Gnathostomes (Jawed Fish)

Gnathostomes

Infraphylum Gnathostomes

• Jawed fish
• Gnath = "jaw", stoma = "mouth"
• Evolution of 2 pairs of fins
• Bigger brain
• Enables more complex movements
• More duplications of Hox genes
• 2 x 2 = 4 clusters of Hox genes
• Added genetic diversity
• Jaws: enabled diverse feeding behavior
• Larger Brain
• Improved vision and smell
• Sharks lost hard skeleton
• Reverted to cartilage
• Evolution of jaws
• Modified 2 pairs of skeletal rods previously used to support pharyngeal slits
• More ways to get food
• Posterior slits became specialized for gas exchange
• Became gill slits

Class Chondrichthyes

• Cartilaginous fish
• Chondro = "cartilage", ichthys = "fish"
• Endoskeleton made out of cartilage
• Do have bony teeth (calcified)
• Cartilaginous skeleton
• Not a primitive character
• Character is derived
• Evolved from ancestors with bony skeletons
• Development of most vertebrates
• Skeleton (cartilaginous) becomes ossified (bony)
• Cartilage is replaced by calcium phosphate
• Cartilage ossifies as embryo develops
• Chondrichthys skips this process

Sharks

Swimming

• propulsion comes from the tail
• Lacks gas-filled swim bladder
• Buoyancy from oil in liver
• Must use muscles to gulp water if not in motion

Skin

• Rough due to dermal placoid scales
• Dermal denticles
• Hydrodynamic
• Scales are homologous to vertebrate teeth
• Unclear which came first

Teeth

• Continuously replaced (conveyor belt of teeth)
• Common in fossil records

Sharks (Continued)

Reproduction

• Copulation
• Claspers: male pelvic fins modified for copulation
• Internal fertilization
• Progeny production
• Oviparous: Lays eggs
• Ovoviviparous: Eggs hatch inside female
• Viviparous: Live birth
• Eggs protected in leathery egg case

Nostrils

• Smelling (not gas exchange)
• Detect changes in electrical field generated by other animal muscle movements
• Uses pores around the head
• Detects vibrations in water
• Uses Lateral Line
• Lateral Line: pores that detect disturbances in the water
• Exists in bony fish as well