The phylum Vetulicolia was erected in 2001 to group the genera
Vetulicola,
Didazoon, and
Xidazoon (later deemed a junior synonym of
Pomatrum). Prior to this the
class Vetulicolida had been defined in 1997 to group
Vetulicola with the previously enigmatic genus
Banffia due to its similar two-part construction, as well as apparent gill slits in a newly discovered specimen. Further work split
Banffia into a separate class called
Banffozoa, which was soon expanded to encompass similar species such as
Heteromorphus. While subsequent studies supported the
monophyly of Vetulicolia, it has also been noted that this would preclude vetulicolians representing a stepwise development of deuterostome characteristics, as the genus with the most such characteristics,
Vetulicola, is one of the most derived in the group. 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.
Cladograms The following cladograms show two possible placements of the Vetulicolia. 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. The possible presence of a notochord in
Nesonektris and the lack of clear evidence for one in
Pikaia is relevant to the validity of the following cladograms. Proponents of a monophyletic Vetulicolia as the sister group of Tunicata note that scoring the notochord as "unknown" in
Nesonektris produces the same topology, albeit with less support for certain nodes. Proponents of a paraphyletic Vetulicolia, who assume that both
Pikaia and
Nesonektris lacked a notochord, similarly note that scoring
Nesonektris as having a notochord did not significantly change their topology. They also noted that fossils of unambiguous crown-group chordates also frequently lack a visible notochord, but the paper cited regarding this lack lists the notochord as one of the last anatomical structures to decay, with other structures preserved in
Pikaia more likely to decay more quickly. Alternatively, a separate paper examining hypotheses regarding a notochord in
Pikaia notes that it may have been lost as other structures evolved to take over its function. }} }}
Classification The following classification is taken from Li
et al. (2018) • ? Genus
Alienum •
S. yunnanense • Class
Heteromorphida (=
Banffozoa ) • Order
Banffiata • Family
Banffiidae • Genus
Banffia •
B. constricta •
B. episoma • Genus
Heteromorphus •
H. confusus (=
Banffia confusa) ; =
H. longicaudatus ) • Genus
Skeemella •
S. clavula • Class
Vetulicolida • Genus
Nesonektris •
N. aldridgei • Order
Vetulicolata • Family
Vetulicolidae • Genus
Vetulicola •
V. cuneata •
V. rectangulata •
V. gantoucunensis •
V. monile •
V. longbaoshanensis • Genus
Ooedigera •
O. peeli • Genus
Beidazoon (=
Bullivetula ) •
B. venustum (=
B. variola ) • Family
Didazoonidae • Genus
Didazoon •
D. haoae • Genus
Pomatrum (=
Xidazoon ) •
P. ventralis (=
X. stephanus ) • Genus
Yuyuanozoon •
Y. magnificissimi History of identification . Click through for detailed description. The current consensus view is that vetulicolians are
stem group chordates, although some researchers continue to raise other possibilities.
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.
Shenzianyuloma has been interpreted as a vetulicolian with both a notochord (a definitively deuterostome trait) and gut diverticula. However, the single fossil of this genus 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
vetulocystids 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. == References ==