Phylum Chordata: The Vertebrates

Traditionally, the phylum Chordata is divided into 7 classes:

Class Agnatha (jawless fish)Class Chondrichthyes (cartilaginous fishes)

Class Osteichthyes (bony fishes)Class Amphibia (amphibians)

Class Reptilia (reptiles)Class Aves (birds)

Class Mammalia (mammals)

Which classes do you think evolved first? Which evolved last?

Chordata: Dorsal Nerve Cord, with surrounding endoskeleton

• OTHER CHARACTERISTICS:• Extrinsic eye muscles. These muscles are attached to the eyeball

and orbital wall, and ensure eye movements Radial muscles in fins. These are small muscles associated with each of the cartilaginous radials of the unpaired and paired fins. They ensure the undulatory movements of the fin web. Atrium and ventricle of heart closely-set. Nervous regulation of heart. The heart in the embryo of the vertebrates is aneural, like the heart of adult hagfishes. In adult vertebrates, however, the heart is innervated by a branch of the vagus nerve. Typhlosole in the intestine. This is a spirally coiled fold of the intestinal wall. In the Gnathostomes, it can be developed into a complex spiral valve. At least two vertical semicircular canals in the labyrinth True neuromasts in the sensory-line system

Evolutionary History

The Family Tree

• Fish evolve first from a deuterostome ancestor during the Cambrian explosion. (~500 MYA)

• Amphibians and reptiles evolve from fish-like ancestors about 400 MYA.

• The first mammals appear about 200 MYA

Early Chordates: The Fishes

• Fish = gills, skull, and 1 or more fins.• Usually have scales, and are ectothermic (“cold

blooded”)

Hagfish: The first

chordates

• Craniates: Only living animals that have a skull but no spinal column.

• Deepest-branching member of phylum chordata.

• Primative eyes, worm-like characteristics

Lampreys• Vertebral column, but no jaws

Hagfish + Lampreys = Class Agnatha, the Jawless Fish

• Other than class Agnatha, all vertebrates have a hinged jaw.

Cartilaginous Fish: Sharks, Skates, and Allies

• Have jaws, cartilaginous skeleton.

Bony Fishes: Good ‘Ole “Fish”• Have bony skeleton, scales, jaw, fins.

Fish Anatomy

Recap: Fishes• Includes 3

classes: jawless fish, cartilaginous fish, and bony fish.

• All fish have gills for respiration, and fins for locomotion.

• All fish are aquatic.

Tiktaalik

• Major fossil discovery in 2004.

• One of many fossil intermediates between fish and amphibians.

• Lived about 380 MYA

Amphibians

• Adults are terrestrial, eggs and young are aquatic.

Amphibian Characteristics

• Webbed feet and long legs for jumping and swimming

Amphibian Characteristics

• Non-amniotic eggs; fertilization is aquatic and external

Amphibian Anatomy

Amphibian Recap

• Semi-aquatic: all have aquatic juvenile stage

• Eggs are laid in water, and fertilization is external, like fish