Gnathifera (from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

'' gnáthos'', “jaw”, and the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

'' -fera'', “bearing”) is a clade of generally small

spiralia 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 ...

ns characterized by complex jaws made of

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

. It comprises the phyla

Gnathostomulida Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. ...

Rotifera The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Ha ...

Micrognathozoa ''Limnognathia maerski'' is a microscopic freshwater animal, discovered living in warm springs on Disko Island, Greenland, in 1994. Since then, it was also found in Crozet Islands of Antarctica.). With an average length of 100 micrometers ...

, and

Chaetognatha The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and can ...

. It may also include the

Cycliophora ''Symbion'' is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-sta ...

. 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.

Description

The most distinctive characteristic of gnathiferans is the presence of complex sclerotized mouthparts made of chitin. In most gnathiferans, the anus opens on the dorsal surface of the animal. In micrognathozoans and gnathostomulids, the anus is transient and only forms during defecation. Unlike other gnathiferans, in chaetognaths and '' Amiskwia'' the anus is located on the ventral surface in a subterminal position.

Development

All known gnathiferans are direct developers. Though gnathiferans are included in Spiralia, rotifers and chaetognaths do not exhibit spiral cleavage. Little is known of the development of micrognathozoans. The development of gnathostomulids is poorly known, but they appear to exhibit spiral cleavage.

Classification

Gnathifera is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians. Before the cladistic era, most gnathiferans were regarded as

aschelminth The Aschelminthes (also known as Aeschelminthes, Nemathelminthes, Nematodes), closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have be ...

s, a grouping now recognized as

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...

. Chaetognaths exhibit numerous morphological similarities to rotifers, suggesting that they may be sister taxa. However, based on molecular data, micrognathozoans may be more closely related to rotifers than chaetognaths. Rotifera comprises four subclades:

Seisonida Seisonidae is a family of rotifers, found on the gills of ''Nebalia'', a marine crustacean. Peculiar among rotifers, males and females are both present and equal in size. Males and females are similar with paired gonads. It is considered to have ...

Acanthocephala Acanthocephala (Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to p ...

Bdelloidea Bdelloidea (Greek ''βδέλλα'', ''bdella'', "leech") is a class of rotifers found in freshwater habitats all over the world. There are over 450 described species of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainly o ...

, and

Monogononta Monogononta is a class of rotifers, found mostly in freshwater but also in soil and marine environments. They include both free-swimming and sessile forms. Monogononts generally have a reduced corona, and each individual has a single gonad, w ...

. 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. Numerous hypotheses of rotifer interrelationships exist. The enigmatic phylum

may belong to Gnathifera, but other studies suggest that it is more closely related to the

Entoprocta Entoprocta (), or Kamptozoa , is a phylum of mostly sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that d ...

Fossil record

The fossil record of gnathiferans is poor. There are no known fossil gnathostomulids. Fossils of the extant rotifer genus ''

Habrotrocha ''Habrotrocha'' is a genus of bdelloid rotifers. References Rotifer genera Bdelloidea {{rotifer-stub ...

'' are known from

Dominican amber Dominican amber is amber from the Dominican Republic derived from resin of the extinct tree '' Hymenaea protera''. Dominican amber differentiates itself from Baltic amber by being nearly always transparent, and it has a higher number of fossil inc ...

dating to the late

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

, but rotifers are otherwise only known from the

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

. By contrast, the chaetognath fossil record, while still patchy, includes numerous

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

specimens.

Protoconodont Protoconodonts are an extinct taxonomic group of conodonts or, possibly, Chaetognaths. Chaetognaths (also known as arrow worms) were thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, how ...

s are stem-group chaetognaths. The earliest protoconodonts date to the

Fortunian The Fortunian age marks the beginning of the Phanerozoic Eon, the Paleozoic Era, and the Cambrian Period. It is the first of the two stages of the Terreneuvian series. Its base is defined as the first appearance of the trace fossil ''Treptichnus p ...

age of the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

, and are among the oldest known bilaterians. The enigmatic Cambrian taxon '' Amiskwia'' is a gnathiferan and may also be a stem-group chaetognath. The Cambrian ectoparasite '' Inquicus'' appears to be a gnathiferan.

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. A similar grouping, Acanthognatha, was suggested in 1998 to unite

gastrotrichs The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, acoelomate animals, and are widely distributed and abundant in freshwater and marine environmen ...

with gnathostomulids and rotifers. However, gastrotrichs are more closely related to lophotrochozoans than gnathiferans.

References

{{Taxonbar, from=Q1135987 Platyzoa Protostome unranked clades