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.
In the most recent research, the three phyla Cycliophora, Entoprocta and Bryozoa makes up a single clade and are the first to branch off from the other lophotrochozoans. The second split is the molluscs, and the third consists of two sister phyla, annelids and nemerteans. Lastly remains the clade that consist of the phoronids and the brachiopods.[16][17]
A number of fossil taxa can be identified as early Lophotrochozoans, even if their precise affinity remains contested. However, relevant Cambrian fossils are debated.
Characteristics
The clade Lophotrochozoa is named after the two distinct characteristics of its members; the
lophophore, a feeding structure consisting of a ciliated crown of tentacles surrounding a mouth, and the developmental stage of the
trochophore larva.
Lophophorata such as
Brachiozoa and
Bryozoa have lophophores, while members of
Trochozoa such as
molluscs and
annelids have trochophore larvae, although some may have none.[19][20]
^Halanych, K. M.; Bacheller, J.; Liva, S.; Aguinaldo, A. A.; Hillis, D. M.; Lake, J. A. (17 March 1995). "18S rDNA evidence that the Lophophorates are Protostome Animals". Science. 267 (5204): 1641–1643.
doi:
10.1126/science.7886451.
PMID7886451.
S2CID12196991.