Chordates and Fish Evolution

Phylogeny of Chordates

• Chordates are bilaterally symmetrical, have true endoskeletons to which muscles are attached, move by muscles attached to bones, have a nerve cord, notochord, pharyngeal gill slits, and a post anal tail
• Echinoderms are pentaradial, have calcareous plates embedded in their skin for muscle attachment, move by tube feet, and lack the chordate features
• Chordates evolved from ancestral deuterostomes
• Vertebrates are chordates with vertebral columns
• Gnathostomes are vertebrates that have jaws and mineralized skeletons
• Osteichthyans are gnathostomes with lungs or lung derivatives

Evolution of Jaws

• The first vertebrates were jawless fish
• Jawed fish evolved from jawless fish

Fish Classes

• Fish are vertebrates with vertebral columns, jaws, paired appendages, internal gills, and efficient blood circulation
• Fish classes include Myxini (hagfish), Petromyzontida (lampreys), Chondrichthyes (cartilaginous fish), Actinopterygii (ray-finned fish), Actinistia (lobe-finned fish), and Dipnoi (lungfish)

Hagfish

• Hagfish belong to the class Myxini
• Hagfish have slime glands

Lampreys

• Lampreys belong to the class Petromyzontida (also called Cephalaspidomorphi)

Chondrichthyes

• Chondrichthyes have improved jaws, cartilaginous skeletons, streamlined bodies, teeth, and lateral line sense organs

Osteichthyes

• Osteichthyes have bony skeletons
• Actinopterygii (ray-finned fish) have fins with parallel thin bony rays and no muscle allowing them to move the fins independently of one another
• Sarcopterygii (lobe-finned fish) have long muscular lobes supported by bones allowing them to move the fins independently of one another
• Actinistia (lobe-finned fish) are coelacanths
• Dipnoi (lungfish) are lobe-finned with lungs

Tiktaalik

• Tiktaalik is a transitional fossil nicknamed the fishapod
• Tiktaalik had fish-like features such as scales, fins, and gills, but also had tetrapod-like features such as a neck, ribs, a flat skull, and eyes on top of its skull

Amphibians

• Amphibians are tetrapods with legs, lungs, and cutaneous respiration, containing pulmonary veins, a divided heart, and two circuits (pulmonary and systemic)
• Frogs and toads have smooth, moist skin or dry skin, long hind legs, no tails, teeth, external fertilization, and tadpole larvae
• Salamanders have smooth, moist skin, four legs, long tails, teeth, internal fertilization, and axolotl larvae
• Caecilians have smooth, moist skin, no legs, small or absent tails, teeth, internal fertilization, and the majority are viviparous

Reptiles

• Reptiles have dry skin, thoracic breathing, amniotic eggs, internal fertilization, and more efficient circulatory systems
• The key innovation of the amniotic egg is extraembryonic membranes including the chorion, allantois, amnion, and yolk sac

Birds

• Birds evolved from bipedal dinosaurs
• Birds have feathers, skeletons, air sacs, efficient circulation, endothermy, and specialized wings
• Features of feathers include the shaft, vane, barbs, and barbules

Mammals

• Mammals have hair, mammary glands, endothermy, efficient circulation, efficient respiration, placentas, specialized teeth, and hooves or horns

Bats

• Bats are flying mammals

Explore the fascinating evolution and classification of chordates and fish in this quiz. Examine the key features, evolutionary stages, and differences among chordates, including the emergence of jaws in vertebrates. Test your knowledge on the various classes of fish and their unique characteristics.