SafekipediaGnathostomata
Adapted from Wikipedia · Discoverer experience
Gnathostomata are jawed vertebrates, which include almost all the vertebrates alive today. They make up about 99% of all living vertebrates. This group includes all kinds of fish, like bony fish and cartilaginous fish, as well as animals that live on land, such as amphibians, reptiles, birds, and mammals.
Most gnathostomes have special features like true teeth, a stomach, and paired body parts such as fins or limbs. They also have a special system that helps them fight off diseases. Scientists believe that gnathostomes evolved from ancestors that already had paired fins. The development of jaws in these animals likely came from changes in the first set of gill supports, which helped them move water over their gills to breathe.
Recent discoveries of very old fossils, such as a 419-million-year-old placoderm named Entelognathus, show that important features of both cartilaginous and bony fish were already present in early gnathostomes. Other fossils, like those of Guiyu oneiros and Psarolepis, suggest that spiny sharks and modern cartilaginous fish are closely related to bony fish. These fossils help scientists understand when and how jaws and teeth first appeared in these animals.
Classification
Gnathostomata is a group of animals that includes most fish and all land animals. It is divided into three main parts: Chondrichthyes, which are cartilaginous fish like sharks; Placodermi, an extinct type of armored fish; and Teleostomi, which includes bony fish, birds, mammals, reptiles, and amphibians. Gnathostomata is closely related to the Agnatha, which are jawless fish.
| Subgroups of jawed vertebrates | ||||
|---|---|---|---|---|
| Subgroup | Common name | Example | Comments | |
| † Placodermi (extinct) | Armoured fish | †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. 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 | 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 | †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. 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, comes from the lower Silurian (Aeronian) of China and it is also the oldest jawed vertebrate with known anatomical features. Coeval to Fanjingshania is the tooth-based acanthodian species Qianodus duplicis that represents the oldest unequivocal toothed vertebrate. | ||
| Osteichthyes | Bony fishes | 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 osteichthyans, which is an extremely diverse and abundant group consisting of 45 orders, with over 435 families and 28,000 species. 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. | ||
| Tetrapoda | Tetrapods | 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. 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
The earliest fish with jaws were the now extinct placoderms and the spiny sharks. Having jaws helped fish survive better than those without jaws. Most fish without jaws went extinct, but some like hagfishes and lampreys survived.
Scientists think jaws developed from parts that supported the gills in earlier fish. These parts changed over time to form the jaw, connecting it to the skull. The oldest clear evidence of fish with jaws comes from fossils found in Guizhou, China, dating back about 439 million years ago.
See also: Fish jaw and Evolution of fish
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