Phylа ChоrdаtаChоrdates are distinguished by fоur embryonic characteristics - a dorsal hollow nerve cord, a notochord, pharyngeal pouches, and a post-anal tail. The notochord is replaced by a vertebral column in vertebrates. The three chordate subphyla are Urochordata (tunicates), Cephalochordata (lancelets), and Vertebrata which includes fishes, amphibians, reptiles, birds and mammals.
FishesFishes were the first vertebrаtes, prоviding the evоlutiоnаry bаse for the invasion of land by amphibians. They are the most diverse vertebrate group, comprising over half of all vertebrate species. The earliest fishes lacked jaws (agnathans like hagfishes and lampreys). The development of jaws occurred in the late Silurian period, evolving from the anterior gill arches made of cartilage.Jawless armored fishes (ostracoderms) and jawed spiny fishes were pioneer vertebrates. By the end of the Devonian period, these were replaced by the two major extant fish groups - cartilaginous fishes (Chondrichthyes like sharks, skates, rays) and bony fishes (Osteichthyes). Sharks have lightweight cartilaginous skeletons reinforced with calcium salts. Bony fishes evolved a heavier bone skeleton, along with key adaptations like the swim bladder to control buoyancy and the operculum covering the gills.The two main bony fish groups are ray-finned (Actinopterygii) and lobe-finned (Sarcopterygii). The fleshy, muscular lobed fins of sarcopterygians were likely predecessors to tetrapod limbs, making them the probable ancestors of amphibians.