Chordate - Definition and Overview

Chordates (phylum Chordata) include the vertebrates, together with several closely related invertebrates. They are united by having, at some stage in their life, a notochord, a hollow dorsal nerve cord, pharyngeal slits and a tail extending past the anus.

The phylum Chordata is broken down into three subphyla: Urochordata, Cephalochordata, and Vertebrata. Urochordate larvae have a notochord and a nerve cord but they are lost in adulthood. Cephalochordates have a notochord and a nerve cord but no vertebrae. In vertebrates, the notochord has been replaced by a bony vertebral column.

The extant groups of chordates are related as shown in the phylogenetic tree, below. They do not match up very well with the traditional groups, and as a result vertebrate classification is in a state of flux, although their relationships are very well understood.

Chordata

• Urochordata (tunicates)
• Cephalochordata (lancelets)
• Craniata (animals with skulls)
  • Myxini (hagfish)
  • Vertebrata (animals with backbones)
    • Cephalaspidomorphi (lampreys)
    • Gnathostomata (jawed vertebrates)
      • Chondrichthyes (cartilaginous fish)
      • Teleostomi (bony fish, ~ Osteichthyes)
        • Actinopterygii (ray-finned fish)
        • Sarcopterygii (lobe-finned fish)
          • Actinistia (coelacanths)
          • Dipnoi (lungfishes)
          • Tetrapoda (four-legged vertebrates)
            • Amphibia (amphibians)
            • Amniotes (amniotic egg)
              • Synapsida
                • Mammalia (mammals)
              • Reptilia
                • Testudines (turtles)
                • Diapsida
                  • Lepidosauria (lizards, snakes, tuatara)
                  • Archosauria
                    • Crocodilia (crocodiles)
                    • Dinosauria
                      • Aves (birds)