Early gnathostomes, the fish

Question 1

What is the palatoquadrate?

Answer 1

The upper jaw.

Question 1

What is the hyoid?

Answer 1

A bone that links the upper jaw to the braincase (upper region) and supports the tongue (lower region).

Question 2

What is the mandible?

Answer 2

The lower jaw.

Question 3

How many dorsal fins did early gnathostomes have?

Answer 3

1

Question 4

Did early gnathostomes have both pelvic and pectoral fins?

Answer 4

No, just pelvic fins.

Question 6

How many sensory capsules are there in the braincase and what are they?

Answer 6

3 - the olfactory, optic and otic capsules.

Question 7

How was the jaw connected to the braincase in early gnathostomes like chondrichthyans?

Answer 7

The lower jaw was connected to the braincase by the hyoid, the upper jaw was loose and swung forwards.

Question 8

Are the gill bones hinged?

Answer 8

Yes.

Question 9

What are the two theories for the origin of jaws?

Answer 9

1. The serial homology scenario: the first two gill plates migrated forwards and upwards to form the upper and lower jaws. The gill slit between them became the spiracle.
2. The new structure scenario: the gills are a new structure derived from the velar skeleton.

Question 10

What are the 4 early radiations of gnathostome?

Answer 10

1. Chondrichthyans
2. Placoderms
3. Acanthodians
4. Osteichthyans

Question 10

What are the 3 major derived characters of chondrichthyans?

Answer 10

1. Heterocercal tail
2. Calcified cartilaginous skeleton
3. Dentacles in the skin

Question 11

What are the 3 major derived characters of chondrichthyans?

Answer 11

1. Heterocercal tail
2. Calcified cartilaginous skeleton
3. Denticles in the skin

Question 12

What kind of teeth do chimeras have?

Answer 12

Broad, slow-growing tooth plates analogous to herbivorous molars.

Question 14

What is the upper jaw of chondrichthyans called? Is it calcified?

Answer 14

The palatopterygoquadrate - it is calcified.

Question 15

What is the lower jaw of chondrichthyans called? Is it calcified?

Answer 15

Meckel’s cartilage - it is not calcified.

Question 16

What is the jaw articulation of chondrichthyans?

Answer 16

The hyoid joins the Meckel’s cartilage (lower jaw) to the braincase.

Question 16

What are the major features of arthrodiran placoderms?

Answer 16

1. Heavy head armour but none on the body

2. Hinge in the head shoulder region to open the mouth wide

Question 17

How many dorsal fins does cladoselache, one of the most primitive sharks, have?

Answer 17

2

Question 18

What are the major features of antiarchan placoderms?

Answer 18

2 dorsal fins

Question 19

What are the 2 major radiations of osteichthyan?

Answer 19

Actinopterygians and sarcopterygians

Question 20

What is characteristic about placoderm fins?

Answer 20

They are supported by an anterior spine

Question 21

What are the 6 key features of actinopterygians?

Answer 21

1. Symmetrical tails
2. Multiple humerus analogues
3. Ganoine body enamel
4. Peg and socket scales
5. No body armour
6. Single dorsal fin

Question 22

What are the 6 key sarcopterygian features?

Answer 22

1. Symmetrical tails
2. Single humerus analogue
3. Cosmine body enamel
4. Reduced body armour
5. Plate-like scales
6. 2 dorsal fins

Question 24

Why do chondrichthyans need heterocercal tails?

Answer 24

Heterocercal tails generate downward force, which is counter-balanced by the pectoral fins. Sharks are not neutrally buoyant so this is an efficient way of swimming.

Question 24

What are the teeth-baring bones in Chondrichthyans?

Answer 24

The palatopterygoquadrate and the Meckel's cartilage

Question 25

Why do osteichthyans have symmetrical tails?

Answer 25

Osteichthyans are neutrally buoyant as they have swimbladders, so do not need to generate lift.

Question 26

What are the teeth-baring bones in osteichthyans?

Answer 26

The premaxilla, maxilla and the dentary.

Question 27

When did early gnathostomes first appear?

Answer 27

400-500mya.

Question 28

How do gnathostome gills work?

Answer 28

By unidirectional flow.

Question 29

When do placoderms originate from?

Answer 29

The Silurian.

Question 30

When were placoderms most diverse?

Answer 30

The Devonian.

Question 31

From which cells are the jaws, hyoid and gill arches derived?

Answer 31

Neural crest cells (ectoderm).

Question 32

Is the calcified cartilaginous skeleton of chondrichthyans the ancestral state?

Answer 32

No, it is believed to be a derived state.

Question 33

What is meant be homologous?

Answer 33

The same evolutionary origin or structure but with a different function.

Question 34

How do we know denticles and teeth are homologous?

Answer 34

They are the same structure, with laminar and vascular layers, a pulp cavity and enamel.

Question 35

What is the homologue of a spiracle in humans?

Answer 35

The Eustacian tube that connects the throat to the middle ear to help equalise pressure.

Question 36

Chondrichthyans have a spiracle - what is it?

Answer 36

A pore between the upper jaw and hyoid element, allowing water flow between the throat and external environment.

Question 37

Which of the 4 groups of early gnathostomes is regarded as the most primitive?

Answer 37

The acanthodians.

Question 38

Sarcopterygians have both dermal and endochondral bone. True or false?

Answer 38

True - they have reduced body armour.

Question 39

Do actinopterygians possess dermal bone?

Answer 39

No.

Question 40

When did the teleosts radiate?

Answer 40

The late Mesozoic.

Question 41

What are the teleosts?

Answer 41

A hugely diverse group of ray-finned fishes (actinopterygians).

Question 42

A modification of the jaw occurred in teleosts that allows them to exploit a range of feeding mechanisms. What happened?

Answer 42

The premaxilla and maxilla became more mobile.

Question 43

Give 3 types of jaw articulation observed in teleosts?

Answer 43

1. Premaxilla and maxilla are fixed (ancestral condition). 2. Maxilla is fixed at the anterior but is free to rotate downwards to expand the cheek. 3. There is mobility in both the premaxilla and the maxilla, allowing the mouth to be extended into a tube-like structure.

Question 44

Actinopterygians evolved symmetrical tails. What was the ancestral condition?

Answer 44

Heterocercal.

Question 45

The evolution of a symmetrical tail in actinopterygians meant that pectoral fins were no longer necessary for lift and could diversify. Why is this?

Answer 45

Actinopterygians lost all body armour and developed swimbladders making them neutrally buoyant, thus they lost their heterocercal tails that created a downward force. Pectoral fins are no longer necessary to counteract this.

Question 47

Is 'fish' an official taxonomic group?

Answer 47

No, it is a paraphyly as it refers to all vertebrates except the tetrapods.