Gnathostomata

Gnathostomata /ˌneɪθoʊstoʊˈmɑːtə/ are the jawed vertebrates. 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 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 neurons. 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.[1]

It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins.[2] In addition to this, some placoderms were shown to have a third pair of paired appendages, that in males had been modified to claspers and basal plates in females, a pattern not seen in any other vertebrate group.[3]

The Osteostraci are generally considered the sister taxon of Gnathostomata.[4][5][6]

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.

Newer research suggests that a branch of Placoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named Entelognathus had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait.[7] The fossil findings of primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived contemporaneously with Entelognathus and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes.[8] It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes.[9] Fossils findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making it impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws.[10][11]

Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians[12] or "shark-like fishes",[13] may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.

Jawed vertebrates
Temporal range: Middle Ordovicianpresent, 462–0 Ma
DunkleosteusSannoble
The placoderm Dunkleosteus,
an early jawed vertebrate
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Olfactores
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Subgroups

Classification

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, birds, mammals, reptiles, and amphibians. Some classification systems have used the term Amphirhina. It is a sister group of the jawless craniates Agnatha.

  Vertebrata  
  Gnathostomata  

  †Placodermi  Dunkleosteus intermedius

  Eugnathostomata  

  Acanthodians, incl. Chondrichthyes (cartilaginous fishes)Carcharodon carcharias drawing

  Osteichthyes  

  Actinopterygii  Atlantic sturgeon flipped

  Sarcopterygii  
  Tetrapoda  

  Amphibia  Deutschlands Amphibien und Reptilien (Salamandra salamdra)

  Amniota  
  Sauropsida  

  Sauropsida  Description des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii)

  Synapsida  

  Mammalia  Phylogenetic tree of marsupials derived from retroposon data (Paucituberculata)

Subgroups of jawed vertebrates
Subgroup Common name Example Comments
Placodermi
(extinct)
Armoured fish Coccosteus BW
Coccosteus
Placodermi (plate-skinned) is an extinct 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 among the first jawed fish; their jaws likely evolved from the first of their gill arches. A 380-million-year-old fossil of one species represents the oldest known example of live birth.[14] The first identifiable placoderms evolved in the late Silurian; they began a dramatic decline during the Late Devonian extinctions, and the class was entirely extinct by the end of the Devonian.
Chondrichthyes Cartilaginous fishes Carcharodon carcharias drawing
Great white shark
Chondrichthyes (cartilage-fish) or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi.
?Acanthodii
(extinct)
Spiny sharks Acanthodes BW
Acanthodes bronni
Acanthodii, or spiny sharks are a class of extinct fishes, sharing features with both bony and cartilaginous fishes, now understood to be a paraphyletic assemblage leading to modern Chondrichthyes.[9] In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They may have been an independent phylogenetic branch of fishes, which had evolved from little-specialized forms close to Recent Chondrichthyes. Acanthodians did, in fact, have a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. They are distinguished in two respects: they were the earliest known jawed vertebrates, and they had stout spines supporting their fins, fixed in place and non-movable (like a shark's dorsal fin). The acanthodians' jaws are presumed to have evolved from the first gill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage. The common name "spiny sharks" is really a misnomer for these early jawed fishes. The name was coined because they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail - hence, "spiny sharks".
Osteichthyes Bony fishes Blue runner
Blue runner
Osteichthyes or bony fishes are a taxonomic group of fish that have bone, as opposed to cartilaginous skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, with over 435 families and 28,000 species.[15] It is the largest class of vertebrates in existence today. Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are about 420 million years ago, which are also transitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.[16]
Tetrapoda Tetrapods Deutschlands Amphibien und Reptilien (Salamandra salamdra)
Fire salamander
Tetrapoda (four-feet) or tetrapods are the group of all four-limbed vertebrates, including living and extinct amphibians, reptiles, birds, and mammals. Amphibians today generally remain semi-aquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs, and many tetrapods have returned to partially aquatic or (in the case of cetaceans and sirenians) fully aquatic lives. The tetrapods evolved from the lobe-finned fishes about 395 million years ago in the Devonian.[17] The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.

