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Gnathostomata(Jawed Vertebrates):
The Gnathostomata, or gnathostomes, are the majority of the Middle Devonian (-380 million years ago) to Recent vertebrates. They differ from all other craniates or vertebrates in having a vertically biting device, the jaws, which consist of an endoskeletal mandibular arch and a variety of exoskeletal grasping, crushing, or shearing organs, i.e. the teeth, and jaw bones. Among Recent vertebrates, the gnathostomes include sharks, rays, chimaeras, ray-finned fishes, lobe-finned fishes and land vertebrates. Extant gnathostomes fall into two major clades, the Chondrichthyes and Osteichthyes. In addition, there are two extinct major gnathostome clades, the Placodermi (Early Silurian-Late Devonian) and the Acanthodii (Latest Ordovician or Earliest Silurian - Early Permian). There may be other fossil gnathostome taxa which fall outside of these four taxa. This could be the case for the Mongolepida, only known from isolated scales from the Early Silurian, and which are provisionally assigned to the chondrichthyans, yet with great reservations. The Chondrichthyes are characterized by a special type of hard tissue lining the cartilages of the endoskeleton: the prismatic calcified cartilage. Another chondrichthyan characteristic is the pelvic clasper, as special copulatory organ derived from the metapterygium, i.e. the posterior part of the pelvic fin. A pelvic clasper may, however, be present in the fossil Placodermi. Chondrichthyans include two major extant clades, the Elasmobranchii and the Holocephali, and a number of fossil clades (Cladoselachidae, Symmoriida, Xenacanthiformes, Iniopterygia, Eugeneodontida) which may fall outside these two clades. The Osteichthyes are characterized by endochondral ("spongy") bone in the endoskeleton, dermal fin rays made up by lepidotrichiae (modified, tile-shaped scales), and three pairs of tooth-bearing dermal bones lining the jaws (dentary, premaxillary and maxillary). The Osteichthyes include two major clades, the Actinopterygii and the Sarcopterygii. The Placodermi are characterized by a dermal armor consisting of a head armor and a thoracic armor. In the thoracic armor, the foremost dermal plates form a complete "ring" around the body and always include at least one median dorsal plate. The Acanthodii are characterized by dermal spines inserted in front of all fins but the caudal one. They also possess minute, growing scales which have a special onion-like structure, i.e. the crown consists of overlying layers of dentine or mesodentine.
Characteristics
Gnathostomes are characterized by: 1.A vertically biting device called jaws, and which is primitively made up by two endoskeletal elements, the palatoquadrate and Meckelian cartilage, and a number of dermal elements called teeth, sometimes attached to large dermal bones.
2.The skull of a gnathostome, or jawed vertebrates (here a shark), are characterized by vertically biting jaws (red) consisting of the palatoquadrate dorsally and the Meckelian cartilage ventrally. The gill arches (green) are situated internally to the gill filaments, and the nasal capsules (blue) open to the exterior by means of paired nostrils. (After Janvier 1996.)
3.Pelvic fins: These are the paired fins or limbs situated just in front of the anus.
4.Interventrals and basiventrals in the backbone. These are the elements of the backbone which lie under the notochord, and match the basidorsals and interdorsals respectively.
5.Gill arches which lie internally to the gills and branchial blood vessels, contrary to the gill arches of all jawless craniates, which are external to the gills and blood vessels. 6.A horizontal semicircular canal in the inner ear.
7.Paired nasal sacs which are independent from the hypophysial tube. In all extant and fossil jawless craniates, the nasal sacs, which contain the olfactory organs, open into a median duct, the nasohypophysial duct, which takes part to the formation of the pituitary gland and either leads postriorly to the pharynx (e.g. in hagfish and galeaspids) or ends as a blind pouch (e.g. in lampreys and osteostracans). In the gnathostomes, this pouch remains as a thin canal in the palate, the buccohypophysial canal, whereas the nasal sacs open separately to the exterior by external nostrils.
8.There are numerous other characteristics of the soft anatomy and physiology (e.g. myelinated nerve fibres, sperms passing through urinary ducts, etc.), which are unique to the gnathostomes among extant craniates, but cannot by observed in fossils. |