Tetrapods

Question 1

When did aquatic vertebrate move to the land?

Answer 1

Devonian period (416-359 MYA)

Question 2

What methods can we use to date rocks and fossils?

Answer 2

• Relative dating; order of decomposition

- Absolute dating; radiometric dating - decay of radioactive isotope; palaeomagnetism

Question 3

There is evidence that the closest ancestors to tetrapods is ___________ fish.

Answer 3

Sarcopterygian

Question 4

Name at least 3 suggested advantages of moving to land.

Answer 4

• Increase in range of habitats & food (vascular plants, snails, arthropods already on land)
• Avoidance of aquatic predators and competitors
• Abundant oxygen
• Raise in body temperature (accelerates metabolism)

Question 5

Name at least 3 disadvantages of leaving water

Answer 5

• Gravity: water supports body with little difference in density. Air less viscous and dense (need support for body for locomotion and prevention of lung collapse)
• More abundant oxygen but need adaptation of respiratory system (lungs)
• Skin to resist desiccation
• Greater fluctuations in temperature (thermoregulation)
• Sense organs respond to different stimuli
• Reproduction
• Blood circulation more difficult (pumping)
• Different food and methods of capture

Question 6

Name at least three tetrapod characteristics

Answer 6

• Paired hind and forelimbs with digits
• Pectoral girdle separated from skull with mobile neck
• Hyomandibular bone (formerly supporting the jaw) becomes stapes conducting sound to ear
• First cervical vertebra modified to allow skull movement

Question 7

Briefly describe the middle ear of tetrapods

Answer 7

• Derived from spiracle

- Hyomandibular (freed from jaw) modified to stapes

Question 8

Briefly describe vision of tetrapods

Answer 8

• Easier in air: flatter lens’ than in fish

- Eyes on top of the head

Question 9

What is the name of the “intermediate”?

What features of lobe-finned fish did it possess?

What feature land-living animals did it possess?

Answer 9

(1) Tiktaalik
(2) fins; scales; primitive jaws; gills and lungs
(3) neck; wrists; flat skull with eyes on top; expanded ribs

Question 10

There are 5 names key to the movement to land, what are they?

Answer 10

• Sarcopterygii
• Eusthenopteron
• Tiktaalik
• Acanthostega
• Ichthyostega

Question 11

What period were terrestrial tetrapods confirmed?

Answer 11

Carboniferous

Question 12

What were the three major events during the movement to land?

Answer 12

• Origin of limbs with digits (also changed vertebral column, braincase, ear)
• Origin of walking
• Origin of terrestriality (adaptations for living on the ground)

Question 13

Name at least three modifications that were considered useful for life on land.

Answer 13

• Paired internal nares (choanae) connect external nostril to buccal cavity
• Air-filled cavity connected to pharynx “lungs”. First appeared in fish before tetrapods (swim bladder later?)
• Bony elements of paired fins

Available for later modification for terrestrial breathing (lungs/double circulation) and support

Question 14

Briefly describe the characteristics found in Acanthostega

Answer 14

• No wrists or ankles
• Paddle-like limbs
• 8 digits
• Internal gills
• Limbs and spine would not support much weight

Question 15

Briefly describe what is significant about the vertebral column of ichthyostega

Answer 15

• Zygapophyses (bony projections) brace against each other and help support spine in absence of water
• Overlapping ribs prevent crushing of lungs but inhibit side-to-side movement

Question 16

How was the skull adapted to breath above water?

Answer 16

• Elongation of the snout, fewer bones meant stronger structures, gives lift out of water
• Strong bones at back of head for muscle attachment
• Fusing of bones of lower jaw for “buccal pump”
• Mouth cavity bellows-like action, gulping air forced into air

Question 17

What was significant about carboniferous tetrapods?

Answer 17

• Bony spine with interlocking vertebrae with centra (obliterating notochord)
• Flex wrists and ankles
• Polydactyl but one group with 5 digits emerged as ancestral to pentadactyl limb of all living tetrapods (might have helped make ankle joint more stable for weight bearing and flexible for walking gait)

Question 18

Briefly describe the pentadactyl limb of tetrapods

Answer 18

• Example of homologous structures
• All tetrapods have 5 digits at some stage in embryonic development, presumably inherited from the common ancestor
• Digits may be lost secondarily at later stages of development
• Origin of tetrapod digits not by new genes but redeployment of gene activity present in earlier fish