Gnathostomata
Encyclopedia
Gnathostomata is the group of vertebrate
s with jaw
s. The term derives from Greek
γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes also have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin
sheathes of neuron
s. Another is an adaptive immune system
that uses V(D)J recombination
to create antigen recognition sites, rather than using genetic recombination in the Variable lymphocyte receptor gene.
The group is traditionally a superclass, broken into three top-level groupings: Chondrichthyes
, or the cartilaginous fish; Placodermi
, an extinct clade
of armored fish; and Teleostomi
, which includes the familiar classes of bony fish, bird
s, mammal
s, reptile
s, and amphibian
s. Some classification systems have used the term Amphirhina
. It is a sister group of the jawless craniates Agnatha
.
New fossil finds suggests thelodonts
as the closest relatives of the Gnathostomata.
It is believed that the jaws evolved from anterior gill
support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively — the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would with time become stronger and tougher, being transformed into real jaws.
Placoderms used sharp bony plates as teeth instead, and newer research indicates the jaws in placoderms evolved independently of those in the other Gnathostomata.
The Gnathostomata first appeared in the Ordovician period
and became common in the Devonian period
.
└─Infraphylum Gnathostomata
├─Class Placodermi
— extinct (armored gnathostomes)
└Microphylum Eugnathostomata
(true jawed vertebrates)
├─Class Chondrichthyes
(cartilaginous fish)
└─(unranked) Teleostomi
(Acanthodii & Osteichthyes)
├─Class Acanthodii
— extinct ("spiny sharks")
└Superclass Osteichthyes
(bony fish)
├─Class Actinopterygii
(ray-finned fish)
└─Class Sarcopterygii
(lobe-finned fish)
└Superclass Tetrapoda
├─Class Amphibia (amphibians)
└(unranked) Amniota (amniotic egg)
├─Class Sauropsida
(reptiles or sauropsids)
│ └─Class Aves
(birds)
└─Class Synapsida
└─Class Mammal
ia (mammals)
Note: lines show evolutionary relationships.
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s with jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
s. The term derives from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes also have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
sheathes of neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s. Another is an adaptive immune system
Adaptive immune system
The adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic growth. Thought to have arisen in the first jawed vertebrates, the adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older innate...
that uses V(D)J recombination
V(D)J recombination
VJ recombination, also known as somatic recombination, is a mechanism of genetic recombination in the early stages of immunoglobulin and T cell receptors production of the immune system...
to create antigen recognition sites, rather than using genetic recombination in the Variable lymphocyte receptor gene.
The group is traditionally a superclass, broken into three top-level groupings: Chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
, or the cartilaginous fish; Placodermi
Placodermi
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were...
, an extinct clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of armored fish; and Teleostomi
Teleostomi
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives...
, which includes the familiar classes of bony fish, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, and amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s. Some classification systems have used the term Amphirhina
Amphirhina
Amphirhina are animals, a phylogenetic classification within the subphylum vertebrata. They are more commonly known as the Branch Gnathostomata, and are described as having double nasal chambers, or nostrils, and jaws. The parallel branch in this naming system is Monorhina , which possess a single...
. It is a sister group of the jawless craniates Agnatha
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
.
New fossil finds suggests thelodonts
Thelodonti
Thelodonts are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armor....
as the closest relatives of the Gnathostomata.
It is believed that the jaws evolved from anterior gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively — the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would with time become stronger and tougher, being transformed into real jaws.
Placoderms used sharp bony plates as teeth instead, and newer research indicates the jaws in placoderms evolved independently of those in the other Gnathostomata.
The Gnathostomata first appeared in the Ordovician period
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and became common in the Devonian period
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
.
Taxonomy and phylogeny
Subphylum Vertebrata└─Infraphylum Gnathostomata
├─Class Placodermi
Placodermi
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were...
— extinct (armored gnathostomes)
└Microphylum Eugnathostomata
Eugnathostomata
-Taxonomy and phylogeny: Subphylum Vertebrata └─Infraphylum Gnathostomata ├─Class Placodermi — extinct └Microphylum Eugnathostomata...
(true jawed vertebrates)
├─Class Chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
(cartilaginous fish)
└─(unranked) Teleostomi
Teleostomi
Teleostomi is a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives...
(Acanthodii & Osteichthyes)
├─Class Acanthodii
Acanthodii
Acanthodii is a class of extinct fishes, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans...
— extinct ("spiny sharks")
└Superclass Osteichthyes
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
(bony fish)
├─Class Actinopterygii
Actinopterygii
The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...
(ray-finned fish)
└─Class Sarcopterygii
Sarcopterygii
The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...
(lobe-finned fish)
└Superclass Tetrapoda
├─Class Amphibia (amphibians)
└(unranked) Amniota (amniotic egg)
├─Class Sauropsida
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
(reptiles or sauropsids)
│ └─Class Aves
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
(birds)
└─Class Synapsida
└─Class Mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ia (mammals)
Note: lines show evolutionary relationships.