References

  1. ^ Cooper MD, Alder MN (February 2006). "The evolution of adaptive immune systems". Cell. 124 (4): 815–22. doi:10.1016/j.cell.2006.02.001. PMID 16497590.
  2. ^ New study showing pelvic girdles arose before the origin of movable jaws
  3. ^ The first vertebrate sexual organs evolved as an extra pair of legs
  4. ^ Zaccone, Giacomo; Dabrowski, Konrad; Hedrick, Michael S. (5 August 2015). Phylogeny, Anatomy and Physiology of Ancient Fishes. CRC Press. p. 2. ISBN 978-1-4987-0756-5. Retrieved 14 September 2016.
  5. ^ Keating, Joseph N.; Sansom, Robert S.; Purnell, Mark A. (2012). "A new osteostracan fauna from the Devonian of the Welsh Borderlands and observations on the taxonomy and growth of Osteostraci" (PDF). Journal of Vertebrate Paleontology. 32 (5): 1002–1017. doi:10.1080/02724634.2012.693555. ISSN 0272-4634.
  6. ^ Sansom, R. S.; Randle, E.; Donoghue, P. C. J. (2014). "Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history". Proceedings of the Royal Society B: Biological Sciences. 282 (1800): 20142245. doi:10.1098/rspb.2014.2245. ISSN 0962-8452. PMC 4298210. PMID 25520359.
  7. ^ Scientists make jaw dropping discovery
  8. ^ Zhu, Min; Yu, Xiaobo; Choo, Brian; Qu, Qingming; Jia, Liantao; Zhao, Wenjin; Qiao, Tuo; Lu, Jing (2012). "Fossil Fishes from China Provide First Evidence of Dermal Pelvic Girdles in Osteichthyans". PLOS ONE. 7 (4): e35103. Bibcode:2012PLoSO...735103Z. doi:10.1371/journal.pone.0035103. PMC 3318012. PMID 22509388.
  9. ^ a b Min Zhu; et al. (10 October 2013). "A Silurian placoderm with osteichthyan-like marginal jaw bones". Nature. 502 (7470): 188–193. Bibcode:2013Natur.502..188Z. doi:10.1038/nature12617. PMID 24067611.
  10. ^ Choi, Charles Q. (17 October 2012). "Evolution's Bite: Ancient Armored Fish Was Toothy, Too". Live Science.
  11. ^ ScienceShot: Ancient Jaws Had Real Teeth
  12. ^ Hanke, Gavin F.; Mark V. H. Wilson (2004). "New teleostome fishes and acanthodian systematics." (PDF). Recent advances in the origin and early radiation of vertebrates. pp. 189–216. ... record of acanthodian fishes is limited to microremains from the latest Ordovician (JANVIER 1996)
  13. ^ Sansom, Ivan J.; Moya M. Smith; M. Paul Smith (February 15, 1996). "Scales of thelodont and shark-like fishes from the Ordovician of Colorado". Nature. 379 (6566): 628–630. Bibcode:1996Natur.379..628S. doi:10.1038/379628a0.
  14. ^ "Fossil reveals oldest live birth". BBC. May 28, 2008. Retrieved May 30, 2008.
  15. ^ Bony fishes SeaWorld. Retrieved 2 February 2013.
  16. ^ Jaws, Teeth of Earliest Bony Fish Discovered
  17. ^ Clack 2012

External links

Adelospondylus

Adelospondylus is an extinct lepospondyl amphibian.

Aggeraspis

Aggeraspis is an extinct genus of arthrodire placoderm fish, which lived during the Early Devonian period in Europe.

Chondrichthyes

Chondrichthyes (; from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') is a class that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).

Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

Cladarosymblema

Cladarosymblema is a genus of prehistoric lobe-finned fish which belonged to the family of Megalichthyidae.

Craniate

A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the much more numerous Gnathostomata (jawed vertebrates).The clade was conceived largely on the basis of the Hyperoartia (lampreys and kin) being more closely related to the Gnathostomata (jawed vertebrates) than the Myxini (hagfishes). This, combined with an apparent lack of vertebral elements within the Myxini, suggested that the Myxini were descended from a more ancient lineage than the vertebrates, and that the skull developed before the vertebral column. The clade was thus composed of the Myxini and the vertebrates, and any extinct chordates with skulls.

However recent studies using molecular phylogenetics has contradicted this view, with evidence that the Cyclostomata (Hyperoartia and Myxini) is monophyletic; this suggests that the Myxini are degenerate vertebrates, and therefore the vertebrates and craniates are cladistically equivalent, at least for the living representatives. The placement of the Myxini within the vertebrates has been further strengthened by recent anatomical analysis, with vestiges of a vertebral column being discovered in the Myxini.

Devonosteus

Devonosteus is an extinct genus of prehistoric sarcopterygian or lobe-finned fish.

Dicksonosteus

Dicksonosteus is an extinct genus of basal arthrodire, placoderm fish, which lived during the Early Devonian period of Spitsbergen, Norway.

Elegantaspis

Elegantaspis reticornis is an arthrodire placoderm fish, which lived during the Early Devonian period in Spitsbergen, Norway.

Eurycaraspis

Eurycaraspis incilis is an extinct petalichthid placoderm from the Middle Devonian of China. It is closely related to Quasipetalichthys.

Gnathostoma

Not to be confused with Gnathostomata (singular: Gnathostoma), a Vertebrate Superclass.

Gnathostoma is a genus of parasitic nematodes. The species Gnathostoma spinigerum and Gnathostoma hispidum can cause gnathostomiasis.

Gnathostomata (echinoid)

The Gnathostomata are a superorder of sea urchins, including the familiar sand dollars.

Gnathostomatans are irregular in shape, but unlike other irregular sea urchins, possess a feeding lantern. The mouth is located in the centre of the lower surface, as it is in most other sea urchins, but the anus is found to one side of the upper surface, rather than being central. The members of the group are adapted for burrowing in soft-bottomed marine environments.

Heintzosteus

Heintzosteus is an extinct genus of placoderm fish, which lived during the Late Devonian period in Spitsbergen, Norway.

Huginaspis

Huginaspis is an extinct genus of placoderm fish, which lived during the Middle Devonian period of Spitsbergen, Norway.

List of animal classes

The following is a list of the classes in each phylum of the kingdom Animalia. There are 107 classes of animals in 33 phyla in this list. However, different sources give different numbers of classes and phyla. For e.g, Protura, Diplura, and Collembola are often considered to be the three orders in the class Entognatha. This list should by no means be considered complete and authoritative and should be used carefully.

List of chordate orders

This page contains a list of all of the classes and orders that are located in the Phylum Chordata.

Litoptychius

Litoptychius is an extinct genus of prehistoric bony fish.

Muranjilepis

Muranjilepis is an extinct genus of prehistoric sarcopterygians or lobe-finned fish.

Subphylum

In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum.

The taxonomic rank of "subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy.

Teleostomi

Teleostomi is an obsolete clade of jawed vertebrates that supposedly 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. The teleostomes include all jawed vertebrates except the chondrichthyans and the extinct class Placodermi.

Recent studies indicate that Osteichthyes evolved from placoderms like Entelognathus, while acanthodians are more closely related to modern chondrichthyes. Teleostomi, therefore, is not a valid, natural clade, but a polyphyletic group of species.The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.

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