Synapsids and Diapsids

Question 1

Describe the 3 membranes in amniotic eggs

Answer 1

• amnion - inner membrane around embryo
• chorion - outer membrane around entire contents of egg
• allantois - outgrowth of embryo hindgut: waste N and gas exchange with surface

Question 2

Describe the shell in amniotic eggs

Answer 2

• calcified and rigid in birds, crocodiles
• leathery in lizards, turtles, monotremes

Question 3

What are examples of Paleozoic amniotes?

Answer 3

• Hylonomus - lizard size, earliest amniote
• Mesosaurus - cat size, secondarily aquatic amniote
• Dimetrodon - alligator size, 1st terrestrial apex predator
• Pareiasaurus - cow size, herbivore

Question 4

Describe the Hylonomus skeleton

Answer 4

• pelvic girdle
• sacral vertebrae fused to an ilium each side
• femur held in socket
• pectoral girdle not fused to vertebrae
• humerus - scapula and coracoid

Question 5

What are the types of skull fenestration?

Answer 5

• anapsid - no fenestration
• synapsid - 1 fenestration
• diapsid - 2 fenestrations

Question 6

Where are fenestrations located in synapsids and diapsids?

Answer 6

post orbital part of skull

Question 7

What is the function of temporal openings?

Answer 7

• space for jaw-closing adductor muscles to bulge
• allows for more muslce mass and attachment points

Question 8

Describe the evolution of the synapsid jaw

Answer 8

• small temporal opening of pelycosaurs - increase cynodonts
• temporal fossa - increased temporalis muscle
• bowed out zygomatic arch - development of masseter muscle
• homodont pelycosaur teeth - heterodont

Question 9

Define fossa

Answer 9

depression

Question 10

Define zygomatic arch

Answer 10

cheekbones

Question 11

Describe basal tetrapods

Answer 11

• sprawling limbs
• evolution of upright posture

Question 12

Describe sprawling limbs in basal tetrapods

Answer 12

• constant muscle action to prevent trunk sagging
• problem increases with size
• wave-like locomotion compresses alternate lungs to aid with breathing

Question 13

How does sprawling limbs problem increase with size?

Answer 13

• mass proporitonal to volume cubed
• muscle strength proportional to cross section squared

Question 14

Describe the evolution of upright posture in basal tetrapods

Answer 14

• progressive regression of ribs on posterior vertebrae: evolution of thoracic cavity, e.g., bounding gait of dog works with lungs
• sprawling becomes increasingly erect

Question 15

Describe the trunk of axial cat skeletons

Answer 15

lumbar and thoracic vertebrae

Question 16

Describe the diapsid evolution of upright posture

Answer 16

• bipedal posture - walking decoupled from breathing
• inspiration - pelvic muscles contract
• expiration - rectus abdominus contracts

Question 17

Describe the evolution of synapsid and diapsid lungs

Answer 17

• synapsids - alveolar lungs, air passages end in alveolar sacs
• diapsids - faveolar lungs: falveoli open from common space, more efficient lungs

Question 18

Describe the convergent evolution of synapsid and diapsid aorta

Answer 18

• right aorta lost in mammal
• left aorta lost in bird
• embryos of both have 2 aortic arches
• probable adaptation to high blood flow

Question 19

Describe the different evolution of synapsid and diapsid kidneys

Answer 19

• synapsid: nephron with loop of Henle to concentrate urine, excrete soluble urea
• diapsid: lacks loop of Henle, insoluble uric acid excreted, water uptake from bladder/cloaca

Question 20

Describe the different evolution of synapsid and diapsid brains

Answer 20

• major difference in advanced brain structures
• forebrain subdivisions for complex behaviour