Olfactores is a clade within the Chordata that comprises the Tunicata (Urochordata) and the Vertebrata (sometimes referred to as Craniata).[3] Olfactores represent the overwhelming majority of the phylum Chordata, as the Cephalochordata (with only 32 known species across 3 genera)[4] are the only chordates not included in the clade. This clade is defined by a more advanced olfactory system which, in the immediate vertebrate generation, gave rise to nostrils.
Etymology
The name Olfactores comes from Latin *olfactores ("smellers," from purposive supine olfactum of olfacio, "to smell," with plural masculine agentive nominalizing suffix -tores), due to the development of pharyngeal respiratory and sensory functions, in contrast with cephalochordates such as the lancelet which lack a respiratory system and specialized sense organs.[3]
Olfactores hypothesis
The long-standing Euchordata hypothesis that Cephalochordata is a sister taxon to Craniata was once widely accepted,[5] likely influenced by significant tunicate morphological apomorphies from other chordates, with cephalochordates even being nicknamed 'honorary vertebrates.'[6]
The name Olfactores was originally introduced in 1991 as part of the now-disproven calcichordate hypothesis.[7] However, studies since 2006 analyzing large sequencing datasets strongly support Olfactores as a clade.[8][9]
Anatomy of ancestral Olfactores
Some studies suggest that the ancestors of Appendicularia and Vertebrata were possibly sedentary-pelagic,[10][11][12] although others recover a free-living chordate ancestor transitioning to free-living vertebrates without an intervening sedentary form.[13] A rudimentary neural crest is present in tunicates, implying its presence in the Olfactores ancestor also, as vertebrates have a true neural crest.[14]
Dunn, C.W.; Hejnol, A.; Matus, D.Q.; Pang, K.; Browne, W.E.; Smith, S.A.; Seaver, E.; Rouse, G.W.; Obst, M.; Edgecombe, G.D.; Sørensen, M.V.; Haddock, S.H.D.; Schmidt-Rhaesa, A.; Okusu, A.; Kristensen, R.M.; Wheeler, W.C.; Martindale, M.Q.; Giribet, G. (2008). "Broad phylogenetic sampling improves resolution of the animal tree of life". Nature. 452 (7188): 745–749. Bibcode:2008Natur.452..745D. doi:10.1038/nature06614. PMID18322464. S2CID4397099.
Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B. J.; Trusler, P.; Hall, M. (2012). "A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the ascidians". Paleontological Journal. 46 (1): 1–11. Bibcode:2012PalJ...46....1F. doi:10.1134/S0031030112010042. S2CID128415270.
Jeffries, R. P. S. (1991). "Two types of bilateral symmetry in the Metazoa: chordate and bilaterian". In Bock, Gregory R.; Marsh, Joan (eds.). Biological Asymmetry and Handedness. Ciba Foundation Symposium. John Wiley and Sons. pp.94–127. doi:10.1002/9780470514160.ch7. ISBN978-0-471-92961-1. PMID1802652.
