Osteichthyes, the bony vertebrates, form a vast clade that includes all animals with a bony internal skeleton. Once regarded as a class of fish, modern cladistics recognizes Osteichthyes as a broader group encompassing not only ray-finned and lobe-finned fish, but also all land-dwelling vertebrates, including amphibians, reptiles, birds, and mammals. The earliest osteichthyan fossils date to the Late Silurian period, over 420 million years ago, marking the rise of mineralized skeletons and complex jaw structures. Today, Osteichthyes represent the overwhelming majority of vertebrate diversity, from tiny freshwater species to whales and humans. Their evolutionary adaptability has allowed them to colonize nearly every aquatic and terrestrial environment.