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Introduction

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.

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