Chordate
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| Chordates | ||||
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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 In vertebrates, the notochord has been replaced by a bony vertebral column.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)
- Reptilia (most modern reptiles)
- Amphibia (amphibians)
Note that snakes are considered tetrapods because they are descended from ancestors who had a full complement of limbs.