Vetulicolia

Vetulicolia[note 1] is the name of a taxon (either phylum[1][2] or subphylum[3] in rank) 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 (or at least openings in the vicinity of the pharynx); 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.[4] The area where the anterior and posterior parts join is constricted.[5]

Vetulicolia
Temporal range: Cambrian Stage 3–Middle Cambrian
Scientific classification
Kingdom: Animalia
Superphylum: Deuterostomia
Class: †Vetulicolia
Orders

Their affinity has been uncertain; they have been considered to represent stem- and crown-group arthropods, stem-group vertebrates,[6] and early deuterostomes. The general scientific consensus before 2001 considered them early limbless arthropods but now considers them early deuterostomes.[7] 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.[3] Research from 2017 indicates Vetulicolia are rather related to Saccorhytus, another basal deuterostome group.[8]

Taxonomy and evolution
Restoration of a generalized didazoonid

As originally proposed, the phylum included the Didazoonidae (Didazoon, Pomatrum, and Xidazoon) and the Vetulicolidae (Vetulicola, and Banffia).[1] Other groups which may be related include the yunnanozoans.[1]

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.[6] 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.[2]

Dominguez and Jefferies have argued, based on morphological analysis, that Vetulicola (and by implication, other Vetulicolians) 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.[9][10] However, recent research have confirmed a position close to urochordates for Vetulicolians.[3]

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.[10] Some groups, like the genus Vetulicola, were more streamlined (complete with ventral keels) than other groups, such as the tadpole-like Didazoonidae.[10]

Because all vetulicolians had mouths which had no features for chewing or grasping, it is automatically assumed that they were not predators.[10] 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.[10]

Classification

Notes
  1. The taxon name, Vetulocolia, is derived from the type genus, Vetulicola, which is a compound Latin word composed of vetuli "old" and cola "inhabitant".
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References
  1. Shu, D. G.; Morris, S. C.; Han, J.; Chen, L.; Zhang, X. L.; Zhang, Z. F.; Liu, H. Q.; Li, Y.; Liu, J. N. (2001-11-22). "Primitive deuterostomes from the Chengjiang Lagerstätte (Lower Cambrian, China)". Nature. 414 (6862): 419–424. Bibcode:2001Natur.414..419S. doi:10.1038/35106514. ISSN 0028-0836. PMID 11719797.
  2. Briggs; et al. (2005). "A new metazoan from the Middle Cambrian of Utah and the nature of the Vetulicolia". Palaeontology. 48 (4): 681–686. doi:10.1111/j.1475-4983.2005.00489.x.
  3. García-Bellido, Diego C; Paterson, John R (2014). "A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group". BMC Evolutionary Biology. 14: 214. doi:10.1186/s12862-014-0214-z. PMC 4203957. PMID 25273382.
  4. Ailin, C.; Hongzhen, F.; Maoyan, Z.H.U.; Dongsheng, M.A.; Ming, L.I. (2003), "A New Vetulicolian from the Early Cambrian Chengjiang Fauna in Yunnan of China", Acta Geologica Sinica, 77 (3): 281–287, doi:10.1111/j.1755-6724.2003.tb00742.x
  5. McMenamin, Mark A. S. (11 August 2019). "Cambrian Chordates and Vetulicolians". Geosciences. 9 (8): 354. Bibcode:2019Geosc...9..354M. doi:10.3390/geosciences9080354.
  6. Shu, Degan (2003). "A paleontological perspective of vertebrate origin". Chinese Science Bulletin. 48 (8): 725–735. doi:10.1007/BF03187041.
  7. Dominguez, Patricio and Jeffries, Richard. (2003). Fossil evidence on the origin of appendicularians. Paper read at International Urochordate Meeting 2003. Abstract at [2] - URL retrieved June 22, 2006.
  8. Han, Jian; Morris, Simon Conway; Ou, Qiang; Shu, Degan; Huang, Hai (2017). "Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China)". Nature. 542 (7640): 228–231. Bibcode:2017Natur.542..228H. doi:10.1038/nature21072. PMID 28135722.
  9. Lacalli, TC (2002), "Vetulicolians - are they deuterostomes? chordates?", BioEssays, 24 (3): 208–11, doi:10.1002/bies.10064, PMID 11891757 Abstract on-line at Pub Ned - accessed August 17, 2009
  10. Aldridge, Richard J. et al. (2007) "The systematics and phylogenetic relationships of vetulicolians," Paleontology, Volume 50
  11. Vinther, J.; Smith, M. P.; Harper, D. A. T. (2011). "Vetulicolians from the Lower Cambrian Sirius Passet Lagerstätte, North Greenland, and the polarity of morphological characters in basal deuterostomes". Palaeontology. 54 (3): 711–719. doi:10.1111/j.1475-4983.2011.01034.x.
  12. Chen, Feng, Ma, Li (2003). "A New Vetulicolian from the Early Cambrian Chengjiang Fauna in Yunnan of China". Acta Geologica Sinica. 77 (3): 281–287. doi:10.1111/j.1755-6724.2003.tb00742.x.CS1 maint: multiple names: authors list (link)
  13. Caron, (2005-2006), Banffia constricta, a putative vetulicolid from the Middle Cambrian Burgess Shales
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