Vetulicolia is the name of a taxon encompassing several extinct 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 ; 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 before 2001 considered them early limbless arthropods but now considers them early deuterostomes. Vetulicolian fossils examined in 2014 show the presence of notochord-like structures, and it was concluded that vetulicolians are crown-group chordates and probably the sister group of modern tunicates. Research from 2017 indicates Vetulicolia are rather related to Saccorhytus, another basal deuterostome group.
Taxonomy and evolution
As originally proposed, the phylum included the Didazoonidae and the Vetulicolidae. Other groups which may be related include the yunnanozoans. The taxonomic placement of the Vetulicolians remains controversial. One researcher 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. 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 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 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.
