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Vetulicolia

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Vetulicolia
Temporal range: about 520–501 Ma Cambrian Stage 3—Drumian (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

Vetulicolia is a group of bilaterian marine animals encompassing several extinct species from the Cambrian,[2] and possibly Ediacaran,[1] periods. As of 2023, the majority of workers favor placing Vetulicolians in the stem group of the Chordata,[3] but some continue to favor a more crownward placement as a sister group to the Tunicata.[2] It was initially erected as a monophyletic clade with the rank of phylum in 2001,[4] with subsequent work supporting its monophyly.[5] However, more recent research suggests that vetulicolians may be paraphyletic and form a basal evolutionary grade of stem chordates.[6]

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.[7]

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.[8] The area where the anterior and posterior parts join is constricted.[9] Notochord-like structures have been found in some vetulicolian fossils.[10]

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.[11] Some groups, like the genus Vetulicola, were more streamlined (complete with ventral keels) than other groups, such as the tadpole-like Didazoonidae.[11]

Because all vetulicolians had mouths which had no features for chewing or grasping, it is assumed that they were not predators.[11] 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.[11]

Taxonomy and evolution

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Banffozoans
Artist's reconstruction of Banffia constricta
Fossil of Skeemella clavula

The phylum Vetulicolia was erected in 2001 to group the genera Vetulicola, Didazoon, and Xidazoon (later deemed a junior synonym of Pomatrum).[12] 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.[13] Further work split Banffia into a separate class called Banffozoa, which was soon expanded to encompass similar species such as Heteromorphus.[14] 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.[12]

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.[6] This is in part due to the Cambroernida, which are basal stem-ambulacrarians, being discovered to share characteristics such as a terminal anus with vetulicolians, despite such characteristics previously being believed to be present in the last common ancestor of deuterostomes.[15]

In this arrangement, the genera assigned to class Banffozoa form the earliest part of the grade, with the Vetulicolida forming the remainder.[6] Within the Vetulicolida, the family Vetulicolidae as defined by Li et al. (2018)[16] is recovered as monophyletic, while the three widely accepted members of the Didazoonidae[17] are in a polytomy with the clade of crownward chordates.

Cladograms

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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.[18]

Next, on the right, the two proposed classes are shown as the earlier (Banffozoa) and later (Vetulicolida) parts of the vetulicolian grade.[19]

Simplified from García-Bellido et al. (2014)
Chordata
Ambulacraria
Simplified from Mussini et al. (2024)
Chordata
(total group)
Ambulacraria
(total group)
"Banffozoa"
"Vetulicolida"

Classification

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The following classification is taken from Li et al. (2018)[16] except where noted.

History of identification

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Evidence for the presence of a pharynx and dorsal/ventral feeding gutters in vetulicolians, supporting their placement among the Deuterostomes. Click through for detailed description.

The current consensus view is that vetulicolians are stem group chordates, although some researchers continue to raise other possibilities.[3] 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 and Vetulicola has further supported overall chordate affinities.[2][10] Other characters that have been used to support a tunicate affinity include the limiting of the notochord to the tail and the presence of a stiff cuticle (tunic).[10]

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.[15] 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.[10]

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.[22] Skeemella, in particular, has been noted as having striking arthropod-like characteristics.[23] 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.[15] 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.[24] The discovery of Shenzianyuloma brought to light a vetulicolian interpreted as having both a notochord (a definitively deuterostome trait) and gut diverticula, although this has not yet been further examined by other researchers.[25]

Protostomia
Deuterostomia
Adapted from cladograms in Aldridge et al. 2007, showing each possible placement for Vetulicolia considered in the paper;[26] note the presence of the Euchordata clade rather than the now-favored Olfactores.

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.[27] 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.[28]

References

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  1. ^ a b c Liu et al. 2024 (Note that while the paper leaves the classification of Alienum uncertain in its systematic paleontology, it also states on page 4 that "Alienum velamenus might belong to a clade of extinct Vetulicolia that formed during the early evolution of Vetulicola." It is tentatively included here on these grounds.)
  2. ^ a b c Gee 2018, pp. 188–189
  3. ^ a b Onai et al. 2023, p. 202
  4. ^ Shu et al. 2001
  5. ^ McMenamin 2019, p. 19
  6. ^ a b c Mussini et al. 2024
  7. ^ Lacalli 2002, p. 208
  8. ^ Chen et al. 2003
  9. ^ a b c McMenamin 2019
  10. ^ a b c d e García-Bellido et al. 2014
  11. ^ a b c d Aldridge et al. 2007
  12. ^ a b Hou et al. 2017, p. 272
  13. ^ Chen & Zhou 1997, pp. 95–96
  14. ^ Aldridge et al. 2007, pp. 151–152 (Note: See Banffozoa#Preference for Banffozoa over Heteromorphida regarding whether Banffozoa or Heteromorphida is the preferred name.)
  15. ^ a b c Mussini et al. 2024, p. 6
  16. ^ a b c Li et al. 2018, pp. 1083–1084 (Note: See Banffozoa#Preference for Banffozoa over Heteromorphida regarding the use of Heteromorphida in this classification.)
  17. ^ "†family Didazoonidae Shu and Han 2001". Paleobiology Database. Retrieved December 8, 2024.
  18. ^ García-Bellido et al. 2014, p. 9
  19. ^ Mussini et al. 2024, pp. 6–7
  20. ^ Shu 2005, p. 2349 (Note: Shu assigned "Form A" to class Heteromorphida, but not to Heteromorphus on account of its very small size, highly sclerotized shell, and details of the anterior section; no order or family was specified; see also Banffozoa#Preference for Banffozoa over Heteromorphida)
  21. ^ "†Nesonektris García-Bellido et al. 2014". Paleobiology Database. Retrieved December 8, 2024.
  22. ^ Caron 2005
  23. ^ Shu 2005, p. 2352
  24. ^ Aldridge et al. 2007, pp. 151–152
  25. ^ McMenamin 2019, pp. 15–16
  26. ^ Aldridge et al. 2007, pp. 156, 158
  27. ^ Giribet & Edgecombe 2020, p. 105
  28. ^ Liu et al. 2022

Works cited

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