Chaetognathifera(as chaetognaths are nested within this clade)
Gnathifera (from theGreekgnáthos, "jaw", and theLatin-fera, "bearing") is a clade of generally smallspiralians characterized by complex jaws made ofchitin. It comprises the phylaGnathostomulida,Rotifera andMicrognathozoa.[2]Chaetognatha has recently been recognised as closely related to the group, with it either being included within Gnathifera[2] or the broader groupChaetognathifera.[3]
Gnathiferans include some of the most abundant phyla. Rotifers are among the most diverse and abundant freshwater animals and chaetognaths are among the most abundant marine plankton.[4][5]
Hypothesis of the possible homology between the jaw parts and nervous system parts of extant and extinct Chaetognathifera taxa. Putative homologies between jaw parts and nervous system respectively have the same color. Questionable, and/or, disputable homologies are in grey. Non oberved part of the nervous system are in dotted lines. Only the anterior part of each organism is represented for the jaws. Modified from Bekkouche and Gąsiorowski 2022[6]
In most gnathiferans, the anus opens on the dorsal surface of the animal.[7][8][9] In micrognathozoans and gnathostomulids, the anus is transient and only forms during defecation.[8][9] Unlike other gnathiferans, in chaetognaths and the extinctAmiskwia the anus is located on the ventral surface in a subterminal position.[10][11]
Both Gnathostomulida and Micrognathozoa areacoelomates, rotifers arepseudocoelomates, and chaetognathans have a truecoelom. Gnathostomulidans and chaetognathans arehermaphrodites, and Micrognathozoa appears to beparthenogenetic as no males have ever been observed, but it has been suggested undiscovered dwarf males could be responsible for fertilization. In rotifers both males and females occurs, except from the classBdelloidea.[12][13][14][15][16][17][18]
Extinct members of thetotal group ofChaetognatha, such asTimorebestia andAmiskwia known from the Cambrian are larger than living chateognaths, and retain jaw-like mouthparts which are not found in living chaetognaths as well as lacking the spines present around the mouth of crown-group chaetognaths. They share with chaetognaths the fins running along the sides of the body and at the end of the tail, and the possession of a unique structure called the "ventral ganglion" not found among other animals which controls the movement and senses. These giant stem-chaetognaths are thought to have been free-swimming (nektonic) and occupied a high trophic position than modern chaetognaths.[19]
All known gnathiferans aredirect developers,[20] except for some rotifers with intermediate stages, which appears to have evolved secondarily.[21] Though gnathiferans are included in Spiralia, rotifers and chaetognaths do not exhibit spiralcleavage.[20][2] Little is known of the development of micrognathozoans.[20] The development of gnathostomulids is poorly known, but they appear to exhibit spiral cleavage.[20][2]
Summary of relationships of gnathiferans in recent studies, with disputed relationships represented as polytomies[2][7][22][23][24]
Gnathifera is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians.[2][23] An alternative phylogeny place Gnathifera into a main spiralian clade Platyzoas.l. as sister clade toMesozoa andPlatyhelminthes.[25] Before the cladistic era, most gnathiferans were regarded asaschelminths, a grouping now recognized aspolyphyletic.
Chaetognaths exhibit numerous morphological similarities to rotifers, suggesting that they may be sister taxa.[7][22] However, based on molecular data, micrognathozoans may be more closely related to rotifers than chaetognaths.[2]
Rotifera comprises four subclades:Seisonida,Acanthocephala,Bdelloidea, andMonogononta. Acanthocephalans were traditionally excluded from Rotifera, but it is now known that rotifers are paraphyletic without including acanthocephalans. Some taxonomists call the clade of rotifers including acanthocephalans Syndermata, but others continue to use Rotifera and regard acanthocephalans as rotifers.[23] Numerous hypotheses of rotifer interrelationships exist.[23][24]
The enigmatic phylumCycliophora may belong to Gnathifera, but other studies suggest that it is more closely related to theEntoprocta.[26]
In 2022, the clade Cucullophora was proposed to includeAmiskwia and crown-group chaetognaths, citing the shared presence of a head hood and body fins, the same study proposed the clade Gynognathifera to include Rotifera and Micrognathozoa.[27]
Amiskwia sagittiformis a basal gnathiferan from Cambrian of Canada, which is much larger than modern gnathiferans
The fossil record of gnathiferans is poor. There are no known fossil gnathostomulids.[28] Fossils of the extant rotifer genusHabrotrocha are known fromDominican amber dating to the lateEocene, but rotifers are otherwise only known from theHolocene.[29][30] By contrast, the chaetognath fossil record, while still patchy, includes numerousPaleozoic specimens.[31]Protoconodonts are stem-group chaetognaths.[32] The earliest protoconodonts date to theFortunian age of theCambrian, and are among the oldest known bilaterians.[33] The enigmatic Cambrian taxonAmiskwia is a gnathiferan and has recently been suggested to be a stem-group chaetognath.[7][10][19]The Cambrian ectoparasiteInquicus appears to be a gnathiferan.[7]
Gnathifera was named in 1995 to unite gnathostomulids and rotifers. Micrognathozoans were soon added to this grouping. Chaetognaths, long considered a distinct lineage with no close relatives, were identified as gnathiferans in 2019.[2]
A similar grouping, Acanthognatha, was suggested in 1998 to unitegastrotrichs with gnathostomulids and rotifers.[34] However, gastrotrichs are more closely related to lophotrochozoans than gnathiferans.[23][2]
^Yin, Xu-Wang; Tan, Bing-Bing; Zhou, Yan-Chun; Li, Xiao-Chun; Liu, Wei (2016). "Development time of male and female rotifers with sexual size dimorphism".Hydrobiologia.767 (1):27–35.doi:10.1007/s10750-015-2472-1.
^abcdHejnol, Andreas (2015). "Gnathifera". In Wanninger, A. (ed.).Evolutionary Developmental Biology of Invertebrates 2: Lophotrochozoa (Spiralia). Springer. pp. 1–12.doi:10.1007/978-3-7091-1871-9_1.ISBN978-3-7091-1870-2.
^abSielaff, Malte; Schmidt, Hanno; Struck, Torsten H.; Rosenkranz, David; Mark Welch, David B.; Hankeln, Thomas; Herlyn, Holger (2016). "Phylogeny of Syndermata (syn. Rotifera): Mitochondrial gene order verifies epizoic Seisonidea as sister to endoparasitic Acanthocephala within monophyletic Hemirotifera".Molecular Phylogenetics and Evolution.96:79–92.Bibcode:2016MolPE..96...79S.doi:10.1016/j.ympev.2015.11.017.PMID26702959.
^Piper, Ross (2013).Animal Earth: The Amazing Diversity of Living Creatures. Thames & Hudson.
^Poinar, G. O.; Ricci, C. (1992). "Bdelloid rotifers in Dominican amber: evidence for parthenogenetic continuity".Experientia.48 (4):408–410.doi:10.1007/bf01923444.S2CID13098228.
^Waggoner, B. M.; Poinar, G. O. (1993). "Fossil habrotrochid rotifers in Dominican amber".Experientia.49 (4):354–357.doi:10.1007/bf01923421.S2CID28087284.
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