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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
25-2
Physical Adaptations: Oxygen content
Oxygen is 20 times more abundant in air so terrestrial animals can obtain oxygen much more easily once they possess lungs and other respiratory structures.
Density
Air is 1000 times less dense
Provides less buoyancy than water
Limbs and skeleton must support more weight
Temperature Regulation
Air fluctuates in temperature more rapidly than water
Animals must adjust to these extremes via behavior or physiological adaptations.
Habitat Diveristy
Variety of terrestrial habitats allows greater opportunities for adaptation
Movement Onto Land
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25-3
Early Evolution of Terrestrial Vertebrates
What structural characteristics evolved in aquatic habitats, that made it possible to explore terrestrial habitats?
Two structures connected to pharynx Air-filled cavity functioned as a swim bladder
Paired internal nares functioned in chemoreception
On land, combination would be used to draw in oxygen-rich air through nares into the air-filled cavity
Bony elements of paired fins Modified for support and movement underwater
On land, would provide same function
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25-4
Adaptations for life on land included the skull, teeth, pectoral girdle and jointed limbs
Tetrapods MUST HAVE:
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
Forward shortening of skull
Longer snout
Early Evolution of Terrestrial Vertebrates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
25-5
Modern Amphibians
Diversity
Over 6000 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
But, they remain tied to water. WHY?
Eggs deposited in water or must be kept moist
Larvae depend on gills for respiration.
Thin skin loses water rapidly
They are ectothermic where their body temperature depends on the environment and this can restricts range
Eggs easily dessicate and must be shed into water or kept moist
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25-6
Caecilians: Order Gymnophiona Approximately 173 living species
Elongate, limbless, burrowing animals with long ribs, and a terminal anus.
Inhabit tropical forests in South America, Africa, India, and Southeast Asia
Feed primarily on worms and small underground invertebrates
Fertilization is internal with the male using a protrusible copulatory organ
Some deposit eggs in the moist
ground near water while others will
guard them by having them develop
in the folds of the body.
In other species, viviparity allows
embryos to obtain nourishment by
eating wall of oviduct
Larva are typically aquatic
Modern Amphibians
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25-7
Salamanders & Newts: Order Caudata (Urodela)
Approximately 553 living species are found primarily in northern temperate regions
Most are small, under 15 cm long while some like the Japanese giant salamander are over a meter long
Limbs are usually about equal length and at right angles to trunk
Burrowing species and some aquatic forms may have lost their limbs
These animals are carnivorous as both larvae and adults and feed on worms, small arthropods and molluscs, or their own eggs
They are ectotherms with a low metabolic rate
Modern Amphibians
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25-8
Salamanders have some very unique breeding behaviors.
Internal fertilization
After courtship, the female will recover the spermatophore, that has been deposited on a leaf or stick, in her cloaca by passing over it.
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
Some are aquatic throughout their life cycle but most have aquatic larvae and terrestrial adults
Entirely terrestrial species will undergo direct development and hatch as miniature adults
Modern Amphibians
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25-9
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
Modern Amphibians
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
25-10
Respiration varies among salamanders & newts.
Extensive vascular nets in skin that exchange both oxygen and carbon dioxide (cutaneous respiration)
At various stages, may also have external gills, lungs, both gills and lungs, or neither
Salamanders with an aquatic stage hatch with gills, 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
Modern Amphibians
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25-11
Frogs and Toads: Order Anura
Approximately 5283 species
Must live near water source
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 tail-less,
jumping adults (except for 1 species)
Modern Amphibians
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25-12
44 families of frogs and
toads
Family Ranidae
Contains the common larger
frogs in North America
Family Hylidae
Includes the tree frogs
Family Bufonidae (true
toads)
Contains amphibians with
thicker skins and prominent
wart like structures.
Modern Amphibians
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25-13
Habitats and Distribution
Genus Rana are common in temperate and tropical regions
Anuran are declining worldwide and becoming geographically fragmented
Malformed limbs are often associated with infection by trematodes or DNA errors.
Modern Amphibians
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25-14
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 reproductive sites 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
Modern Amphibians
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25-15
Reproduction and Development
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
Eggs usually laid in large masses
Development begins immediately
Tadpole may hatch in 6–9 days and have a long, finned tail, no legs, and internal and external gills
Herbivorous tadpoles feed with horny jaws and use a ventral adhesive disc for clinging to objects
Modern Amphibians
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25-16
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 reabsorbed
Intestine becomes shorter
Mouth transforms to the adult condition
Lungs develop and gills are resorbed
Modern Amphibians
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
25-18
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
Marsupial frogs carry eggs in a pouch on the back
These mate on land and eggs hatch directly into froglets
Modern Amphibians