gnathostomes

Gnathostomes are jawed vertebrates, a principal clade within the vertebrate lineage that includes most fish and all tetrapods. Their defining feature is the presence of jaws derived from pharyngeal arches, allowing efficient biting and processing of prey. They also typically possess paired pectoral and pelvic fins, a more complex skull, and a vertebral column with a mineralized endoskeleton in many lineages. The origin of jaws marks a major evolutionary transition that enabled a wide range of feeding strategies and ecological roles.

Within gnathostomes, two major living lineages are recognized: Chondrichthyes, the cartilaginous fishes (sharks, rays, and chimaeras), which retain a predominantly cartilaginous skeleton and tooth-like dentitions; and Osteichthyes, the bony fishes, which have a bone-based skeleton and include both ray-finned fishes (Actinopterygii) and lobe-finned fishes (Sarcopterygii). The lobe-finned group is of particular significance because it gave rise to the tetrapods, the vertebrates that colonized land. Many gnathostomes possess a lateral line system for detecting movement and vibrations in water, and some lineages have developed lungs or swim bladders for buoyancy control.

The fossil record shows gnathostomes first appearing in the Paleozoic, with early jawed fishes in the Silurian and Devonian, including transitional forms such as acanthodians. The group underwent substantial diversification during the Devonian, contributing to the broad array of aquatic and, in the case of tetrapods, terrestrial life seen today.