Icthyology - Early Fish Evolution

Opportunities in Water

• Support Medium

• Thermal Stability

• Maximum O2 Transfer

Obstacles in Water

• Dense

• Higher Heat Loss in Body

• Less O2 than in Air

What is FISH? (Working Definition)

• Vertebrate, primary aquatic

• Breathe via gills, supplemented by primate lungs (often)

  • Sometimes replaced by advanced lungs

• Excluding terrestrial vertebrate classes 

What is FISH? (Examples)

Trouts, anglerfish, eels, possibly hagfish 

Fin Facts

• 25K + species ( ½ of all vertebrates)

• About 55% marine, 44% freshwater, 1% euryhaline

Chordata Characteristics

• Notochord

• Dorsal (Hollow) Nerve Cord

• Pharyngeal Gill Slits

• Post-Anal Tail

Subphylum Cephalochordate (“Amphioxus”)

• Lancelets

• Have 4 Chordata Characteristics

• Now believed to resemble ancestral Chordata

Subphylum Urochordata

• Sea Squirts, Tunicates

• Adults - sessile filter feeders

• Larvae - mobile and bilateral

• Much stronger case for being ancestral to Chordata

Process of Heterochrony

• “Other Timing”

• Yield Reproductive “Larvae” with chordata characteristics

Agnatha (Jawless Fish)

• Hagfish and Lampreys

• Earliest Vertebrates

• Scavengers or Ectoparastic

Sea Lampreys

Petromizon marinus

Hagfishes

• No true vertebral column

• kept notochord

• might not be vertebrates (no obv. cranium or column)

Agnatha (Phylogeny)

• NOT Monophyletic

• does not contain all its ancestors

“Ammocoete” Larvae (Lampreys)

Early Chordata Characteristics

Super-Class Ostracodermii - Extinct “Armored” Jawless Fishes

• No Modern Fin Arrangement

• Bony armor protects against invertebrate predators

• possibly oldest vertebrates

Super-Class Gnathostomata - ALL Jawed Fishes

• Gill Arches into Pharyngeal Jaw (in mouth jaw)

• Class Placodermi - Extinct Armored Fishes w/ Jaws

Class Placodermi - Extinct Armored Fishes w/ Jaws

• ie.) Dunkleosteus

• Origins 440 MYA

• Connection to Modern Fishes Unknown (Dead-end side branch)