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Earliest Amphibia
• Very much like crossopterygians, plus
• Legs• No other changes!!!
• Already had lungs• Very large, heavy• Carnivores, food in
water.
Crossopts have fleshy fins to push through swamp vegetation.
Classic earliest amphibian
Large, still has tail fins
Heavy armored head
Aquatic carnivore
Changes: add limbs – no other change as crossopterygians had lungs.
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
Modern amphibia
• Small –• Reduced head, skeleton, weight• Some better lungs (toads) some no lungs but
skin respiration, some external feather gills• Lay “fish” egg.
• Big fossils or their eggs eaten by early reptiles? Only little inconspicuous ones remain.
3 modern groups: frogs (anura = tailless) Urodela = salamanders
Apoda (Gymnophiona) = legless salamanders
All small, smooth skin, loss of armor, soft gelatinous egg
Why become a reptile(why lay eggs on land)
• Free of water – metamorphosis necessary
• 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
• 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)
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
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