The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian. The two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively. The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed vertebrates (the Gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine. As in most vertebrates, fish jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. The jaw is derived from the most anterior two pharyngeal arches supporting the gills, and usually bears numerous teeth. It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increased respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and amphibians) that pumps water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans), in feeding, was selected for and became a very important function in vertebrates. Many teleost fish have substantially modified their jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved. Jawed vertebrates and jawed fish evolved from jawless fish, and the cladogram below for jawed vertebrates is a continuation of the cladogram in the section above. Spiny sharksЖ Spiny sharks (extinct) were the earliest known jawed fishes. They resembled sharks but were an independent branch. Further information: Acanthodii and List of acanthodians Spiny sharks, class Acanthodii, are extinct fishes which share features with both bony fishes and cartilaginous fishes. Despite being called "spiny sharks," acanthodians predate sharks. They evolved in the sea at the beginning of the Silurian Period, some 50 million years before the first sharks appeared. Eventually competition from bony fishes proved too much, and the sp
ny sharks died out in Permian times about 250 Ma. 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 cartilaginous fishes. PlacodermsЖ Placoderms (extinct) were armoured jawed fishes (compare with the ostracoderms above) Further information: Placodermi and List of placoderms Placoderms, class Placodermi (plate skinned), are extinct armoured prehistoric fish, which appeared about 430 Ma in the late Silurian. They were mostly wiped out at the end of the Devonian 360 Ma, though a few survived another 5 million years. Their head and thorax were covered with massive and often ornamented armoured plates. The rest of the body was scaled or naked, depending on the species. The armour shield was articulated, with the head armour hinged to the thoratic armour. This allowed placoderms to lift their heads, unlike ostracoderms. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. Bony fishes Further information: Osteichthyes, List of transitional fossils#Bony Fish, and List of prehistoric bony fish External videos Chordate Evolution and Bony Fish YouTube The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Both the bony fishes and cartilaginous fishes may have arisen from either the spiny sharks or the placoderms. A subclass of the Osteichthyes, the ray-finned fishes (Actinopterygii), have become the dominant group of fishes in the post-Paleozoic and modern world, with some 30,000 living species. However, another subclass of Osteichthyes, the Sarcopterygii, including lobe-finned fishes including coelacanths and lungfish) and tetrapods, was the most diverse group of bony fishes in the Devonian. Sarcopterygians are basally characterized by internal nostrils, lobe fins containing a robust internal skeleton, and cosmoid scales. The bony (and cartilaginous) fish groups that emerged after the Devonian, were characterised by steady improvements in foraging and locomotion.