Nephrozoa is a major clade of bilaterians, divided into the protostomes and the deuterostomes, containing almost all animal phyla and over a million extant species. Its sister clade is the Xenacoelomorpha. The Ambulacraria (conventionally deuterostomes) was formerly thought to be sister to the Xenacoelomorpha, forming the Xenambulacraria as basal Deuterostomes, or basal Bilateria invalidating Nephrozoa and Deuterostomes in earlier studies. The coelom, the digestive tract and excretory organs, and nerve cords developed in the Nephrozoa. It has been argued that, because protonephridia are only found in protostomes, they can not be considered a synapomorphy of this group. This would make Nephrozoa an improper name, leaving Eubilateria as this clade's name.
Chordates (which include all the vertebrates) are deuterostomes. It seems very likely that the 555 million year oldKimberella 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 558 million years ago, and hence well before the start of the Cambrian 541 million years ago.
^"Animal Evolution: Interrelationships of the Living Phyla. Third Edition. By Claus Nielsen. Oxford and New York: Oxford University Press. $135.00 (hardcover); $69.99 (paper). x + 402 p. + 9 pl.; ill.; systematic and subject indexes. 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.
^New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.", in Vickers-Rich, Patricia; Komarower, Patricia (eds.), The Rise and Fall of the Ediacaran Biota, Special publications, 286, London: Geological Society, pp. 157-179, doi:10.1144/SP286.12, ISBN9781862392335, OCLC156823511CS1 maint: uses authors parameter (link)
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