Evolution of tetrapods

1. Evolution of tetrapods Amphibia through reptiles

2. The Crossopterygian fish, Eusthenopteron

3. 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.

4. Series heart to parallel heart and separation of oxygenated and unoxygenated blood.

6. 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.

8. Very much like crossopterygians, plus Legs 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.

11. Note: 8 toes

12. Eryops – one of earliest amphibia. 6 ft long, carnivorous probably couldn’t lift chin off ground for very long. Poor limbs.

13. 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??

14. 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.

15. Surviving amphibia: frogs, toads, salamanders, legless salamanders. .

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

18. Earliest reptiles • Microsaurs • a reptile because • no larva (tadpoles) • lay reptilian type egg?

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

20. 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

21. 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)

22. amplexus

23. Urchin gastrulation; total and equal clevage Frog gastrulation, total but unequal

24. Gastrulation in chicken or reptile; partial clevage, primitive streak

25. Outgrowth to form membranes

26. 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

27. Amphibian to reptile – monophyletic due to Complexity of egg difference 4 extra embryonic membranes, no intermediates.

28. Living Reptiles: Turtles Lizards and snakes crocodiles Extinct: Plesiosaurs (aquatic) Ichthyosaurs (aquatic) dinosaurs mammal like reptiles Descendents birds (dinosaurs) mammals ( mammal like reptiles)

31. Anapsids; solid cheek, no hole – primitive reptiles and turtles.

32. Turtles: shell is vertebrae and ribs, limb girdles inside!

35. Ichthyosaurs – totally aquatic

39. plesiosaurs

41. Lizards and snakes

47. Pelvic structure separates dinosaurs into two groups.