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Vetulicolia

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Vetulicolia
Temporal range: about 520–501 Ma Cambrian Stage 3Drumian (Possible Ediacaran record)[1]
Restorations and fossils of vetulicolians
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade?: Vetulicolia
Shu et al. 2001
Type species
Vetulicola cuneata
Hou, 1987
Classes
  • Banffozoa
  • Vetulicolida

Vetulicolia is a group of bilaterian animals encompassing several extinct species from the Cambrian,[2] and possibly Ediacaran,[1] periods. While initially erected as a monophyletic clade with the rank of phylum in 2001[3], more recent research indicates that it is a basal evolutionary grade within the stem group of the Chordata.[4]

Etymolygy

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The taxon name, Vetulicolia, is derived from the type genus, Vetulicola, which is a compound Latin word composed of vetuli "old" and cola "inhabitant". It was named after Vetulicola cuneata, the first species of the group described in 1987.[5]

Description

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Schematic of V. rectangulata
pharynx and alimentary canal: dashed line; ventral food grooves: dotted line; gill slits: pink

The vetulicolian body plan comprises two parts: a voluminous rostral (anterior) forebody, tipped with an anteriorly positioned mouth and lined with a lateral row of five round to oval-shaped openings on each side, which have been interpreted as gills (or at least orifices in the vicinity of the pharynx); and a caudal (posterior) section that primitively comprises seven body segments and functions as a tail. All vetulicolians lack preserved appendages of any kind, having no legs, feelers or even eye spots.[6] The area where the anterior and posterior parts join is constricted.[7] Notochord-like structures have been found in some vetulicolian fossils.[8]

Ecology and lifestyle

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From their superficially tadpole-like forms, leaf or paddle-shaped tails, and various degrees of streamlining, it is assumed that all vetulicolians discovered to date were swimming animals that spent much, if not all, of their time living in water.[9] Some groups, like the genus Vetulicola, were more streamlined (complete with ventral keels) than other groups, such as the tadpole-like Didazoonidae.[9]

Because all vetulicolians had mouths which had no features for chewing or grasping, it is assumed that they were not predators.[9] 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.[9]

Taxonomy and evolution

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Banffozoans
Artist's reconstruction of Banffia constricta

The phylum Vetulicolia was erected in 2001 to group the genera Vetulicola, Didazoon, and Xidazoon (later deemed a junior synonym of Pomatrum).[10] 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.[11] Further work split Banffia into a separate class, called either Banffozoa[4] or Heteromorphida (for the new banffiid that had since been reassigned to a new genus, Heteromorphus),[12] or Banffozoa, depending on the source.[13] 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.[10]

A 2024 phylogenetic analysis by Mussini and colleagues found vetulicolians to be a paraphyletic group of stem-chordates, lying outside a clade formed by Yunnanozoon, Cathaymyrus, Pikaia and crown-chordates. In this arrangement, the genera assigned to class Banffozoa form the earliest part of the grade, with the Vetulicolida forming the remainder.[4] The family Vetulicolidae is labeled based on Li et al. (2018)[14], while the three widely accepted members of Didazoonidae[15] are in a polytomy with the clade of crownward chordates.

In the following slightly simplified clade, the two proposed classes are shown as the earlier (Banffozoa a.k.a. Heteromorphida) and later (Vetulicolida) parts of the vetulicolian grade. The relationship to Cambroernida is also shown due to its importance in establishing the Vetulicolia as early stem group chordates rather than near the tunicates:[4]

Deuterostomia
"Banffozoa"
"Vetulicolida"

Classification

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The following classification is taken from Li et al. (2018)[14] except where noted.

  • Phylum Vetulicolia Shu et al. 2001[7]
    • ? Genus Alienum Liu et al. 2024[1]
      • A. velamenus Liu et al. 2024
    • Genus Shenzianyuloma McMenamin 2019[7]
    • Class Heteromorphida Shu 2005 (= Banffozoa Caron 2006)
      • Order Banffiata Aldridge et al. 2007
    • Class Vetulicolida Chen and Zhou 1997
        • Genus Nesonektris García-Bellido et al. 2014[8][16]
          • N. aldridgei García-Bellido et al. 2014
      • Order Vetulicolata Hou and Bergström 1997
        • Family Vetulicolidae Hou and Bergström 1997
        • Family Didazoonidae Shu and Han in Shu et al., 2001
          • Genus Didazoon Shu and Han in Shu et al., 2001
            • D. haoae Shu and Han in Shu et al., 2001
          • Genus Pomatrum Luo and Hu, in Luo et al. 1999 (= Xidazoon Shu, Conway Morris and Zhang in Shu et al. 1999)
            • P. ventralis Luo and Hu, in Luo et al. 1999 (= X. stephanus Shu, Conway Morris and Zhang in Shu et al. 1999)
          • Genus Yuyuanozoon Chen, Feng and Zhu in Chen et al., 2003

History of identification

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The current consensus view is that vetulicolians are stem group chordates, although some researchers continue to raise other possibilities.[17] The possible identification of an endostyle bolstered theories of a tunicate affinity, but was later retracted, while the tentative identification of a notochord in Nesonektris has further supported overall chordate affinities.[2]

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 rathre 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.[18]

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 be within Protostomia instead.[19] 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.[18]

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.[20] 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.[21]

References

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  1. ^ a b c Liu et al. 2024
  2. ^ a b Gee 2018, pp. 188–189
  3. ^ Shu et al. 2001
  4. ^ a b c d Mussini et al. 2024
  5. ^ Briggs & Fortey 2005
  6. ^ Chen et al. 2003
  7. ^ a b c McMenamin 2019
  8. ^ a b García-Bellido et al. 2014
  9. ^ a b c d Aldridge et al. 2007
  10. ^ a b Hou et al. 2017, p. 272
  11. ^ Chen & Zhou 1997, pp. 95–96
  12. ^ Aldridge et al. 2007, p. 168
  13. ^ Mussini et al. 2024, p. 9, footnote 54 (Note: Banffozoa is credited as established in 2006, while Heteromorphida was published with a date of October 2005. However, as cited by some workers such as the referenced footnote, the Banffozoa paper is given a date of June 2005, making it unclear how to apply the principle of priority. Note also that the principle of priority does not formally apply to taxonomic ranks above family.)
  14. ^ a b Li et al. 2018, pp. 1083–1084
  15. ^ "†family Didazoonidae Shu and Han 2001". Paleobiology Database. Retrieved December 8, 2024.
  16. ^ "†Nesonektris García-Bellido et al. 2014". Paleobiology Database. Retrieved December 8, 2024.
  17. ^ Onai et al. 2023, p. 202
  18. ^ a b Mussini et al. 2024, p. 6
  19. ^ Caron 2006
  20. ^ Giribet & Edgecombe 2020, p. 105
  21. ^ Liu et al. 2022

Works cited

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