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 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 In vertebrates, the notochord has been replaced by a bony vertebral column.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 (kingdom Animalia) A large phylum comprising the animals that possess a rod of flexible tissue (notochord), which is protected in higher forms by a vertebral column.
Mammalia (phylumChordata, superclass Gnathostomata) A class of homoiothermic animals in which the head is supported by a flexible neck, typically with seven vertebrae, articulating through two occipital condyles, the side wall of the skull is formed by the alisphenoid bone, the lower jaw is...
Light-Sensitive Voltage Responses in the Neurons of the Cerebral Ganglion of Ciona savignyi (Chordata: Ascidiacea).
These fossils are highly significant because they imply the contemporary existence of the tunicates and craniates in the Early Cambrian during the so-called Cambrian Explosion of animal life.
In the Tree of Life project, conodonts are placed as a subgroup of vertebrates.
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