Bony fish split into ray-fins and our lobe-fin kin
Osteichthyes, from the Greek words for bone and fish, is the clade of vertebrates whose inner skeletons are built mainly of bone. It splits into the ray-finned fishes, which make up most fish alive today, and the lobe-finned fishes, whose descendants include every land vertebrate, so in the modern cladistic sense humans count as bony fish too.
The living fish in the group number about 28,000 species in 45 orders and more than 435 families, set against cartilaginous sharks and rays and the extinct placoderms and acanthodians. The oldest fossils, from the early Silurian about 436 million years ago, show teeth arranged halfway between shark rows and true bony-fish rows. Oddly, those pioneers had barely begun to ossify; what set them apart was a pair of gut pouches that later became swim bladders in one branch and lungs in the other. The common ancestor also underwent a whole-genome duplication.
Those pouches first let fish gulp air in stagnant water. Ray-fins turned them into adjustable buoyancy bags, while the elpistostegalians, lobe-fins leading toward tetrapods, refined them into the lungs of amphibians, reptiles, birds and mammals. Jointed bones inside lobe-fin paddles became limbs, and bony gill covers let fish pump water over their gills without swimming. Their scales sit beneath the skin, in cosmoid, ganoid and thin teleost forms, the last split into cycloid and ctenoid types.
Older schemes ranked Osteichthyes as a class defined by swim bladder, three pairs of gill arches under a bony cover and a bony skeleton, which left it paraphyletic because tetrapods were excluded. Since 2013 widely cited papers treat it as a clade including tetrapods, making it synonymous with Euteleostomi, as palaeontologists already did.
Lifestyles vary widely. Most are cold-blooded, but opah, swordfish and tuna evolved warmth independently. Diets include plants, filtered plankton, detritus and even blood. Some species are hermaphrodites or reproduce by parthenogenesis, and male seahorses brood eggs in a belly pouch.
Source: Osteichthyes
