Actinopterygians: The Ray-Finned Fishes—An Explosion of Diversity

Abstract

Living ray-finned fishes number approximately 30,000 species, roughly equal to modern lobe-finned fishes plus tetrapods combined. The fossil record of ray-finned fishes extends to the Early Devonian (ca. 415 Ma), although the oldest taxa for which the morphology of the otic region is known in any great detail are 35 million years younger. Early actinopterygians have consolidated saccular otoliths, open vestibular fontanelles, and a persistent otico-occipital fissure. These general osteichthyan conditions are joined by specializations like the lateral cranial canal, a diverticulum that extends through the loop of the posterior semicircular canal and is thought to be unrelated to hearing. This anatomy persists into early members of the dominant modern radiation of ray-finned fishes, Neopterygii. Neopterygian sub-clades, including teleosts, show parallel losses of persistent fissures, fragmentation of the endocranium into separate ossification centers, and reduced bony enclosure of the inner ear. Teleosts show remarkable specializations for hearing based on connections between the gas-filled swim bladder and the otic capsule. Cretaceous fossils of clupeomorphs (herrings) and otophysans (catfishes, minnows, characins, and Neotropical knifefishes) document the assembly of the specialized anatomical features associated with hearing in both groups.