Gnathifera (from the

Greek Greek may refer to: 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 of all kno ...

'' gnáthos'', “jaw”, and the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

'' -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 Taxon, taxa. The term ''Spiralia'' is applied to those phyla that exhibit canonical spiral clea ...

ns characterized by complex jaws made of

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

. 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 and Micrognathozoa.

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, they are mostly pelagic; however about 20% of the known species ...

has recently been recognised as closely related to the group, with it either being included within Gnathifera or the broader group Chaetognathifera. 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

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 the extinct '' Amiskwia'' the anus is located on the ventral surface in a subterminal position. Both Gnathostomulida and Micrognathozoa are acoelomates, rotifers are pseudocoelomates, and chaetognathans have a true

coelom The coelom (or celom) is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, i ...

. Gnathostomulidans and chaetognathans are

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

s, and Micrognathozoa appears to be

parthenogenetic Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

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 Bdelloidea (from Greek language, Greek βδέλλα, ''bdella'' 'leech') is a Class (biology), class of rotifers found in freshwater habitats all over the world. There are over 450 described species of bdelloid rotifers (or 'bdelloids'), disti ...

. Extinct members of the

total group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

, such as ''

Timorebestia ''Timorebestia koprii'' is an extinct species of stem-group chaetognath (arrow worm relative) that lived about 520 million years ago, in the Cambrian. Its fossils are known from the Sirius Passet Lagerstätte in Greenland, and it was first descri ...

'' 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.

Development

All known gnathiferans are direct developers. Though gnathiferans are included in Spiralia, rotifers and chaetognaths do not exhibit spiral

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

. 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. An alternative phylogeny place Gnathifera into a main spiralian clade Platyzoa ''s.l.'' as sister clade to

Mesozoa The Mesozoa are minuscule, worm-like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. A 2017 study recovered Mesozoa ...

and

Platyhelminthes Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called f ...

. Before the cladistic era, most gnathiferans were regarded as aschelminths, a grouping now recognized as

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

. 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,

Acanthocephala Acanthocephala ( Greek , ' 'thorn' + , ' 'head') is a group 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 t ...

, 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 Sessility (zoology), sessile forms. Monogononts generally have a reduced Rotifer#Anatomy, corona, and e ...

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

Entoprocta Entoprocta (), or Kamptozoa , is a phylum (biology), phylum of mostly Sessility (zoology), sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whos ...

. 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.

Fossil record

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

dating to the late

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

, but rotifers are otherwise only known from the

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

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

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

specimens.

Protoconodont Protoconodonts are an extinct group of Cambrian animals known from fossilized phosphatic tooth-like structures. They were originally described as an informal group of early conodonts, though more recent studies consider them to be more closely re ...

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 ...

age of the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

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

List of Genera

*'' Amiskwia'' *'' Capinatator'' *''

'' *'' Inquicus'' *'' Protosagitta'' *'' Ankalodous'' *'' Eognathacatha'' *'' Paucijaculum'' - One of the only post-

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

gastrotrichs The gastrotrichs (phylum Gastrotricha), common name, commonly referred to as hairybellies or hairybacks, are a group of Microscopic scale, microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant ...

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

References

{{Taxonbar, from=Q1135987 Protostome unranked clades Taxa described in 1995