Spiralia

Spiralia Temporal range: Cambrian–Recent [1] PreꞒ Ꞓ O S D C P T J K Pg N Possible Ediacaran fossil [2]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
Clade:	Protostomia
Clade:	Spiralia sensu Edgecombe et al. 2011
Clades
Gnathifera Platytrochozoa

The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. ^[3] The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most members of the Lophotrochozoa. ^[4]

Distribution of spiralian development across phylogeny

Members of the molluscs, annelids, platyhelminths and nemerteans have all been shown to exhibit spiral cleavage in its classical form. Other spiralian phyla (rotifers, brachiopods, phoronids, gastrotrichs, and bryozoans) are also said to display a derived form of spiral cleavage in at least a portion of their constituent species, although evidence for this is sparse. ^[5]

Lophotrochozoa within Spiralia

Previously, spiral cleavage was thought to be unique to the Spiralia in the strictest sense—animals such as molluscs and annelids which exhibit classical spiral cleavage. The presence of spiral cleavage in animals such as platyhelminths could be difficult to correlate with some phylogenies. ^[6]

Evidence of a close relationship between molluscs, annelids and lophophorates was found in 1995 and Lophotrochozoa was defined as the group containing these taxa and all the descendants of their last common ancestor. ^[7] More recent research has established the Lophotrochozoa as a superphylum within the Metazoa. ^[8] With this understanding, the presence of spiral cleavage in polyclad platyhelminths, as well as the more traditional Spiralia, has led to the hypothesis that spiral cleavage was present ancestrally across the Lophotrochozoa as a whole. ^[5] With the introduction of Platytrochozoa and Rouphozoa, the cladogram is as follows, with an indication approximately how many million years ago (Mya) the clades radiated into newer clades. ^{[9] [10] [11] [12] [13] [14] [15]}

Protostomia	Ecdysozoa Kimberella † Spiralia Gnathifera Platytrochozoa Mesozoa Rouphozoa Gastrotricha Platyhelminthes Lophotrochozoa Annelida Mollusca Kryptotrochozoa Lophophorata Brachiozoa Brachiopoda Phoronida Entoprocta Cycliophora Ectoprocta (Bryozoa) Nemertea 580 mya
610 mya

An alternative phylogeny was given in 2019, with a basal grouping Mollusca with Entoprocta grouping named Tetraneuralia, and a second grouping of Nemertea with Platyhelminthes named Parenchymia as sister of Annelida. In their proposal and according to the original definition, Lophotrochozoa may become a senior synonym for Platytrochozoa. ^{[1] [16] [17] [18]}

Protostomia	Ecdysozoa   Spiralia   (s.l.)  / Gnathifera   Lophotrochozoa  /    Tetraneuralia     Mollusca Entoprocta     Gastrotricha   Lophophorata       Ectoprocta Phoronida Brachiopoda Annelida   Parenchymia     Platyhelminthes Nemertea   Platytrochozoa  /  Spiralia   (s.s.)   Gnathospiralia

In 2019 the Rouphozoa was recovered again as a basal Platytrochozoa clade. ^[19]

A 2022 study supported the Trochozoa and Platyzoa hypotheses, as shown below. ^[20] The same year another study placed bryozoans, entoproctans and cycliophorans in the group Polyzoa as one of the earliest branches among Lophotrochozoa. ^[21]

Protostomia	Ecdysozoa Spiralia Trochozoa Mollusca Nemertea Annelida Brachiozoa Brachiopoda Phoronida Bryozoa Entoprocta Cycliophora Platyzoa Gastrotricha Platyzoa Gnathifera Platyhelminthes Mesozoa Dicyemida Orthonectida

References

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13. Laumer, Christopher E.; Bekkouche, Nicolas; Kerbl, Alexandra; Goetz, Freya; Neves, Ricardo C.; Sørensen, Martin V.; Kristensen, Reinhardt M.; Hejnol, Andreas; Dunn, Casey W. (2015). "Spiralian Phylogeny Informs the Evolution of Microscopic Lineages". Current Biology . 25 (15): 2000–2006. doi: 10.1016/j.cub.2015.06.068 . PMID   26212884.
14. Lu, Tsai-Ming; Kanda, Miyuki; Satoh, Noriyuki; Furuya, Hidetaka (2017-05-29). "The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution". Zoological Letters. 3: 6. doi: 10.1186/s40851-017-0068-5 . PMC   5447306 . PMID   28560048.
15. Luo, Yi-Jyun; Kanda, Miyuki; Koyanagi, Ryo; Hisata, Kanako; Akiyama, Tadashi; Sakamoto, Hirotaka; Sakamoto, Tatsuya; Satoh, Noriyuki (2017-12-04). "Nemertean and phoronid genomes reveal lophotrochozoan evolution and the origin of bilaterian heads". Nature Ecology and Evolution . 2 (1): 141–151. doi: 10.1038/s41559-017-0389-y . PMID   29203924.
16. Marlétaz, Ferdinand; Peijnenburg, Katja T. C. A.; Goto, Taichiro; Satoh, Noriyuki; Rokhsar, Daniel S. (2019-01-10). "A New Spiralian Phylogeny Places the Enigmatic Arrow Worms among Gnathiferans". Current Biology . 29 (2): 312–318.e3. doi: 10.1016/j.cub.2018.11.042 . ISSN   0960-9822. PMID   30639106.
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19. Laumer, Christopher E.; Fernández, Rosa; Lemer, Sarah; Combosch, David; Kocot, Kevin M.; Riesgo, Ana; Andrade, Sónia C. S.; Sterrer, Wolfgang; Sørensen, Martin V.; Giribet, Gonzalo (2019-07-10). "Revisiting metazoan phylogeny with genomic sampling of all phyla". Proceedings of the Royal Society B: Biological Sciences . 286 (1906): 20190831. doi:10.1098/rspb.2019.0831. ISSN   0962-8452. PMC   6650721 . PMID   31288696.
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21. Polyzoa is back: The effect of complete gene sets on the placement of Ectoprocta and Entoprocta - Science