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Neopterygian node O (defining node of the CLUPEOCEPHALA): Endoskeletal gill arch elements fused with tooth plates, ural centrum one's neural arch is reduced or absent, articular and angular bone co-ossified.
There are over 350 extant species of clupeomorphs - all herring-like - and the 150 extinct forms known extend back to the Early Cretaceous. These are typically small marine flocking fish feeding on plankton (as the Clupea - or herring - above and the anchovy). A recognisable type of abdominal scute, a unique organisation of the tail vertebrae and a specialised air sac are characteristic traits of most clupeomorphs. The latter trait is the most unusual: clupeomorphs' air sac extends to the exoccipital region of the braincase and is functional in these fish's sense of hearing.
