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vetulicolia
Vetulicolia〔The taxon name, Vetulocolia, is derived from the type genus, ''Vetulicola'', which is a compound Latin word composed of ''vetuli'' "old" and ''cola'' "inhabitant".〕 is an extinct taxon, either phylum〔 or subphylum〔 in rank, encompassing several Cambrian organisms. The vetulicolian body comprises two parts: a voluminous anterior forebody, tipped with an anteriorly positioned mouth, and lined with a row of five round to oval-shaped features on each lateral side, which have been interpreted as gills - or at least openings in the vicinity of the pharynx, and a posterior section that primitively comprises seven segments, and functions as a tail. All vetulicolians lack preserved appendages of any kind, having no legs, feelers, or even eyes.〔 〕 The area where the anterior and posterior parts join is constricted. Their affinity has been uncertain; they have been considered to represent stem- and crown-group arthropods, stem-group vertebrates, and early deuterostomes. The general scientific consensus formerly (before 2001) considered them early limbless arthropods, but now considers them early deuterostomes.〔Dominguez, Patricio and Jeffries, Richard. (2003). Fossil evidence on the origin of appendicularians. Paper read at International Urochordate Meeting 2003. Abstract at () - URL retrieved June 22, 2006.〕 Recently examined Vetulicolian fossils show the presence of notochord-like structures. Therefore, it can be concluded that vetulicolians are crown-group chordates and probably the sister group of modern tunicates.
==Taxonomy and evolution==
As originally proposed, the phylum included the Didazoonidae (''Didazoon'', ''Pomatrum'', and ''Xidazoon'') and the Vetulicolidae (''Vetulicola'', and ''Banffia'').〔 Other groups which may be related include the yunnanozoans.〔
The taxonomic placement of the Vetulicolians remains controversial. One researcher have argued that the vetulicolians probably represent an early side-branch of deuterostomes, and that this implies that segmentation in cephalochordates and vertebrates may be derived from the common ancestor of protostomes and deuterostomes.〔 However, the researchers who described ''Skeemella'' from the Middle Cambrian of Utah regard it as having affinity to Vetulicolia, but also as having arthropod features, thus confounding assignment of Vetulicolia to Deuterostomia.
Dominguez and Jefferies have argued, based on morphological analysis, that ''Vetulicola'' (and by implication, other Vetulicolians) is a urochordate, and probably a stem-group larvacean. Some question the relation to tunicates and larvaceans, as there is no evidence of segmentation in tunicates, larval or adult, that is comparable to segmentation in vetulicolians, that the anus of urochordates is within the atrium, while that of vetulicolians is positioned at the terminal end of the tail, and, perhaps most importantly, there is no exhalant siphon, or analogous structure, seen in vetulicolians.〔 Abstract on-line at (Pub Ned ) - accessed August 17, 2009〕〔Aldridge, Richard J. et al. (2007) "The systematics and phylogenetic relationships of vetulicolians," ''Paleontology'', Volume 50〕 However, recent research have confirmed a position close to urochordates for Vetulicolians.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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