There are two classes in the superclass Osteichthyes. There are Class Actinopterygii, the ray-finned fishes and Class Sarcopterygii, the lobe-finned fishes.
Osteichthyans are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones, but this characteristic has been lost or modified in many modern species. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by fissure. Osteichthyans have a lung or swim bladder. They do not have fin spines, but instead support the fin with lepidotrichia (bone fin rays). They also have an operculum, which helps them breathe without having to swim. They also have a swim bladder which helps the body create a neutral balance between sinking and floating. They also are able to see in color, unlike most other fish.
For the ClassActinopterygii or known as ray-finned fishes are an enormous assemblage containing all familiar bony fishes that more than 23,600 species. Earliest actinopterygians, known as palaeoniscids, were small fishes, with large eyes, heterocercal tail, and thick, interlocking scales with an outer layer of ganoin. These fishes had a single dorsal fin and numerous bony rays derived from scales stacked end to end, distinctively different in appearance from the lobe-finned fishes with which they shared the Devonian waters. Palaeoniscids are represented by fossil fragments as early as the late Silurian, and flourished throughout the late Paleozoic era, during the same period that ostracoderms, placoderms, and acanthodians disappeared and sarcopterygians declined in abundance. This morphological specializations evolving in the actinopterygian lineage gave them ecological superiority over most other fishes.
From these earliest...