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