Amniotes & reptiles

Question 1

Describe the 3 membranes of an amniotic egg

Answer 1

Has 3 extraembryonic membranes:
Allantois → storage for waste, vascularised, left behind
Aminon → grows around developing embryo, sec filled with amniotic fluid
Chorion → surrounds everything

Question 2

Describe the shell of an amniotic egg and how it differs across taxa

Answer 2

Leathery ancestral (e.g turtles, snakes), but highly specialised in birds (calcified)
Function:
- Defence, gas/water exchange
- Many amniotes also bury eggs to limit dessciation

Question 3

Give 4 derived features of amniotes

Answer 3

1. Skeletal = head rotation
2. Reduced skin permeability
3. Costal ventilation
4. Temporal fenestration

Question 4

What are the skeletal characters that allow head rotation in amniotes

Answer 4

axis & atlas (vertebrae in neck) = head rotation

Question 5

Describe reduced skin permeability in amniotes

Answer 5

Thicker, keratinised, more lipids
Skin elaborations: scales, hair, feathers (homologous → epidermal placode)
Alpha (found in all amniotes) vs beta keratin (found only in sauropsids)

Question 6

Why did amniotes evolve holes (fenestrations) in their skulls?

Compare to non-amniotes

Answer 6

• Non-amniote tetrapod = relatively flat skulls bc they use buccal pumping to breathe
• Because the amniotes developed costal ventilation = smaller more dome shaped skull → differentiation of adductor muscle into two
• This allows them to close jaw and apply pressure
• Non-amniotes: Muscles attached inside
• Amniotes: thought that small hole in skull allowed muscles to attach to the outside → enlargement of hole = larger muscles

Question 7

Describe the Chelonia (turtles/tortoises)

Answer 7

→ Beta keratin scutes on surface of carapace and plastron
Ribs fused to carapace
No teeth (but ancestral forms did have)
Neck → can pull into carapace = defence
Odontochelys fossil (220 MYA) had plastron but not carapace → suggests they evolved in the aquatic environment (debated)

Question 8

What species make up the lepidosaurs?

Answer 8

• Tuatara
• Lizards & Snakes (squamates)

Question 9

Describe the skulls of the lepridosaurs

Answer 9

• Lizards classic diapsid
• Snakes lack temporal fenestrae (mod, classified as diapsid though)
• Tuatara appear anapsid (loss fenestrae secondarily?)

Question 10

Snakes & lizards have kinetic skulls. What does this mean?

Answer 10

very flexible due to wide gape

Question 11

How are snakes thought to have evolved from lizards?

Answer 11

• Leglessness has evolved many times in the lizards
• Snakes are very specialised legless lizards, thought to have branched in Cretaceous due to a digging stage
• Led to a change in gene expression (e.g Sonic Hedgehog and ZRS genes)
• Eupodophis → fossil form that shows back legs but no front limbs (shows that ancestors lost their front limbs first)

Question 12

Name some key features of the extant Archosaurs

Answer 12

Alligators, gharials & crocodiles (Crocodylia)
- Classically diapsid skull
- Teeth in sockets (thecodont)
- Secondary palate
- Semi-aquatic predators
- egg-laying
- Sprawling gait

Question 13

Why is the secondary palate useful in Crocodylia?

Answer 13

Means they can breathe with just nostrils above the water