Chordates range in time from Cambrian to Quaternary and possess a spinal cord or notochord at some time during their lifetimes.
Vertebrates possess a spinal cord enclosed by vertebrae. Vertebrates possess an internal skeleton of bone or cartilage and well-developed nervous, circulatory, digestive, and muscular systems. The most commonly preserved portions of the internal skeleton are the bones and teeth of the animal. The teeth and bones are composed of apatite, which is a variety of calcium phosphate.
This class includes modern jawless vertebrates such as lamprey and hagfish. Fossil ostracoderms were jawless fish with platy armor. Class Agnatha ranges from Cambrian to Recent, but ostracoderms became extinct in the Devonian.
Placoderms were primitive jawed fish with platy armored skin. This class ranges from late Silurian to Carboniferous.
Cartilaginous fishes include modern sharks, rays, and skates. Since the skeleton is composed of nonmineralized material, usually only the teeth and spines are fossilized. Chondrichthyes range from Silurian to Recent.
This class includes the bony fish. Major groups are (1) extinct forms called acanthodians: (2) ray-finned fish, including most modern gamefish and food fish; and
(3) lobe-finned fish, which have bone structure in their fins that is similar to the limbs of other vertebrates. Lobe-finned fish called crossopterygians are considered ancestral to amphibians. The range of this class is Silurian to Recent.
The amphibians were the first vertebrates to live on land, although they need moisture in which to lay eggs. Juveniles are aquatic with gills; adults possess lungs. Labyrinthodont amphibians were common Paleozoic fossils. Modern representatives are frogs and salamanders. Amphibians range from Devonian to Recent.
Reptiles were the first vertebrates to conduct their lives fully on land, although many returned to aquatic environments. The key to this ability was the self contained, amniotic egg. Modern representatives of Reptilia include turtles, crocodiles, snakes, and lizards. Fossil members include dinosaurs, flying reptiles, marine reptiles, phytosaurs, therapsids and many others. Reptiles range from upper Carboniferous to Recent.
Birds are warm-blooded, egg-laying vertebrates with an external covering of feathers. The earliest fossil bird, Archaeopteryx, is from the Jurassic. Hesperornis was a Cretaceous diving bird. Birds are very rare as fossils. Somepaleontologists consider birds to be descendants of small theropod dinosaurs. This class ranges from Jurassic to Recent.
Mammals have hair, are warm-blooded, and nurse their young. They range from late Triassic to Recent. There are three major modern groups:
Monotremes are egg-laying mammals and include the duckbill platypus and the echidna.
Marsupials are pouched mammals and include the kangaroo, koala, and opossum.
Placental mammals are born at a more advanced stage of development than marsupials, nourished within the female by the placenta. Examples are rodents, elephants, whales, horses, rabbits, bats, monkeys, and humans.