gnathostome

Gnathostomes, or jawed vertebrates, form a large clade of animals defined by the presence of true jaws, evolved from the first pharyngeal arches, and typically accompanied by paired pectoral and pelvic fins. This feature distinguishes gnathostomes from jawless vertebrates (agnathans), such as lampreys and hagfishes. The group is highly diverse and includes most living vertebrates.

The origin of gnathostomes is placed in the early Paleozoic era, roughly 440–420 million years ago, among early jawed fishes such as placoderms and possibly other stem groups. From this lineage, two major living lineages developed: Chondrichthyes (cartilaginous fishes, including sharks, rays, and chimaeras) and Osteichthyes (bony fishes), the latter subdividing into ray-finned and lobe-finned fishes. The lobe-finned lineage gave rise to the tetrapods, which include amphibians, reptiles, birds, and mammals.

Anatomically, jaws allowed new feeding strategies and ecological roles, while paired fins improved maneuverability in aquatic environments. Skeletal tissues vary among gnathostomes: many have mineralized or partially mineralized skeletons, while others retain cartilaginous endoskeletons. Additional shared features often include a more complex gill-arch system and a more elaborate nervous system relative to jawless vertebrates.

In summary, gnathostomes encompass the vast majority of vertebrate diversity, including both cartilaginous and bony fishes and all tetrapods, and they are defined by the critical innovation of jaws and associated morphological traits that facilitated diversification.