vertevrates : the arrival of jaws

Question 1

Gnathostomes

Answer 1

jawed vertebrates

Question 2

Thelodonts (as ancestors of Gnathostomes)

Answer 2

• stomach
• scales like teeth and modern shark placoid scales

Question 3

Thelodonts (ancestors of Gnathostomes)

Answer 3

• stomach
• scales like teeth and modern shark placoid scales
• broad scale pectoral fin

Question 4

Osteotracomorphs (ancestors of Gnathostomes)

Answer 4

• ossified bone around the eye
• cellular bone
• slit shaped gill opening
• paired fin structure
• 2 dorsal fins
• epicercal tail

Question 5

Placodermi (Gnathostomata class)

Answer 5

• bony, ornamented plates over 30-50% of the body
• big jaws, teeth and large gape: ‘craniovertebral joint”
• quite diverse (200 genera)
• Dunkleosteus : 1m x 6m or larger
• Antiarchs : pectoral fins enclosed in bone-like arms
• evolved toward reduced body armour
• limitations :
  
  • no replacement dentition
  • jaw could not generate suction forces

Question 6

Chondrichthyes (Gnathostomata class)

Answer 6

• 970 living species
• defining features :
  
  • prismatic calcification of endoskeletal cartilage
  • pelvic claspers
• specialised as marine predators
• dominated ancient seas

Question 7

Acanthodii (Gnathostomata class)

Answer 7

• “spiny sharks”
• small and stout
• bony spines before all fins
• cartilaginous skeleton
• body covered in small scales
• large head and eyes
• water column feeders :
  
  • streamlined, round bodies
  • reduced armour

Question 8

Sarcopterygii (Gnathostomata class)

Answer 8

key traits :
- fleshy, lobed fins
- enamelled teeth
- cosmoid scales
- only 8 extant fishes
- 24,742 extant species

Question 9

Actinopterygii (Gnathostomata class)

Answer 9

• 26,891 living species
• sister group to Sarcopterygii
• monophyletic but no strong derived characters :
  
  • scales : ganoid, cycloid, ctenoid or absent
  • spiracle absent
  • gular plate absent