Gnathifera (Spiralia)

Gnathifera (from the Greek gnáthos, “jaw”, and the Latin -fera, “bearing”) is a clade of generally small spiralians characterized by complex jaws made of chitin. It comprises the phyla Gnathostomulida, Rotifera and Micrognathozoa.^[2] Chaetognatha has recently been recognised as closely related to the group, with it either being included within Gnathifera^[2] or the broader group Chaetognathifera.^[3]

Gnathifera Temporal range: Fortunian – Recent PreꞒ Ꞓ O S D C P T J K Pg N
Pseudosagitta maxima (Chaetognatha)
Brachionus quadridentatus (Rotifera)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Clade:	Bilateria
Clade:	Nephrozoa
Clade:	Protostomia
Clade:	Spiralia
Clade:	Gnathifera Ahlrichs, 1995
Phyla
Gnathostomulida Micrognathozoa Rotifera Pararotatoria Seisonidae Acanthocephala(?) †Juracanthocephalus Acanthocephala(cladistically included) Eurotatoria †Inquicus †Fimbulispina[1] †Dakorhachis?[1] Chaetognatha s.l †Timorebestia Cucullophora †Amiskwia Chaetognatha
Synonyms
Chaetognathifera (as chaetognaths are nested within this clade)

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]

Description

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 extinct Amiskwia the anus is located on the ventral surface in a subterminal position.^[10][11]

Both Gnathostomulida and Micrognathozoa are acoelomates, rotifers are pseudocoelomates, and chaetognathans have a true coelom. Gnathostomulidans and chaetognathans are hermaphrodites, and Micrognathozoa appears to be parthenogenetic 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 class Bdelloidea.^{[12][13][14][15][16][17][18]}

Extinct members of the total group of Chaetognatha, such as Timorebestia and Amiskwia 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 callled 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]

Development

All known gnathiferans are direct developers.^[20] Though gnathiferans are included in Spiralia, rotifers and chaetognaths do not exhibit spiral cleavage.^[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]

Classification

Spiralia Gnathifera Gnathostomulida Micrognathozoa Chaetognatha Rotifera Seisonida Acanthocephala Bdelloidea Monogononta Platytrochozoa

Summary of relationships of gnathiferans in recent studies, with disputed relationships represented as polytomies[2][7][21][22][23]

Gnathifera is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians.^[2][22] An alternative phylogeny place Gnathifera into a main spiralian clade Platyzoa s.l. as sister clade to Mesozoa and Platyhelminthes.^[24] Before the cladistic era, most gnathiferans were regarded as aschelminths, a grouping now recognized as polyphyletic.

Chaetognaths exhibit numerous morphological similarities to rotifers, suggesting that they may be sister taxa.^[7][21] However, based on molecular data, micrognathozoans may be more closely related to rotifers than chaetognaths.^[2]

Rotifera comprises four subclades: Seisonida, Acanthocephala, Bdelloidea, and Monogononta. 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.^[22] Numerous hypotheses of rotifer interrelationships exist.^[22][23]

The enigmatic phylum Cycliophora may belong to Gnathifera, but other studies suggest that it is more closely related to the Entoprocta.^[25]

In 2022, the clade Cucullophora was proposed to include Amiskwia 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.^[26]

Fossil record

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.^[27] Fossils of the extant rotifer genus Habrotrocha are known from Dominican amber dating to the late Eocene, but rotifers are otherwise only known from the Holocene.^[28][29] By contrast, the chaetognath fossil record, while still patchy, includes numerous Paleozoic specimens.^[30] Protoconodonts are stem-group chaetognaths.^[31] The earliest protoconodonts date to the Fortunian age of the Cambrian, and are among the oldest known bilaterians.^[32] The enigmatic Cambrian taxon Amiskwia is a gnathiferan and has recently been suggested to be a stem-group chaetognath.^[7][10][19]The Cambrian ectoparasite Inquicus appears to be a gnathiferan.^[7]

List of Genera

• Amiskwia
• Capinatator
• Timorebestia
• Inquicus
• Protosagitta
• Ankalodous
• Eognathacatha
• Paucijaculum - One of the only post-Cambrian fossil chaetognaths
• Dakorhachis?
• Habrotrocha
• Titerina
• Eoserratosagitta
• Fimbulispina
• Juracanthocephalus

History

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 unite gastrotrichs with gnathostomulids and rotifers.^[33] However, gastrotrichs are more closely related to lophotrochozoans than gnathiferans.^[22][2]