The adaptation that made bony fish what they are was an internal, ossified skeleton. The skeleton afforded the fish greater mobility to feed, which it largely did by sucking and filtering through feeding mechanisms not unlike the jawless lampreys of today. For a period, thick armor developed on many bony fish to protect them from environmental hazards before a muscular skeleton evolved to afford them the mobility to evade hazards.
Bony fish are the ancestors of all living vertebrates, having first made their appearance approximately 450 million to 500 million years ago. It took them another 15 million years, according to evolutionary biologists, to universally develop the first two major adaptations of this class: jaws and a hardened brain casing. Jaws evolved from bony gill arches and eventually led fish to become predators. With the greater behavioral diversity available through jaws, the role of the brain became more important and required a well-protected central nervous system -- the durable brain casing. The first great period of proliferation for fish was the Devonian Period, approximately 360 million to 408 million years ago.
About 200 million years ago, bony fish developed another adaptation, apparently in at least four different places: the swim bladder. A swim bladder is a pocket in the fish's abdomen where carbon dioxide concentrated from cell respiration is collected, which the fish can expel or retain to achieve neutral buoyancy at varying depths. This gave bony fish access to a greater variety of aquatic habitats and led to a burst in species diversity.
The next major adaptation for bony fish were hardened scales, as various species began to prey on one another, renewing the need for external protection without a loss of mobility. These small, overlapping structures are composed of materials similar to both fingernails and teeth. Keratin, the material in human hair and nails, forms the inner portion of scales; dentin, the hardened surface material on teeth, is on the outside of the scales. Some scales developed smoothly, some had built-in spines and others were accompanied by independently growing protective spines.