Colonisation of the land

Question 1

What are sarcopterygians?

Answer 1

Lobe-finned fishes.

Question 2

What are the 5 main synapomorphies of sarcopterygians?

Answer 2

1. A single humerus analogue
2. 2 dorsal fins
3. Cosmine body enamel
4. Reduced body armour
5. V. muscular pectoral girdle

Question 3

When did early tetrapods originate?

Answer 3

The end of the Devonian.

Question 4

What are the 3 extant groups of sarcopterygian?

Answer 4

1. Coelacanths
2. Lungfish
3. Tetrapods

Question 5

What environment do coelacanths live in?

Answer 5

Marine.

Question 6

What environment do lungfish live in?

Answer 6

Freshwater.

Question 7

What are the 3 extant species of lungfish?

Answer 7

1. African
2. South American
3. Australian

Question 8

Which species of lungfish is 100% aquatic?

Answer 8

Australian.

Question 9

Which species of lungfish is an obligate air-breather?

Answer 9

African.

Question 10

Why do lungfish have both lungs and gills?

Answer 10

The lungs evolved as an adaptation to living in stagnant water or in water bodies that frequently dry up.

Question 11

Give 3 facts about coelacanths.

Answer 11

1. They live in caves at depths of 300m
2. They give birth to live young
3. Their swimbladders are full of fat

Question 12

Which group is generally larger (in average body size), actinopterygians or sarcopterygians?

Answer 12

Sarcopterygians.

Question 13

Which evolved first, lungs or swimbladders?

Answer 13

Lungs, which then were exapted into swimbladders.

Question 14

What is the most common limb condition among tetrapods regarding the number of digits?

Answer 14

Pentadactyly.

Question 15

List 3 possible reasons for the colonisation of the land.

Answer 15

1. Aridity
2. Predator evasion
3. Exploitation of new prey

Question 16

What is the traditional hypothesis of land colonisation?

Answer 16

Limbs evolved afterwards, as in a fish crawled up onto land with fins and subsequently developed fins.

Question 17

What is the new hypothesis of land colonisation?

Answer 17

Limbs evolved before fish left the water.

Question 18

List 3 reasons why acanthostega serves as support for the new hypothesis of land colonisation?

Answer 18

1. It had 8 digits but was typically ‘fishy’
2. It had the ancestral lateral line system
3. It had internal gills

Question 19

How does acanthostega’s lateral line system prove it is not a regressive tetrapod, as many opposing the new land colonisation hypothesis would argue?

Answer 19

It has the ancestral lateral line system of a tube beneath the skin connected to several tubes that meet the surface. The derived lateral line system of regressive tetrapods is a groove/channel along the surface of the skin.

Question 20

How do acanthostega’s internal gills prove it is not a regressive tetrapod, as many opposing the new land colonisation hypothesis would argue?

Answer 20

Internal gills are possessed by true fish, whereas regressive tetrapods have external gills. This is because they lost their gills when they colonised the land and had to evolve new ones.

Question 21

Acanthostega is one of the most primitive tetrapods but it is still described as what?

Answer 21

A highly modified fish.

Question 22

What is the lateral line system for?

Answer 22

Detecting pressure changes in the water.

Question 23

Why doesn’t the lateral line system work in air? Give 2 reasons.

Answer 23

1. Air is not dense enough

2. It creates too much water loss

Question 24

If the new hypothesis of land colonisation is true, what is it an example of and why?

Answer 24

Exaptation - acanthostega’s limbs were used for paddling/digging but have been converted into locomotive structures.

Question 25

List 3 major extinct early tetrapods.

Answer 25

1. Pandrichthyes
2. Eustenopteron
3. Tiktaalik