limbless, burrowing animals
Inhabit tropical forests in South America,
Africa, India, and Southeast Asia
Bodies have many vertebrae, long ribs, no
limbs, and terminal anus
Feed primarily on worms and small
underground invertebrates
Fertilization is internal
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Male has a protrusible copulatory organ
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Modern Amphibians
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Eggs deposited in moist ground near water
Some species have aquatic larvae
In others, larval development occurs within
egg
Eggs are guarded and develop in folds of the
body in some
In other species, viviparity allows embryos to
obtain nourishment by eating wall of oviduct
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Modern Amphibians
Salamanders: Order Caudata (Urodela)
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Approximately 553 living species are found
primarily in northern temperate regions
Most are small, under 15 cm long
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Japanese giant salamander is 1.5 meters long
Limbs usually at right angles to trunk
Forelimbs and hindlimbs about equal in
length
Burrowing species and some aquatic forms
may have lost their limbs
Carnivorous as both larvae and adults
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Feed on worms, small arthropods, and molluscs
Ectotherms with a low metabolic rate
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Modern Amphibians
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Breeding Behavior
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Some aquatic throughout life cycle
Most have aquatic larvae and terrestrial adults
Internal fertilization in most
Female recovers in cloaca a spermatophore
deposited on a leaf or stick
Aquatic species lay eggs in clusters or stringy
masses
Completely terrestrial species deposit eggs in
small, grape-like clusters under logs or in soft
earth
Terrestrial species undergo direct development
 Hatch as miniature adults
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Modern Amphibians
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Some North American newts have aquatic larvae
that metamorphose into terrestrial juveniles that
again metamorphose into secondarily aquatic,
breeding adults
Some newt populations skip the terrestrial “red
eft” stage and remain entirely aquatic
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Modern Amphibians
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Respiration
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Wide array of respiratory mechanisms
Extensive vascular nets in skin that exchange
both oxygen and carbon dioxide
At various stages, may also have external gills,
lungs, both gills and lungs, or neither
Salamanders with aquatic stage hatch with gills
and which are lost at metamorphosis
Several diverse lineages fail to undergo
metamorphosis and retain gills and a fin-like tail
In species with lungs, lungs are present from birth
and become functional following metamorphosis
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Modern Amphibians
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Aquatic amphiumas lose gills and respire by lung
 Hold nostrils above water surface
Many species of Plethodontidae are lungless and
strictly terrestrial
Respiratory gases may also be exchanged across
vascularized lining of mouth cavity
Lungless salamanders likely evolved in cold
streams where lungs would have been too
buoyant
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Modern Amphibians
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Paedomorphosis
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Descendants retain into adulthood features that
were present only in pre-adult stages of ancestors
Eliminating ancestral adult morphology is a trend
found in salamander evolution
Non-metamorphic species that retain gills, etc. are
perennibranchiate
Obligate perennibranchiates, like the mudpuppy,
have never been observed to metamorphose
Others reach sexual maturity with larval
morphology but can change depending on the
conditions
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Modern Amphibians
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In Mexico and the U.S.
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Ambystoma tigrinum may stay in a gilled stage as
an axolotl
When ponds dry up, may metamorphose into a
terrestrial form and migrate to a new pond
Axolotls treated with thyroid hormone will
metamorphose artificially
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Paedomorphosis may be partial
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Pituitary gland appears to be the controlling factor in
natural populations
Amphiumas shift to lungs but otherwise remains
larval in body form
In some species, larval appendages have been
maintained to preserve a climbing ability
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Modern Amphibians
Frogs and Toads: Order Anura (Salientia)
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Approximately 5283 species
Dates from Triassic period, 250 million years
ago
Must live near water source
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Reproduction mode requires water
Skin is water-permeable skin
Ectothermy prevents anurans from
inhabiting polar and subarctic habitats
All have a tailed larval stage and tailless,
jumping adults (except for 1 species)
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Modern Amphibians
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Eggs of most hatch into tadpoles with a long,
finned tail, no legs, internal and external gills
and specialized mouthparts for (usually)
herbivorous feeding
Tadpoles look and act entirely different from
adult frogs
Perennibranchiate condition never occurs in
frogs and toads
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Modern Amphibians
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44 families of frogs and toads
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Family Ranidae
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Family Hylidae
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Includes the tree frogs
Family Bufonidae (true toads)
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Contains the common larger frogs in North America
Contains toads with thicker skins and prominent warts
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Modern Amphibians
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Habitats and Distribution
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Genus Rana are common in temperate and tropical
regions
Rana sylvatica, wood frog, spends most of its time
on damp forest floors, returns to pools for breeding
Bullfrogs and green frogs occur in or near
permanent water and swamps
Leopard frog is widespread and commonly studied
in laboratories
Pickerel frog is restricted to certain localities
Anuran are declining worldwide and becoming
geographically fragmented
 Cause is not known
Malformed limbs are often associated with infection
by trematodes
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Modern Amphibians
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Life Cycle
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Most larger frogs are solitary until breeding
season
During breeding season, males are especially
noisy when trying to attract a female
Hold forelimbs near body when swimming with
powerful hindlimbs
Surface to breathe with only the head and
foreparts exposed
During winter in temperate climates
 Hibernate in soft mud in bottom of pools
During hibernation period, the little energy they
use is provided from stored glycogen and fat
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Modern Amphibians
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Frost-tolerant frogs
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Many are easy prey
Defend themselves by aggression, concealment,
and poison glands
