Classification and Evolution of Fishes

Classification of Fishes

• AGNATHA: Jawless fish (e.g., Lampreys, Hagfish).
• CHONDRICHTHYES: Jawed fish with cartilaginous skeletons (e.g., Sharks, Rays).
• OSTEICHTHYES: Fish with bony skeletons (e.g., Lungfish, Trout).

Characteristics of Agnatha

• No jaws, cartilaginous skeleton.
• Oral sucker.
• Predators and filter feeders.
• Fresh and salt water; some are anadromous.

Characteristics of Chondrichthyes

• Cartilaginous skeleton, ventral mouth.
• Placoid scales.
• 5-7 gill slits, no operculum.
• No swim bladder, internal fertilization.
• Spiral valve intestines.
• Heterocercal fin, urea waste.

Characteristics of Osteichthyes

• Bony skeleton, terminal mouth.
• Cycloid/ctenoid scales.
• External fertilization.
• 4 gills with operculum.
• Protrusible jaws.
• Swim bladder.
• Homocercal fin, ammonia waste.

Evolution of Fish

• Natural selection: Traits improve adaptation.

Ostracoderms

• First vertebrates; no jaws or paired fins.
• Bony armor, cartilaginous skeleton.
• Heterocercal tail.
• Small bottom dwellers.

Placoderms

• Earliest jawed fishes.
• Heavy skeletons; Devonian period.

Evolution of Chondrichthyes

• Cartilaginous skeleton, teeth replaced in rows.
• Unsegmented fin rays.
• Ventral nostril.
• Spiral valve intestine, claspers.
• Two lines: Elasmobranchii (sharks, rays), Holocephali (chimaeras).

Evolution of Elasmobranchii

• Predators; rare fossils.
• Gill openings, spiracle, placoid