Gnathostomata

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Jawed vertebrates
Temporal range:
Early SilurianPresent, 435–0 Ma[1] (Possible Late Ordovician record, 444 Ma)[1]
Gnathostomata.jpg
Example of jawed vertebrates: Dunkleosteus (Placodermi), Spotted wobbegong (Chondrichthyes), Silver arowana (Osteichthyes) and a Nile crocodile (Tetrapoda).
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Olfactores
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Subgroups

Gnathostomata (/ˌnæθˈstɒmətə/; from Greek: γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living gnathostomes have true teeth (a characteristic which has subsequently been lost in some), paired appendages (pectoral and pelvic fins, arms, legs, wings, etc.),[2] the elastomeric protein of elastin,[3] and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons, and an adaptive immune system that has the discrete lymphoid organs of spleen and thymus,[4] and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.[5]

It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins.[6] Until recently these ancestors, known as antiarchs, were thought to have lacked pectoral or pelvic fins.[6] In addition to this, some placoderms (extinct fish with bony plates) were shown to have a third pair of paired appendages, that had been modified to claspers in males and basal plates in females—a pattern not seen in any other vertebrate group.[7]

The Osteostraci (bony armored jawless fish) are generally considered the sister taxon of Gnathostomata.[2][8][9]

Jaw development in vertebrates is likely a product of the supporting gill arches. This development would help push water into the mouth by the movement of the jaw, so that it would pass over the gills for gas exchange. The repetitive use of the newly formed jaw bones would eventually lead to the ability to bite in some gnathostomes.[10]

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.[11] 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.[12] It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes.[11] Fossil findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making them 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.[13][14]

Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians[15] or "shark-like fishes",[16] 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.

Classification[edit]

Gnathostomata is traditionally a infraphylum, broken into three top-level groupings: Chondrichthyes, or the cartilaginous fish; Placodermi, an extinct grade 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.jpg

  Eugnathostomata  

  Acanthodians, incl. Chondrichthyes (cartilaginous fishes)White shark (Duane Raver).png

  Euteleostomi  

  Actinopterygii  Atlantic sturgeon flipped.jpg

  Sarcopterygii  
  Tetrapoda  

  Amphibia  Salamandra salamandra (white background).jpg

  Amniota  
  Sauropsida  

  Sauria  Deinosuchus riograndensis.png

  Synapsida  

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

Subgroups of jawed vertebrates
Subgroup Common name Example Comments
Placodermi
(extinct)
Armoured fish Coccosteus BW.jpg 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.[17] 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 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.jpg 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.[11] 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". The earliest recorded acanthodian, Fanjingshania renovata,[18] comes from the lower Silurian (Aeronian) of China and it is also the oldest jawed vertebrate with known anatomical features.[18] Coeval to Fanjingshania is the tooth-based acanthodian species Qianodus duplicis[19] that represents the oldest unequivocal toothed vertebrate.
Osteichthyes Bony fishes Blue runner.jpg Osteichthyes (bone-fish) 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.[20] 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.[21]
Tetrapoda Tetrapods Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg 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 reptillian snakes and mammalian 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.[22] 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.

Evolution[edit]

Vertebrate classes
Spindle diagram for the evolution of fish and other vertebrate classes.[23] The earliest classes that developed jaws were the now extinct placoderms and the spiny sharks.

The appearance of the early vertebrate jaw has been described as "a crucial innovation"[24] and "perhaps the most profound and radical evolutionary step in the vertebrate history".[25][26] Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period. However studies of the cyclostomes, the jawless hagfishes and lampreys that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved.[27][28]

The customary view is that jaws are homologous to the gill arches.[29] In jawless fishes a series of gills opened behind the mouth, and these gills became supported by cartilaginous elements. The first set of these elements surrounded the mouth to form the jaw. The upper portion of the second embryonic arch supporting the gill became the hyomandibular bone of jawed fish, which supports the skull and therefore links the jaw to the cranium.[30] The hyomandibula is a set of bones found in the hyoid region in most fishes. It usually plays a role in suspending the jaws or the operculum in the case of teleosts.[31]

References[edit]

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  2. ^ a b 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.
  3. ^ Rodriguez-Pascual, Fernando (27 October 2021), "The Evolutionary Origin of Elastin: Is Fibrillin the Lost Ancestor?", in Sashank Madhurapantula, Rama; Orgel P.R.O., Joseph; Loewy, Zvi (eds.), Extracellular Matrix - Developments and Therapeutics, Biochemistry, vol. 23, IntechOpen, doi:10.5772/intechopen.95411, ISBN 978-1-83968-235-3, S2CID 233943453
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  6. ^ a b Zhu, Min (4 January 2012). "An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates". Biology Letters. 8 (3): 453–456. doi:10.1098/rsbl.2011.1033. PMC 3367742. PMID 22219394 – via Research Gate.
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  9. ^ 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): 2014–2245. doi:10.1098/rspb.2014.2245. ISSN 0962-8452. PMC 4298210. PMID 25520359.
  10. ^ Gridi-Papp, Marcos (2018). "Comparative Oral+ENT Biology" (2018). Pacific Open Texts. 4. Pacific Open Texts.
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  12. ^ 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.
  13. ^ Choi, Charles Q. (17 October 2012). "Evolution's Bite: Ancient Armored Fish Was Toothy, Too". Live Science.
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  15. ^ Hanke, Gavin; Wilson, Mark (January 2004). "New teleostome fishes and acanthodian systematics". Journal of Vertebrate Paleontology: 187–214 – via Research Gate.
  16. ^ Sansom, Ivan J.; Smith, Moya M.; Smith, M. Paul (15 February 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. S2CID 4257631.
  17. ^ "Fossil reveals oldest live birth". BBC. 28 May 2008. Retrieved 30 May 2008.
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  19. ^ Andreev, Plamen S.; Sansom, Ivan J.; Li, Qiang; Zhao, Wenjin; Wang, Jianhua; Wang, Chun-Chieh; Peng, Lijian; Jia, Liantao; Qiao, Tuo; Zhu, Min (September 2022). "The oldest gnathostome teeth". Nature. 609 (7929): 964–968. doi:10.1038/s41586-022-05166-2. ISSN 1476-4687.
  20. ^ Bony fishes Archived 6 June 2013 at the Wayback Machine SeaWorld. Retrieved 2 February 2013.
  21. ^ Jaws, Teeth of Earliest Bony Fish Discovered
  22. ^ Clack 2012
  23. ^ Benton 2005.
  24. ^ Kimmel, C. B.; Miller, C. T.; Keynes, R. J. (2001). "Neural crest patterning and the evolution of the jaw". Journal of Anatomy. 199 (1&2): 105–119. doi:10.1017/S0021878201008068. PMC 1594948. PMID 11523812.
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  29. ^ For example: (1) both sets of bones are made from neural crest cells (rather than mesodermal tissue like most other bones); (2) both structures form the upper and lower bars that bend forward and are hinged in the middle; and (3) the musculature of the jaw seem homologous to the gill arches of jawless fishes. (Gilbert 2000)
  30. ^ Gilbert (2000). Evolutionary Embryology.
  31. ^ Clack, J. A. (1994). "Earliest known tetrapod braincase and the evolution of the stapes and fenestra ovalis". Nature. 369 (6479): 392–394. Bibcode:1994Natur.369..392C. doi:10.1038/369392a0. S2CID 33913758.

External links[edit]