Early Tetrapods

Origin of the Amphibia and Reptiles

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.

Earliest Amphibia:

arose from crossopterygian fish

With assymetric fins

Classic earliest amphibian

Large, still has tail fins

Heavy armored head

Aquatic carnivore

Changes: add limbs – no other change as crossopterygians had lungs.

Eusthenopteron Ichthyostega

Eryops

Early Amphibians

lousy legs, heavy skull, aquatic carnivores.

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.

Axolotl. External “skin” gills unrelated to internal fish gills.

3 modern groups: frogs (anura = tailless) Urodela = salamanders

Apoda (Gymnophiona) = legless salamanders

All small, smooth skin, loss of armor, soft gelatinous egg

Earliest reptiles

• Microsaurs

• a reptile because

• no larva

• lay eggs?

Larval fossil amphibian: poorly ossified limbs, in some impression of external gills.

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)

Urchin gastrulation; total and equal clevage

Frog gastrulation, total but unequal

Gastrulation in chicken; partial clevage, primitive streak

Outgrowth to form membranes

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

Monophyly of reptiles – all share complex egg type