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Vetulicolia is a group of bilaterian marine encompassing several extinct species from the Cambrian, and possibly Ediacaran, periods. As of 2023, the majority of workers favor placing Vetulicolians in the stem group of the Chordata, but some continue to favor a more crown group placement as a sister group to the Tunicata. It was initially erected as a monophyletic clade with the taxonomic rank of phylum in 2001, with subsequent work supporting its monophyly. However, more recent research suggests that vetulicolians may be paraphyletic and form a basal evolutionary grade of stem chordates.
Because all vetulicolians had mouths which had no features for chewing or grasping, it is assumed that they were not predators. Since vetulicolians possessed gill slits, many researchers regard these organisms as planktivores. The sediment infills in the guts of their fossils have caused 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.
The earliest vetulicolians appear to have lived in shallow water, with the first deeper water specimens appearing in the Balang biota and possibly in the Qingjiang biota.The first deep water vetulicolians were described by Shuhan Ma in:
A 2024 phylogenetic analysis by Mussini and colleagues found vetulicolians to be a paraphyletic group of Stem group-chordates, lying outside a clade formed by Yunnanozoon, Cathaymyrus, Pikaia and Crown group-chordates. This is in part due to the Cambroernida, which are basal stem-ambulacrarians, being discovered to share with vetulicolians a lack of crown-group chordate characters such as a post-anal tail, despite such characteristics previously being believed to be present in the last common ancestor of deuterostomes. However, ascidian larvae have been noted to have endoderm extending to the terminal end, which could suggest that the ancestral tunicate also had a terminal anus.
Other possible placements are suggested by the Centroneuralia hypothesis, which features a paraphyletic Deuterostomia with chordates as the sister-group to protostomes. If proven true, pharyngeal slits would no longer require a deuterostome placement and vetulicolians could prove to be stem protostomes that lost the post-anal tail. In such a scenario, Banffozoa could be a more derived stem protostome group than Vetulicolida.
First, on the left, a monophyletic Vetulicolia is shown as the sister group to Tunicata, but with all internal relationships unresolved.
Next, on the right, the two proposed classes are shown as the earlier (Banffozoa) and later (Vetulicolida) parts of the vetulicolian grade. Within the Vetulicolida, the family Vetulicolidae as defined by Li et al. (2018) is recovered as monophyletic, while the three widely accepted members of the Didazoonidae are in a polytomy with the clade of crownward chordates. Unusually, the second phylogeny places Nesonektris, which bears a notochord, as less derived than Pikaia which lacks a notochord (while the latter may simply not preserve the notochord, as it preserves the nerve cord and gonads this is unlikely in favour of it simply not having a notochord)
Recent research has strengthened the arguments for placing vetulicolians in the chordate stem lineage rather than near the tunicates. Like vetulicolians, members of the basal ambulacrarian clade Cambroernida have a terminal anus rather than a post-anal tail. Since Ambulacraria is the sister-group of the chordates within the deuterostomes, this suggests that the last common ancestor of both groups lacked a post-anal tail. However, ascidian larvae have been noted to have endoderm extending to the terminal end, which could suggest that tunicates also lacked post-anal tails ancestrally.
Some workers have questioned the inclusion of Banffozoa within this group due to their lack of gill slits and apparent gut diverticula, and have theorized that they may fit within Protostomia instead. Skeemella, in particular, has been noted as having striking arthropod-like characteristics. However, Herpetogaster, the most basal cambroernid, is thought to have non-serialized pores for pharyngial openings. If banffozoans are the most basal vetulicolians, this could explain why they also lack serialized pharyngeal structures. Additionally, a comprehensive review of the Vetulicolia in 2007 did not find evidence of gut diverticula in their material while acknowledging the previous report regarding Banffia. Shenzianyuloma has been interpreted as a vetulicolian with both a notochord (a definitively deuterostome trait) and gut diverticula. However, this fossil has is of unusual provenance (a "crystal and fossil vendor"), and has not yet been examined by other researchers.
Vetulicolians were thought to be stem arthropods when Vetulicola was first discovered, but around 2001 the focus of most theories shifted towards stem deuterostomes due to the discovery of pharyngial gill slits (a deuterostome characteristic), as well as the mounting evidence that vetuicolians have no appendages of any kind. A theory grouping both vetulicolians and vetulocystida with Saccorhytus was disproven when the alleged pharyngial openings of Saccorhytus were shown to be remnants of spines that had broken off; the saccorhytids are now considered to be ecdysozoans.
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