Chordate

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? Chordates
Yellowfin tuna, Thunnus albacares
Scientific classification
Kingdom: Animalia Superphylum: Deuterostomia Phylum: Chordata Bateson, 1885
Typical Classes
Subphylum Urochordata Ascidiacea Thaliacea Larvacea Subphylum Cephalochordata Subphylum Vertebrata Myxini Conodonta Hyperoartia Cephalaspidomorphi Pteraspidomorphi Placodermi Chondrichthyes Acanthodii Actinopterygii Sarcopterygii Amphibia Sauropsida Synapsida Aves Mammalia

Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a muscular tail extending past the anus. Some scientists argue, however, that the true qualifier should be pharyngeal pouches rather than slits.

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 vertebra. In all vertebrates except for Hagfish, the dorsal hollow nerve cord has been surrounded with cartilaginous or bony vertebrae and the notochord generally reduced.

The chordates and two sister phyla, the hemichordates and the echinoderms, make up the deuterostomes, a superphylum.

It has been theorised that the phylum originated as a result of paedomorphy occurring in a primitive ancestral form.

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 not very well understood.

Phylum Chordata

• Subphylum Urochordata - Tunicates
• Subphylum Cephalochordata - Lancelets
• (unranked)Craniata (animals with skulls)
• Subphylum Vertebrata (Vertebrates - animals with backbones)
  • Class Myxini or Hyperotreti (hagfish)
  • Class Conodonta (Conodonts)
  • Class Cephalaspidomorphi (Paleozoic jawless fish)
    • Hyperoartia (lampreys and kin)
  • Class Pteraspidomorphi (Paleozoic jawless fish)
  • Infraphylum Gnathostomata (jawed vertebrates)
  • Class Placodermi (Paleozoic armoured forms)
  • Class Chondrichthyes (cartilaginous fish)
    • (unranked) Teleostomi (bony fish and their descendents)
      • Class Acanthodii (Paleozoic "spiny sharks")
      • Class Actinopterygii (ray-finned fish - Osteichthyes in part)
      • Class Sarcopterygii (lobe-finned fish - Osteichthyes in part))
        • Superclass Tetrapoda (four-legged vertebrates)
          • Class Amphibia (amphibians)
          • Series Amniota (amniotic egg)
            • Class Synapsida (mammal-like reptiles)
              • Class Mammalia (mammals)
              • Class Sauropsida - (reptiles)
              • Subclass Anapsida
                • • Order Testudines (turtles)
                • Subclass Diapsida (most modern reptiles)
                  • Superorder Lepidosauria (lizards, snakes, tuatara)
                  • Division Archosauria ("ruling reptiles")
                    • Order Crocodilia (crocodiles, alligators, caimans, gharials)
                    • Superorder Dinosauria
                      • Class Aves (birds)

Wikispecies has information related to:

Chordata

Wikibooks Dichotomous Key has more about this subject:

Chordata