The teleosts are an extremely diverse group of fishes, with well over 23,000 living species that are classified in 40 orders (Nelson, 1994). This enormous diversity is clearly impossible to survey in detail and only the main groups can be mentioned. The teleosts are characterized by modifications to the tail that give it a symmetrical (homocercal) appearance, but with the vertebral column not running into the upper lobe. Specialized elements, the uroneurals,run alongside the last caudal (ural) vertebrae and help support the upper lobe of the tail. In addition, teleosts have a mobile pre-maxilla, not seen in basal neopterygians, and some
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