Vetulicolia

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Vetulicolia[note 1] is an extinct phylum[1] encompassing several Cambrian organisms. Their bodies comprise two parts; their front is voluminous and is tipped with a large "mouth" and a row of five round to oval-shaped features on each side which have been interpreted as gills - or at least openings in the vicinity of the pharynx. Their posterior section comprises seven segments. They lack preserved appendages of any kind.[2] The area where the anterior and posterior parts join is constricted. Their affinity is very uncertain; they have been considered to represent stem- and crown-group arthropods, stem-group vertebrates, and early deuterostomes.

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Taxonomy and evolution

As originally proposed, the phylum included the Didazoonidae (Didazoon, Pomatrum, and Xidazoon) and the Vetulicolidae (Vetulicola, and Banffia).[1] Other groups which may be related include the yunnanozoans.[1]

The taxonomic placement of the Vetulicolians remains controversial. Shu (2003) has 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. Briggs et al. (2005) described Skeemella from the Middle Cambrian of Utah 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. However, 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.[3][4]

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