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Evolution of tetrapods. Amphibia through reptiles. The Crossopterygian fish, Eusthenopteron. Origin of lungs. A gill pouch that fails to open to the outside Carries vagus nerve into the chest Requires a major change in circulation.

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evolution of tetrapods

Evolution of tetrapods

Amphibia through reptiles

origin of lungs
Origin of lungs
  • A gill pouch that fails to open to the outside
  • Carries vagus nerve into the chest
  • Requires a major change in circulation.
concept of preadaptation
Concept of “Preadaptation”
  • Crossopterygian fish had lungs for breathing in low oxygen water.
  • Crossopterygians had muscular fins for pushing through swamp vegetation
  • Both features allow them to live on land.
earliest amphibia
Very much like crossopterygians, plus


No other changes!!!

Already had lungs

Very large, heavy

Carnivores, food in water.

Earliest Amphibia

Crossopterygians (Sarcopterygians) had fleshy fins to push through swamp vegetation.


Eryops – one of earliest amphibia. 6 ft long, carnivorous

probably couldn’t lift chin off ground for very long. Poor limbs.

why come out on land
Story 1.

dry period – need to find another water hole

Practice walking to it.

Story 2

young ones come into shallow water and land to avoid bigger carnivorous ones

Why come out on land??
modern amphibia
Modern amphibia
  • Small –
  • Reduced head, skeleton, weight
  • Some better lungs (toads) some no lungs but skin respiration, some ext feather gills
  • Lay “fish” egg.
  • Big original ones eaten by reptiles? Only little inconspicuous ones remain.
earliest reptiles
Earliest reptiles
  • Microsaurs
  • a reptile because
  • no larva (tadpoles)
  • lay reptilian type egg?
why become a reptile why lay eggs on land
Why become a reptile(why lay eggs on land)
  • Free of water – metamorphosis unnecessary
  • No double life; plant eater in water, carnivore on land (More food available on land now (insects))
  • Avoid nasty predatory fish
  • Egg safety
needed to become a reptile
Needed to become a reptile

No metamorphosis (some salamanders)

Lay eggs on land ( some salamanders)

Internal fertilization (behavior change)

Develop egg shell (soft in some reptiles (turtles), harder in birds. Calcium deposition.

Development of extra-embryonic membranes. (the biggie)


Urchin gastrulation; total and equal clevage

Frog gastrulation, total but unequal


Development of the 4 extra embryonic membranes

Growth outwards of ectoderm and mesoderm

Forms amnion and chorion

Growth outwards of endoderm and mesoderm forms yolk sac.

Later outgrowth of endoderm and mesoderm forms the allantois


Amphibian to reptile – monophyletic due to

Complexity of egg difference

4 extra embryonic membranes, no intermediates.


Living Reptiles:


Lizards and snakes



Plesiosaurs (aquatic)

Ichthyosaurs (aquatic)


mammal like reptiles


birds (dinosaurs)

mammals ( mammal like reptiles)