Amniotes

Question 1

Amniote Origins and Diversification

Answer 1

• Tetrapods that are NOT amphibians
  Why/how did they evolve??
• Late carboniferous; 312 mya
• Via Anthracosaurs (Reptilliomorphs)
  2 early diversifications:
• *Synapsids (to mammals)
• *Sauropsids -> *Diapsids (to reptiles & birds)
  The Mesozoic was “The Age of Reptiles”
• Dinosaurs!
  Truly terrestrial

Question 2

The Early Split

Answer 2

• *Synapsids (mammals) & *Sauropsids (birds/reptiles)
• Diverged very early in Amniote evolution (312-306 mya)
  - e.g. *Protoclepsydrops (early fossil synapsid - 1st known?) & *Paleothyris (early sauropsid) both from Joggins Fossil cliffs in NS!

Question 3

Amniotes Synapomorphies

Answer 3

1. Amniotic egg
2. Thicker and more waterproof skin
3. Rib ventilation of the lungs

Question 4

Enclosing “the Pond”

Answer 4

1. The *amnion is an *extraembryonic membrane that encloses a *fluid-filled cavity – i.e., *“the pond”
2. Embryonic development occurs in the *amnion (larger & faster too)
3. Most evolved a *shelled egg that did not depend on water!!
   - *Protective & *semipermeable, (allows gasses, limits water loss)
4. Paleozoic amniote diversification gave rise to all *nonavian reptiles, birds, & mammals

Question 5

1. Amniotic Egg

Answer 5

• All amniotes have eggs with *4 extraembryonic membranes
  1. *Chorion (O2/CO2)
  2. * Allantois (O2/CO2, waste)
  3. *Amnion (fluid, cushion)
  4. *Yolk sac (food)
• Better protection & nourishment

Question 6

2. Thicker & More Waterproof Skin

Answer 6

• Mostly gas-proof; cannot breathe through the skin
• Keratinized (scales, hair, feathers, claws…)

Question 7

3. Rib Ventilation of the Lungs

Answer 7

1. Anamniote: “Mouth breather”
   - Mouth/throat muscular contraction & expansion
   - *Pushing air into lungs
   - *Positive pressure ventilation
2. Amniote: Aspiration
   - Rib/muscular contraction & expansion
   - *Sucking air into lungs
   - *Negative pressure ventilation

Question 8

4. Jaws

Answer 8

• Jaws efficiently designed for applying *crushing or gripping force to prey
• Move from fast closure/suction feeding to *large muscles to grip and chew
• E.g., Pliosaurs!

Question 9

Skulls: Holes for Jaw Musculature

Answer 9

*Anapsids (ancestral)
- Have a skull with *no temporal opening behind the orbits
- E.g. ancestral, and secondarily derived in turtles (truly diapsids)

*Diapsids
- Skull has “two pairs of temporal opening (fenestra)
- one pair below the cheeks and another above
- Gave rise to all other traditional “reptiles” and to birds
- e.g., dinosaurs, snakes, lizards, crocodiles
- Turtles appear anapsid, but secondarily lost diapsid condition