Because of their characteristic endoskeleton (internal skeleton), the chordates are well represented in the fossil record by bones and teeth.  They have been the dominant large land animals throughout time and are generally found at the top of the food chain.  Along with insects and plants, the group successfully colonized the land.  The phylum is represented by the following extant (living) classes:

Class OSTEIICHTHYES (bony fishes)

more fish fossils

Class CHONDRICHTHYES (sharks and rays)

more pictures of sharks' teeth

Class REPTILIA (snakes, turtles, and dinosaurs) a few dinosaur pictures

Class AVES (birds)

Class MAMMALIA (mammals - to which we belong)