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| Chordates Fossil range: Late Ediacaran – Recent |
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 Yellowfin tuna, Thunnus albacares
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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 cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. Some scientists argue that the true qualifier should be pharyngeal pouches rather than slits.citation needed
The phylum Chordata is broken down into three subphyla: Urochordata, Cephalochordata, and Vertebrata. Urochordate larvae have a notochord and a nerve cord but these are lost in adulthood. Cephalochordates have a notochord and a nerve cord but no vertebrae. 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 three sister phyla, the hemichordates, the echinoderms and the xenoturbellidae, make up the deuterostomes, a superphylum. It is the largest grouping of deuterostomes.
The extant groups of chordates are related as shown in the phylogenetic tree below. Many of the taxa listed do not match traditional classes because several of those classes are paraphyletic. Different attempts to organize the profusion of chordate clades into a small number of groups, some with and some without paraphyletic taxa, have thrown vertebrate classification into a state of flux. Also, the relationships of some chordate groups are not very well understood.
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Classification
Taxonomy
- Phylum Chordata
- Subphylum Cephalochordata — (lancelets, 30 species)
- Subphylum Urochordata or Tunicata — (tunicates, 3,000 species)
- Class Ascidiacea — (sessile filter-feedeing tunicates)
- Class Thaliacea — (free-floating tunicates; solitary or colonial)
- Class Appendicularia or Larvacea — (free-floating tunicates; solitary)
- Class Sorberacea — (sessile carnivorous tunicates)
- (Unranked) Craniata (animals with skulls; 57,739 species)
- Class Myxini or Hyperotreti (hagfish; 65 species)
- Class Cephalaspidomorphi (Paleozoic jawless fish)
- Subphylum Vertebrata (vertebrates — animals with backbones; 57,674 species)
- Infraphylum Agnatha (jawless vertebrates; 100+ species)
- Class Conodonta (Conodonts)
- Class Hyperoartia (Lampreys)
- Class Pteraspidomorphi (Paleozoic jawless fish)
- Class Thelodonti (Paleozoic jawless fish)
- Infraphylum Gnathostomata (jawed vertebrates)
- Class Placodermi (Paleozoic armoured forms)
- Class Chondrichthyes (cartilaginous fish; 900+ species)
- (Unranked) Teleostomi
- Class Acanthodii (Paleozoic "spiny sharks")
- Superclass Osteichthyes (bony fishes; 30,000+ species)
- Class Actinopterygii (ray-finned fish; about 30,000 species)
- Class Sarcopterygii (lobe-finned fish)
- Superclass Tetrapoda (four-legged vertebrates; 18,000+ species)
- Infraphylum Agnatha (jawless vertebrates; 100+ species)
Phylogeny
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Note: Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. Chordata is a phylum.
Origins
The origin of chordates is currently unknown. The first clearly-identifiable chordates are reduced fish- or lancelet-like specimens from the Cambrian. Most speculations about their origin fit into one or more of these categories:
- A sediment-dwelling worm-like animal that evolved a flatter body and/or fins for swimming.
- A sessile tubular filter-feeder that evolved into a free-swimming animal via usage of fins. (Tunicates, considered a chordate, are sessile filter feeders that have a tadpole-like larvae.)
- A drifting or swimming larva of some other kind of animal that eventually retained its swimming features into adulthood.
The notochord's stiffness in many chordates may have evolved to facilitate the effectiveness of alternating muscle contractions for swimming (in S-shaped movements). In other words, in order to bend the body, a muscle needs a rigid structure to pull against, and notochord at early stage is the main feature. Lack of a stiff body part would merely result in the shorting of the animal during muscle contractions instead of the bending motions needed for swimming.
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