The phylum ''Chordata'' consists of three subphyla: ''Urochordata'', represented by tunicates; ''Cephalochordata'', represented by lancelets; and ''Craniata'', which includes Vertebrata. Chordates are monophyletic, meaning that ''Chordata'' contains all and only the descendants of a single common ancestor which is itself a chordate, and that craniates' nearest relatives are cephalochordates. The Chordates arose from a more general superphylum '''Deuterostomia''', which consists of Chordata, Hemichordata,Echinodermata and Xenoturbellida. The Deuterostomes split from Protostomes ~550 mya in the Cambrian era. It is supposed that Chordates arose in the Mid-Cambrian period, however there is controversy regarding that. The controversy arises mainly due to the fact that fossils of early chordates are very rare.
Chordates form a phylum of creatures that are based on a bilateral body plan, and is defined by having at some stage in their lives all of the following: 1) A notochord, 2) A dorsal neural tube 3) Pharyngeal slits 4) A muscular tail that extends backwards behind the anus and 5) An endostyle