Lophotrochozoa (/ləˌfɒtroʊkoʊˈzoʊə/, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence.[2][3] The clade includes animals like annelids, molluscs, bryozoans, brachiopods, and platyhelminthes.[4]
Lophotrochozoa was defined in 1995 as the "last common ancestor of the three traditional lophophorate taxa (brachiopods, bryozoans, and phoronid worms), the mollusks and the annelids, and all of the descendants of that common ancestor".[5] It is a cladistic definition (a node-based name), so the affiliation to Lophotrochozoa of spiralian groups not mentioned directly in the definition depends on the topology of the spiralian tree of life, and in some phylogenetic hypotheses, Lophotrochozoa may even be synonymous to Spiralia. Nemertea and Orthonectida (if not directly considered as part of Annelida) are probably lophotrochozoan phyla; Dicyemida, Gastrotricha, and Platyhelminthes may be lophotrochozoans or placed in the Rouphozoa clade outside Lophotrochozoa; Chaetognatha, Gnathostomulida, Micrognathozoa, and Syndermata are probably gnathiferans and so placed as a basal spiralian clade outside Lophotrochozoa; Cycliophora could be a gnathiferan or a lophotrochozoan phylum. One of the candidate hypotheses is presented below.
The Lophotrochozoa has basal Cycliophora and Mollusca groups, and more derived Lophophorate, Nemertea and Annelida groups.[6][7]
With the introduction of Platytrochozoa and Rouphozoa, one candidate phylogeny is pictured below – though other studies recover a range of alternative possibilities:[8][9][10][11][12][13][14][15]
A number of fossil taxa can be identified as early Lophotrochozoans, even if their precise affinity remains contested. However, relevant Cambrian fossils are debated.
The clade Lophotrochozoa is named after the two distinct characteristics of its members; the feeding structure lophophore, which is a ciliated crown of tentacles surrounding a mouth, and the developmental stage trochophore larvae. Lophophorata such as Brachiozoa and Bryozoa have lophophores, while members of Trochozoa such as molluscs and annelids have trochophores, although some may have none.[16][17]