The phylum ''Chordata'' consists of three … 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. fossils of early chordates are very rare. +