 The teleosts or Teleostei ( Greek : teleios , " complete " + osteon , " bone " ) are by far the largest infraclass in the class Actinopterygii , the ray @-@ finned fishes , and make up 96 percent of all fish . This diverse group arose in the Triassic period and members are arranged in about 40 orders and 448 families . Over 26 @,@ 000 species have been described . Teleosts range from giant oarfish , measuring 25 ft ( 7 @.@ 6 m ) or more , and ocean sunfish weighing over 2 @.@ 0 long tons ; 2 @.@ 2 short tons ( 2 t ) , to the minute male anglerfish Photocorynus spiniceps , just 0 @.@ 24 in ( 6 @.@ 2 mm ) long . As well as torpedo @-@ shaped fish built for speed , teleosts can be flattened vertically or horizontally , be elongated cylinders or take specialised shapes as in anglerfish and seahorses . Teleosts dominate the seas from pole to pole and inhabit the ocean depths , estuaries , rivers , lakes and even swamps . 
