Jaw attachments and gnathostome relationships
The palatoquadrate in gnathostomes may be attached to the neurocranium, the main portion of the skull which enclosed the brain and sensory organs, in various ways. In early sharks, such as Cobelodus from the Late Caboniferous of North America, there is a double attachment with links fore and all, the amphistylic condition, The amphistylic pattern has been modified in two main ways. In most modern fishes, the palatoquaddrate contacts the neurocranium at the front only, and the jaw joint is entirely braced by the hyomandibular. On opening the jaw, the palatoquadrate can slide forwords, which increases the gape. This is the hyostylic jaw suspension condition. The second modification has been to exclude the hyomandibular from support of the jaw, and to fuse the palatoquadrate firmly to the neurocranium, the autostylic condition. This is typical of certain fish groups, as well as the land vertebrates. Living gnathosome are grouped in the classes Chondrithyes and Osteichthyes (Bony fishes and Tetrapods), and two extinct groups are the Class Acanthodii of the Silurian to Permian, and the Class Placodermi of the Devonian. Acanthodians and osteichthyes share various features of the braincase and they are tentatively regarded as sister-groups in the cladogram The placement of placoderms has been more difficult. They are currently regarded by different authors as the sister group of chondrichthyans, of osteichthyans, or of both groups together, so the three groups are shown in an unresolved branching pattern