CHAPTER 25 Early Tetrapods and Modern...

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CHAPTER 25

Early Tetrapods and Modern Amphibians

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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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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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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


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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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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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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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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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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


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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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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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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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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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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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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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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


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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

Marsupial frogs carry eggs in a pouch on the back

These mate on land and eggs hatch directly into froglets

Modern Amphibians

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