Many species have suffered from changes in the
environment and climate brought about by humans
Climatic changes that reduce water depth at
oviposition sites
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Prepare for freezing by accumulating glucose and glycerol
in body fluids
Protection from damaging effects of ice-crystal formation
Increases ultraviolet exposure of embryos
Also, makes them more susceptible to fungal infection
Decline of some amphibians may be caused by
other amphibians such as Bufo marinus
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Modern Amphibians
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Integument and Coloration
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Frog skin is thin, moist and attached loosely to the
body at a few points
Skin is composed of an outer stratified epidermis
and an inner spongy dermis
Epidermal layer is shed periodically
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More terrestrial amphibians have heavier deposits
of keratin
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Amphibian keratin is soft
Epidermis has two types of integumentary glands
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Contains deposits of keratin
Mucous glands secrete protective waterproofing
Large serous glands produce a whitish, watery poison
Dendrobatid frogs of South America secrete
highly toxic skin poisons
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Modern Amphibians
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Specialized pigment cells, chromatophores,
produce skin color in frogs
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Primarily in dermis
Xanthophores (upper region)
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Iridophores (middle region)
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Silvery light-reflecting pigment that acts like a mirror
Melanophores (lower region)
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Yellow, orange or red pigments
Black or brown melanin
Green hue is an interaction of xanthophores
containing yellow pigment and underlying
iridophores
Many can adjust color to match with
background and camouflage themselves
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Modern Amphibians
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Skeletal and Muscular Systems
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Well-developed endoskeleton of bone and cartilage
provides for body support and muscular
movements
Movement to land posed new mechanical stress
problems
Dramatic changes in the musculoskeletal system
for jumping and swimming
Vertebral column lost much flexibility in order to
transmit force from limbs to body
Extremely shortened body
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Only nine trunk vertebrae and a urostyle
Caecilians have not moved toward tetrapod
locomotion and have as many as 285 vertebrae
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Modern Amphibians
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Front of the skull, containing the brain, eyes, and
nose is lightweight and flattened
Back of the skull, which contained the gill
apparatus in fishes, is reduced
Posterior limbs have three main joints
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Foot generally has five rays
Hand is four-rayed
Both have several joints in the digits
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Hip, knee and ankle
System derived from the pattern in rhipidistian lobefinned fish
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Modern Amphibians
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Limb musculature
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Anterior, ventral group pulls the limb forward and toward
the midline
Posterior, dorsal group draws the limb backward and
away from body
Myomeres highly modified to support head and
brace the vertebral column
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Modern Amphibians
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Respiration and Vocalization
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Amphibians use 3 respiratory surfaces for gas
exchange in air
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Skin provides cutaneous breathing
Mouth provides buccal breathing
Lungs are usually present in adults
Frogs and toads depend on lung breathing more
than salamanders
Skin is critical during winter hibernation
Carbon dioxide is mostly lost across skin while
oxygen is absorbed across the lungs
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Modern Amphibians
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Lungs supplied by pulmonary arteries and return
blood to the left atrium
Frog lungs
 Ovoid, elastic sacs
 Inner surfaces divide into a network of smaller
chambers
Absorptive surface is 20 cm2/cc of air compared to
300 cm2 for humans
Positive Pressure Breathing
 Air moved into lungs by force
 Rhythmic throat movements gulp air and force
it backward
 Rib cage does not expand to draw air into the
lung
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Modern Amphibians
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Vocal cords are located in the larynx
 Much more developed in males than females
Air is passed back and forth over vocal cords
between the lungs to a large pair of vocal sacs
Most species have unique sound patterns
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Modern Amphibians
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Circulation
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Circulation is closed with a single pressure pump
moving blood through the peripheral network
Main difference in circuitry is the shift from gill to
lung breathing
Elimination of gills reduced one obstacle to blood
flow in the arterial circuit
Conversion of sixth aortic arch into a pulmonary
artery provided a blood circuit to lungs
Separation of oxygenated blood from
deoxygenated blood circuit is only partial
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Modern Amphibians
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Frog Heart
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Single undivided ventricle and two separate atria
Blood from body enters through the sinus venosus
and right atrium
Blood from the lung enters left atrium
Both atria contract at the same time
 Pumps blood into ventricle
Ventricle contracts
 Moving blood moves to the lungs or body
Although there is no septum
 Deoxygenated blood goes primarily to the lungs
 Oxygenated blood goes mostly to the body
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Modern Amphibians
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Right and left atria contract asynchronously
 Oxygenated and deoxygenated blood remains
mostly separated when entering ventricle
Blood separation aided by
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Spiral valve which divides systemic and pulmonary flows in
the conus arteriosus
Different blood pressure in the pulmonary and systemic
blood vessels leaving the conus arteriosus
Exact mechanism and precision of separation of
oxygenated and deoxygenated blood in the conus
arteriosus remain unclear
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Modern Amphibians
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Feeding and Digestion
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Most adult are carnivorous
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Catch prey with a tongue that is attached at the
front of mouth
Free end of tongue is glandular
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Do not bite or chew
Short digestive tract produces enzymes for
digesting fats, carbohydrates and proteins
Larval stages of tadpoles are usually herbivorous
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Sticky secretion adheres to prey
Any teeth that are present function to hold prey
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Feed on insects, spiders, worms, slugs, etc.
