It is now assumed that Gnathostomata evolved from ancestors that already possessed two pairs ofpaired fins.[7] Until recently these ancestors, known asantiarchs, were thought to have lacked pectoral or pelvic fins.[7] In addition to this, someplacoderms were shown to have a third pair of paired appendages, that had been modified toclaspers in males and pelvic basal plates in females — a pattern not seen in any other vertebrate group.[8] ThejawlessOsteostraci are generally considered the closestsister taxon of Gnathostomata.[3][9][10]
Jaw development in vertebrates is likely a product of bending the first pair ofgill arches. This development would help suck water into the mouth by the movement of the jaw, so that it would then pass over the gills viabuccal pumping forgas exchange. The repetitive use of the newly formed jaw bones would eventually lead to the ability to bite in some gnathostomes.[11]
Newer research suggests that a branch ofplacoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm namedEntelognathus had a bony oral skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in cartilaginous fish is a derived trait.[12] The fossil findings of primitive bony fishes such asGuiyu oneiros andPsarolepis, which lived contemporaneously withEntelognathus 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.[13] It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes.[12] 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.[14][15]
LateOrdovician-aged microfossils of what have been identified as scales of eitheracanthodians[16] or "spiny sharks",[17] may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by theearly Silurian, and become abundant by the start of theDevonian.
†Placodermi(plate-skinned) is anextinctclass of armouredprehistoric fish, known fromfossils, which lived from the lateSilurian to the end of theDevonian Period. Theirhead andthorax were covered by articulated armoured plates and the rest of the body wasscaled or naked, depending on the species. Placoderms were among the firstjawedfish; their jaws likely evolved from the first of theirgill arches. A 380-million-year-old fossil of one species represents the oldest known example of live birth.[18] The first identifiable placoderms evolved in the late Silurian; they began a dramatic decline during theLate Devonian extinctions, and the class was entirely extinct by the end of the Devonian.
Chondrichthyes(cartilage-fish) or cartilaginous fishes are jawed fish with paired fins, pairednares, scales, a heart with its chambers in series, and skeletons made ofcartilage rather thanbone. The class is divided into two subclasses:Elasmobranchii (sharks,rays andskates) andHolocephali (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 intoTeleostomi.
†Acanthodii, or spiny sharks are a class of extinct fishes, sharing features with bothbony andcartilaginous fishes, now understood to be a paraphyletic assemblage leading to modernChondrichthyes.[12] In form they resembled sharks, but theirepidermis was covered with tiny rhomboid platelets like the scales ofholosteans (gars,bowfins). They may have been an independent phylogenetic branch of fishes, which had evolved from little-specialized forms close to recentChondrichthyes. Acanthodians did, in fact, have a cartilaginousskeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with adentine spine. They are distinguished in two respects: they were the earliest known jawedvertebrates, and they had stoutspines supporting theirfins, fixed in place and non-movable (like ashark'sdorsal fin). The acanthodians' jaws are presumed to have evolved from the firstgill 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,[19] comes from the lowerSilurian (Aeronian) ofChina and it is also the oldest jawedvertebrate with known anatomical features.[19] Coeval toFanjingshania is the tooth-based acanthodian speciesQianodus duplicis[20] that represents the oldest unequivocal toothed vertebrate.
Osteichthyes(bone-fish) or bony fishes are ataxonomic group of fish that have bone, as opposed tocartilaginous skeletons. The vast majority of fish are osteichthyans, which is an extremely diverse and abundant group consisting of 45 orders, with over 435 families and 28,000 species.[21] 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 alsotransitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.[22]
Tetrapoda(four-feet) or tetrapods are the group of all four-limbedvertebrates, including living and extinctamphibians,reptiles,birds, andmammals. Amphibians today generally remain semi-aquatic, living the first stage of their lives as fish-liketadpoles. Several groups of tetrapods, such as the reptilliansnakes and mammaliancetaceans, have lost some or all of their limbs, and many tetrapods have returned to partially aquatic or (in the case of cetaceans andsirenians) fully aquatic lives. The tetrapods evolved from thelobe-finned fishes about 395 million years ago in theDevonian.[23] 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 amongpalaeontologists at present.
Spindle diagram for the evolution of fish and other vertebrate classes.[24] The earliest classes that developed jaws were the now extinctplacoderms and thespiny sharks.
The appearance of the early vertebrate jaw has been described as "a crucial innovation"[25] and "perhaps the most profound and radical evolutionary step in the vertebrate history".[26][27]Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period. However studies of thecyclostomes, the jawlesshagfishes andlampreys that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved.[28][29]
The ancestor of all jawed vertebrates have gone through two rounds of whole genome duplication. The first happened before the gnathostome and cyclostome split, and appears to have been an autopolyploidy event (happened within the same species). The second occurred after the split, and was an allopolyploidy event (the result of hybridization between two lineages).[30]
The customary view is that jaws arehomologous to thegill arches.[31] In jawless fishes a series ofgills opened behind the mouth, and these gills became supported bycartilaginous 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.[32] Thehyomandibula is a set of bones found in thehyoid region in most fishes. It usually plays a role in suspending the jaws or theoperculum in the case ofteleosts.[33]
While potentially olderOrdovician records are known, the oldest unambigious evidence of jawed vertebrates areQianodus andFanjingshania from the early Silurian (Aeronian) ofGuizhou, China around 439 million years ago, which are placed asacanthodian-gradestem-chondrichthyans.[34][35]
^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,ISBN978-1-83968-235-3,S2CID233943453
^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.S2CID4257631.
^For example: (1) both sets of bones are made fromneural crest cells (rather thanmesodermal 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)
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