24 Chordates Cont.

Question 1

Acanthostega

Answer 1

• Transitional form b/w fish/tetrapod
• Prehistoric tetrapod of the Late Devonian Period and among the first tetrapods to have recognizable limbs
• had a neck so could move its head around
• had digits and fish tail
• shallow water predator
• fin skeleton looks similar to limbed animals but no walking so only used for support

Question 2

Amphibians

Answer 2

larval stage = tadpole
- aquatic herbivore
- gills
- lateral line
- long finned tail
metamorphosis: gills/tail/lateral line system disappear and legs/lungs/digestive system appear to adapt for a carnivorous diet

Question 3

Amniotes

Answer 3

• **O2 and CO2 diffuse across shell ***
• includes reptiles (with birds) and mammals
• KEY for evolution: amniotic egg allows them to move away from water
• extra embryonic membranes function in gas exchange, waster storage, transfer of nutrients
• shelled egg retains water so can be laid in dry place
• 4 membranes
  1 amnion
  2 allantois
  3 chorion
  4 yolk sac

Question 4

Amniotes membranes

Answer 4

1. amnion: protects the embryo in a fluid-filled cavity that cushions against mechanical shock
2. allantois: a disposal sac for certain metabolic wastes produced by the embryo
3. chorion: along with the membrane of the allantois, exchanges gases between the embryo and the air
4. yolk sac: contains the yolk, a stockpile of nutrients (add’l nutrients stored in the albumen/ “egg white”)

Question 5

Reptiles

Answer 5

1. Scales of keratin, waterproof skin - prevent dehydration.
   - Reptiles cannot breathe through skin, so all gas exchange occurs via lungs.
2. Shelled amniotic eggs from internal fertilization. Shell forms around fertilized egg in the reproductive tract.
3. ectotherms. Ectotherms absorb external heat (i.e. sunlight).
   - One advantage: a reptile can survive on <10% of calories required by a mammal of equivalent size
4. Oldest reptile fossils are from the late Carboniferous, about 320 MYA (dinosaurs and pterosaurs).

Question 6

Reptile Evolution

Answer 6

• More widespread, numerous, & diverse during Mesozoic
• Oldest fossils date back to Carboniferous period, ~320 MYA

Question 7

Reptile Reproduction

Answer 7

• fertilization is always internal before shell is secreted
• some lizards and snakes form a placenta that nourishes the embryo and give birth to bodies not eggs

Question 8

4 Groups of Modern Reptiles

Answer 8

Turtles
Tuataras
Lizards/Snakes
- Lizards are the most numerous group.
- Snakes are descendants of lizards & have vestigial pelvic and limb bones.
Crocodiles/Alligators
- most closely related to dinosaurs