Chordates (which include all the vertebrates) are deuterostomes. It seems very likely that the 555 million year oldKimberella was a protostome. If so, this means that the protostome and deuterostome lineages must have split sometime before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 538.8 million years ago.
^"Animal Evolution: Interrelationships of the Living Phyla. Third Edition. By Claus Nielsen. Oxford and New York: Oxford University Press ISBN: 978-0-19-960602-3 (hc); 978-0-19-960603-0 (pb). 2012". The Quarterly Review of Biology. 87 (3): 258. September 2012. doi:10.1086/666815. ISSN0033-5770.
^Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y. (2007), "New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications", in Vickers-Rich, Patricia; Komarower, Patricia (eds.), The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 157–179, doi:10.1144/SP286.12, ISBN978-1-86239-233-5, OCLC156823511