Chordate

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 muscular 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 In vertebrates, the notochord has been replaced by a bony vertebral column.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 or Hyperotreti (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)
        • Anapsida
          • Testudines (turtles)
        • Diapsida
          • Reptilia (most modern reptiles)
            • Lepidosauria (lizards, snakes, tuatara)
          • Archosauria
            • Crocodilia (crocodiles)
            • Dinosauria
          • Aves (birds)

Note that snakes are considered tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations