Chordata

Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates.

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses, but the current consensus is that chordates are monophyletic, in other words contain all and only the descendants of a single common ancestor which is itself a chordate, and that craniates' nearest relatives are cephalochordates. All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that these are vertebrates. Because the fossil record of chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However the use of molecular phylogenetics for dating evolutionary transitions is controversial.

Origins
The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, and chordates are deuterostomes. It seems very likely that Kimberella was a member of the protostomes. If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least, and hence well before the start of the Cambrian. The Ediacaran fossil Ernettia, from about, may represent a deuterostome animal.

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids) are quite common from the start of the Cambrian,. The Middle Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. Another Chenjiang fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes — although it also had short tentacles round its mouth. Haikouichthys and Myllokunmingia, also from the Chenjiang fauna, are regarded as fish. Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate. On the other hand fossils of early chordates are very rare, since non-vertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.

A consensus family tree of the chordates The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected. Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but open the possibility that tunicates (urochordates) are "basal deuterostomes", in other words surviving members of the group from which echinoderms, hemichordates and chordates evolved. Most researchers agree that, within the chordates, craniates are most closely related to cephalochordates, but there also reasons for regarding tunicates (urochordates) as craniates' closest relatives. One other phylum, Xenoturbellida, appears to be basal within the deuterostomes, in other words closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.

Since chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques, in other words by analysing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before and the earliest chordates around. However molecular estimates of dates often disagree with each other and with the fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged.

Taxonomy
The following schema is from the third edition of Vertebrate Palaeontology. While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.


 * Phylum Chordata
 * Subphylum Tunicata (Urochordata) — (tunicates, 3,000 species)
 * Subphylum Cephalochordata (Acraniata) — (lancelets, 30 species)
 * Subphylum Vertebrata (Craniata) (vertebrates — animals with backbones; 57,674 species)
 * Class 'Agnatha'Paraphyletic (jawless vertebrates; 100+ species)
 * Subclass Myxinoidea (hagfish; 65 species)
 * Subclass Petromyzontida (Lampreys)
 * Subclass Conodonta
 * Subclass Pteraspidomorphi (Paleozoic jawless fish)
 * Order Anaspida
 * Order Thelodonti (Paleozoic jawless fish)
 * Infraphylum Gnathostomata (jawed vertebrates)
 * Class Placodermi (Paleozoic armoured forms)
 * Class Chondrichthyes (cartilaginous fish; 900+ species)
 * Class Acanthodii (Paleozoic "spiny sharks")
 * Class Osteichthyes (bony fishes; 30,000+ species)
 * Subclass Actinopterygii (ray-finned fish; about 30,000 species)
 * Subclass Sarcopterygii (lobe-finned fish)
 * Superclass Tetrapoda (four-legged vertebrates; 18,000+ species)
 * Class Amphibia (amphibians; 6,000 species)
 * Series Amniota (with amniotic egg)
 * Class Reptilia — (reptiles; 8,225+ species)
 * Subclass Anapsida (extinct "proto-reptiles" and possibly turtles)
 * Subclass Synapsida (mammal-like "reptiles"; 4,500+ species, progenitors of mammals)
 * Subclass Diapsida (majority of reptiles, progenitors of birds)
 * Class Aves (birds; 8,800–10,000 species)
 * Class Mammalia (mammals; 5,800 species)