Digestive tract is relatively long
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Modern Amphibians
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Nervous System and Special Senses
Nervous System
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Brain has 3 fundamental parts
 Forebrain or telencephalon
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Midbrain or mesencephalon
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Perceives vision
Hindbrain or rhombencephalon
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Interprets smells
Perceives hearing and balance
Brain is gradually assuming more information
processing independent of spinal cord
 However, a headless frog still has highly
coordinated behavior based on spinal cord alone
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Modern Amphibians
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Forebrain contains the olfactory center
 Remainder of forebrain has little importance
Complex integrative activities occur in the
midbrain optic lobes
Hindbrain is divided into an anterior cerebellum
and a posterior medulla
Cerebellum, which coordinates equilibrium and
movement, is not well developed in frogs
All sensory neurons except those for vision and
smell pass through the medulla which is on the
anterior end of the spinal cord
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Modern Amphibians
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Medulla has centers for auditory reflexes,
respiration, swallowing, and vasomotor control
Special Senses
 Pressure-sensitive lateral line found only in
amphibian larvae and aquatic adults
 Ear specialized for detecting airborne sounds
 Large tympanic membrane or eardrum passes
vibrations to inner ear via the columella
 Inner ear has a utricle with three semicircular
canals and a saccule with a lagena
 Lagena is covered with a tectorial membrane that
is similar to the mammalian cochlea
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Modern Amphibians
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Frogs are sensitive to low-frequency sound
energy under 4000 Hz (cycles per second)
Except for blind caecilians, vision is dominant
sense in many amphibians
Lachrymal glands and eyelids
 Keep eye moist, free of dust, and protected
Cornea and lens
 Bend light rays to focus an image on the retina
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Modern Amphibians
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At rest, the fish eye focuses on near objects and
the frog eye focuses on distant objects
Amphibian retina contains rods and cones
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Cones provide frogs with color vision
Iris can rapidly change aperture to adjust to light
levels
Upper eyelid is fixed
Lower eyelid is folded into a transparent
nictitating membrane
Other sensory receptors include chemical
receptors in skin, taste buds on tongue, and
olfactory epithelium in nasal cavity
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Modern Amphibians
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Reproduction and Development
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Frogs and toads are ectothermic
Breed, feed, and grow during warm seasons
In spring, males call to attract females
When eggs are mature, females enter the water and
the males clasp them in amplexus
After fertilization, jelly layers of egg absorb water
and swell
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Development begins immediately
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Eggs usually laid in large masses
Tadpole may hatch in 6–9 days
Tadpole head has horny jaws for feeding and a
ventral adhesive disc for clinging to objects
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Modern Amphibians
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3 pairs of external gills develop into internal gills
covered with a flap of skin
On right side of a tadpole, operculum fuses with
body wall
On left side, a spiracle remains
 Water enters mouth, flows past gills, and then out
spiracle
Metamorphosis
 Hindlegs are first to appear
 Forelegs temporarily hidden in folds of operculum
 Tail is resorbed
 Intestine becomes shorter
 Mouth transforms to the adult condition
 Lungs develop and gills are resorbed
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Modern Amphibians
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Males migrate back to breeding ponds or streams
Tropical anurans have different reproductive
strategies
 Some lay eggs in foam masses that float on
surface of water
 Some deposit eggs on leaves over-hanging
ponds and streams into which tadpoles drop
 Other place eggs in water trapped in tree
cavities or water-filled chambers of bromeliads
 Poison-dart frog tend to their eggs
 Tadpoles hatch on their back and can be
carried for varying lengths of time
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Modern Amphibians
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Marsupial frogs carry eggs in a pouch on the back
Eleutherodactylus mate on land and eggs hatch
directly into froglets
Some salamanders have a strong homing instinct,
returning to the same pool to breed
Stimulation to migrate depends on hormone
changes and sensitivity to temperature and
humidity
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Modern Amphibians
Classification
Class Amphibia
 Order Gymnophiona
 Order Caudata
 Order Anura
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