Vetulicolia

Vetulicolia is an extinct taxon, either phylum or subphylum in rank, encompassing several Cambrian organisms. The vetulicolian body comprises of two parts; a voluminous anterior forebody, tipped with a 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 of 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. 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. However, recent research have confirmed a position close to urochordates for Vetulicolians.

Ecology and Lifestyle
From their superficially tadpole-like forms, leaf or paddle-shaped tails, and various degrees of streamlining, it is assumed that all vetulicolians discovered thus far were swimming animals that spent much, if not all, of their time living in the water column. Some groups, like the genus Vetulicola, were more streamlined (complete with ventral keels) than other groups, such as the tadpole-like Didazoonidae. Because all vetulicolians had mouths which had no features for chewing or grasping, it is automatically assumed that they were not predators. Because of their gill slits, many researchers regard the vetulicolians as being planktivores. The sediment infills in the guts of their fossils have led some to suggest that they were deposit feeders. This idea has been contested, as deposit feeders tend to have straight guts, whereas the hindguts of vetulicolians were spiral-shaped. Some researchers propose that the vetulicolians were "selective deposit-feeders" which actively swam from one region of the seafloor to another, while supplementing their nutrition with filter-feeding.

Classification

 * Phylum Chordata
 * Subphylum Vetulicolia
 * Class Vetulicolida
 * Genus Nesonektris
 * N. aldridgei
 * Family Vetulicolidae
 * Genus ''Vetulicola
 * V. rectangulata
 * V. cuneata
 * V. gangtoucunensis
 * V. monile
 * Genus Ooedigera
 * O. peeli
 * Genus Yuyuanozoon
 * Y. magnificissimi
 * Family Beidazooidae
 * Genus Beidazoon
 * B. venustum (synonym = Bullivetula variola)
 * Family Didazoonidae
 * Genus Didazoon
 * D. haoae
 * Genus Xidazoon
 * X. stephanus
 * Genus Pomatrum
 * P. ventralis
 * ?Class Banffozoa
 * Genus Skeemella
 * Sk. clavula
 * Family Banffiidae
 * Genus Banffia
 * B. constricta
 * Class Heteromorphida
 * Family Heteromorphidae
 * Genus Heteromorphus
 * H. longicaudatus (synonym=Banffia confusa)
 * "Form A" (unnamed species)