The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most (but not all) members of the Lophotrochozoa.
Distribution of spiralian development across phylogeny
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.
Evidence of the relationship between molluscs, annelids and lophophorates was found in 1995. More recent research has established the Lophotrochozoa as a superphylum within the Metazoa.
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.
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.
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.
^ abHejnol, A. (4 August 2010). "A Twist in Time--The Evolution of Spiral Cleavage in the Light of Animal Phylogeny". Integrative and Comparative Biology. 50 (5): 695-706. doi:10.1093/icb/icq103. PMID21558233.
^Boyer, Barbara C.; Henry, Jonathan Q.; Martindale, Mark Q. (1 November 1996). "Dual Origins of Mesoderm in a Basal Spiralian: Cell Lineage Analyses in the Polyclad Turbellarian Hoploplana inquilina". Developmental Biology. 179 (2): 329-338. doi:10.1006/dbio.1996.0264. PMID8903349.
^Hankeln, Thomas; Wey-Fabrizius, Alexandra; Herlyn, Holger; Witek, Alexander; Weber, Mathias; Nesnidal, Maximilian; Struck, Torsten (2014). "Phylogeny of platyzoan taxa based on molecular data". In Wägele, J. Wolfgang; Bartolomaeus, Thomas (eds.). Deep Metazoan Phylogeny: The Backbone of the Tree of Life. Walter de Gruyter GmbH. pp. 105-125.
^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. PMID26212884.