Early gnathostomes, the fish Flashcards
What is the palatoquadrate?
The upper jaw.
What is the hyoid?
A bone that links the upper jaw to the braincase (upper region) and supports the tongue (lower region).
What is the mandible?
The lower jaw.
How many dorsal fins did early gnathostomes have?
1
Did early gnathostomes have both pelvic and pectoral fins?
No, just pelvic fins.
How many sensory capsules are there in the braincase and what are they?
3 - the olfactory, optic and otic capsules.
How was the jaw connected to the braincase in early gnathostomes like chondrichthyans?
The lower jaw was connected to the braincase by the hyoid, the upper jaw was loose and swung forwards.
Are the gill bones hinged?
Yes.
What are the two theories for the origin of jaws?
- 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.
- The new structure scenario: the gills are a new structure derived from the velar skeleton.
What are the 4 early radiations of gnathostome?
- Chondrichthyans
- Placoderms
- Acanthodians
- Osteichthyans
What are the 3 major derived characters of chondrichthyans?
- Heterocercal tail
- Calcified cartilaginous skeleton
- Dentacles in the skin
What are the 3 major derived characters of chondrichthyans?
- Heterocercal tail
- Calcified cartilaginous skeleton
- Denticles in the skin
What kind of teeth do chimeras have?
Broad, slow-growing tooth plates analogous to herbivorous molars.
What is the upper jaw of chondrichthyans called? Is it calcified?
The palatopterygoquadrate - it is calcified.
What is the lower jaw of chondrichthyans called? Is it calcified?
Meckel’s cartilage - it is not calcified.
What is the jaw articulation of chondrichthyans?
The hyoid joins the Meckel’s cartilage (lower jaw) to the braincase.
What are the major features of arthrodiran placoderms?
- Heavy head armour but none on the body
2. Hinge in the head shoulder region to open the mouth wide
How many dorsal fins does cladoselache, one of the most primitive sharks, have?
2
What are the major features of antiarchan placoderms?
2 dorsal fins
What are the 2 major radiations of osteichthyan?
Actinopterygians and sarcopterygians
What is characteristic about placoderm fins?
They are supported by an anterior spine
What are the 6 key features of actinopterygians?
- Symmetrical tails
- Multiple humerus analogues
- Ganoine body enamel
- Peg and socket scales
- No body armour
- Single dorsal fin
What are the 6 key sarcopterygian features?
- Symmetrical tails
- Single humerus analogue
- Cosmine body enamel
- Reduced body armour
- Plate-like scales
- 2 dorsal fins
Why do chondrichthyans need heterocercal tails?
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.
What are the teeth-baring bones in Chondrichthyans?
The palatopterygoquadrate and the Meckel’s cartilage
Why do osteichthyans have symmetrical tails?
Osteichthyans are neutrally buoyant as they have swimbladders, so do not need to generate lift.
What are the teeth-baring bones in osteichthyans?
The premaxilla, maxilla and the dentary.
When did early gnathostomes first appear?
400-500mya.
How do gnathostome gills work?
By unidirectional flow.
When do placoderms originate from?
The Silurian.
When were placoderms most diverse?
The Devonian.
From which cells are the jaws, hyoid and gill arches derived?
Neural crest cells (ectoderm).
Is the calcified cartilaginous skeleton of chondrichthyans the ancestral state?
No, it is believed to be a derived state.
What is meant be homologous?
The same evolutionary origin or structure but with a different function.
How do we know denticles and teeth are homologous?
They are the same structure, with laminar and vascular layers, a pulp cavity and enamel.
What is the homologue of a spiracle in humans?
The Eustacian tube that connects the throat to the middle ear to help equalise pressure.
Chondrichthyans have a spiracle - what is it?
A pore between the upper jaw and hyoid element, allowing water flow between the throat and external environment.
Which of the 4 groups of early gnathostomes is regarded as the most primitive?
The acanthodians.
Sarcopterygians have both dermal and endochondral bone. True or false?
True - they have reduced body armour.
Do actinopterygians possess dermal bone?
No.
When did the teleosts radiate?
The late Mesozoic.
What are the teleosts?
A hugely diverse group of ray-finned fishes (actinopterygians).
A modification of the jaw occurred in teleosts that allows them to exploit a range of feeding mechanisms. What happened?
The premaxilla and maxilla became more mobile.
Give 3 types of jaw articulation observed in teleosts?
- Premaxilla and maxilla are fixed (ancestral condition).
- Maxilla is fixed at the anterior but is free to rotate downwards to expand the cheek.
- There is mobility in both the premaxilla and the maxilla, allowing the mouth to be extended into a tube-like structure.
Actinopterygians evolved symmetrical tails. What was the ancestral condition?
Heterocercal.
The evolution of a symmetrical tail in actinopterygians meant that pectoral fins were no longer necessary for lift and could diversify. Why is this?
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.
Is ‘fish’ an official taxonomic group?
No, it is a paraphyly as it refers to all vertebrates except the tetrapods.
