Early Tetrapods and Modern Amphibians

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Early Tetrapods and Modern Amphibians

• Early tetrapods had to accommodate to the
  following differences when adapting to land
• 1) prevention of dessication
• 2) oxygen in a different medium (air)
• 3) density (gravity)
• 4) temperature fluctuations
• 5) habitat diversity

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Major Developments For Movement To Land Lungs
and Limbs

• Vascular plants, pulmonate snails, and insects
  have already adapted to land
• 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
• 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
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Figure 17_03
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Modern Amphibians

• Descended from lissamphibians diverged into the
  3 groups of caecilians, salamanders, and
  frogs/toads
• 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 (Apoda)

• Limbless and elongate (snake-like)
• Burrow underground found in tropical rainforests

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Salamanders (Caudata)

• 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

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Figure 17_10
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Frogs and Toads (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
• Toads are stockier, with thicker skin (warts),
  often with poison glands, and are more terrestrial

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