Early Tetrapods and Modern Amphibians
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Early tetrapods had to accommodate to
the following differences when adapting
to land
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1)
2)
3)
4)
5)
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prevention of dessication
oxygen in a different medium (air)
density (gravity)
temperature fluctuations
habitat diversity
Major Developments For Movement To Land –
Lungs and Limbs
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Vascular plants, pulmonate snails, and insects have already adapted to
land
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Involves lobe-finned ancestor
Devonian period (400 million years ago); mild temps with alternating
droughts and floods
Surviving fishes in shallow water 1) developed lungs from out growth of
pharynx
2) increased blood flow by developing pulmonary circulation (tetrapod
double circulation with systemic and pulmonary circuits
3) developed four weight bearing limbs from stout, fleshy appendages of
lobed fins
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Devonian followed by Carboniferous period which was a warm and moist
climate
 Amphibians went through much adaptive radiation feeding on abundant
insects
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Figure 17_01
Figure 17_03
Modern Amphibians
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Descended from lissamphibians; diverged into the 3
groups of caecilians, salamanders, and frogs/toads
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Possess:
1) 4 digits on fore-limbs, 5 on rear; caecilians are
limbless
2) often with webbed feet with no nails or claws
Ectothermic
Smooth, moist, glandular skin
Respiration by skin, lungs, or gills
Double circulation
Gelatinous eggs
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Caecilians (Order Gymnophiona; aka
Apoda)
Limbless and elongate (snake-like)
 Burrow underground; found in tropical
rainforests
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Salamanders (Order Urodela; aka Caudata)
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Tailed with 4-legged body plan of ancestral forms
Internal fertilization; female accepts spermatophore from male
Eggs often found in water
Some species exhibit paedomorphosis; the retention of larval
characteristics while reaching sexual maturity
1) remain aquatic
2) retention of gills
Figure 17_10
Frogs and Toads (Order Anura; aka Salientia)
Largest group of amphibians
 Specialized legs for jumping with webbed feet
 No tail, but possess during larval stages
 Undergo major metamorphosis from tadpole to adult
 Males court females; undergo copulatory embrace called
amplexus
 Many unique reproductive strategies
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Toads are stockier, with thicker skin (“warts”), often with
poison glands, and are more terrestrial