The Agnatha and the evolution of the jaw

Question 1

Name and describe an extinct group of Agnathans

Answer 1

Ostracoderms
- Approx 500 MYA
- Dermal exoskeleton
- Moveable plates around mouth
- Notochord
- Dorsal fin, some pectoral fins
- Coexisted with gnathostomes for 50MY
- Extinct in late Devonian

Question 2

What are present day agnathans?

Answer 2

Lampreys & hagfishes (cyclostomes)

Question 3

Describe Lampreys anatomy

Answer 3

• Predaceous as adults (latch onto prey and weaken)
• Produce anticoagulant from oral gland

Internal:
- Notochord
- Arcualia
- Tidal ventilation (Velum stops water from flowing into mouth)
- Nasohyophyseal opening (basically a nostril) to detect chemical cues
- Well developed eyes and pineal eyes

Question 4

Describe lampreys reproduction

Answer 4

• Adult life in sea, return to freshwater to breed (anadromous)
• Adults find streams by detecting pheromones released by larvae
• Larvae burrow in slit and stay for 3-7 years (filter feeding through mucus produced by gill openings)

Question 5

Describe Hagfish and their anatomy

Answer 5

• Found in marine deep sea
• Produce mucus when attacked (defence/predacious?)
  Anatomy:
• Notochord
  -Arcualia ventral in tail
• Blood isomotic to seawater (only verts with this!)
• Adenohypophysis and olfactory sac close together - separated by velum
• Slime glands
• Eyes covered by skin usually

Question 6

What parts evolved to form the jaw?

Answer 6

Muscularised pharynx - branchial arches → formed the jaw
Anterior branchial arch (mandibular arch)