Amniotes

Question 1

Amniotic egg membranes

Answer 1

3 additional membranes surrounding the egg - coming from the embryo (extra embryonic)
•amnion grows out from embryo and surrounds it
•chorion grows out from the embryo and surrounds the embryo and the yolk sac
•allantois grows out from the hind region of embryo - where N waste is deposited, bladder forms from this, left behind in egg case

Question 2

Other characteristics of amniotic egg

Answer 2

•egg-laying species have membranes surrounded in albumen - source of water and protein, provides protection
•surrounded in shell if egg-laying - leathery or like bird egg
•pores in shell when laid on land - subject to some desiccation, which is why they have an albumen
-often buried in damp places or in ground
-hard shell more capable of dealing with extremities of land

Question 3

Mammalian membranes

Answer 3

•placental and marsupial mammals are non egg-laying, but have homologous membranes
-some mammals like monotremes still lay eggs

Question 4

Why would these membranes have evolved?

Answer 4

Provides a greater SA for gaseous exchange
•allantois is thought to be vascularised so allows for gaseous exchange
•provides support and allows larger eggs to be laid, along with larger adults to evolve larger - only a theory
•some amphibians lay eggs on land, evolution of these membranes is not only about laying eggs on land

Question 5

Derived amniote features

Answer 5

1. Skin more waterproof, thicker and more keratin, lipids make more waterproof
2. Have elaborations on the skin, like scales, hairs or feathers (all homologous, just differentially expressed)
   - formed by thickenings on epidermal surface - the anatomical placode
3. Costal (rib) ventilation
4. Changes in skull morphology

Question 6

Costal ventilation

Answer 6

Use their ribs to ventilate the lungs
•powerful movement making pressure differences on the inside and outside to draw air into the lungs, and out again
•very efficient and has allowed amniotes to have longer neck (can draw air for longer distances)
•changes in nervous development - nerves branched and complex, controlling forelimbs = more movement and dexterity

Question 7

Skull morphology changes

Answer 7

Different types of skull
•anapsid skull (no fenestra)
•synapsid skull - one arch and hole
•diapsid skull - 2 holes and 2 arches, can have secondary modifications
-always debated turtles, now thought they were diapsid but holes covered up
-mammals hole combined with orbit
•Sauropsid ancestral condition had no holes - evolved holes convergently to mammals
•birds also combined hole and orbit

Question 8

Fenestration

Answer 8

Allows bigger muscles through fenestra - for more powerful jaw muscles and feeding
•amniotes have costal ventilation, so skull can become smaller and dome-shaped = more developemnt of muscle
-amphibians had to have broader heads previously for ventilating lungs

Question 9

Muscles of skull in amniotes

Answer 9

• has become 3 different types of muscle controlling the jaw
• the muscles are now attaching on top of skull through holes
• muscles expanding through holes gave selective advantage as muscles could become bigger and expand
• jaw more controlled, can contract and produce force onto what they’re eating

Question 10

Diapsids split…

Answer 10

1. Archosaurs (dinosaurs, birds and crocodiles)
2. Lepidosaurs (snakes, lizards and tuatara)
   Diverged from each other in Permian
   Turtles are own separate group