Studies of the skeletal system (osteology) and comparative anatomy have produced most of the information used in the classification of protacanthopterygian fishes. The Protacanthopterygii once contained a large number of primitive orders of fishes, including fishes now classified in, for example, the orders Salmoniformes, Esociformes, Aulopiformes, and Myctophiformes, no two of which are considered each others’ closest relatives. The skeleton and external anatomy continue to provide a wealth of characters for systematic ichthyologists; yet focus on the significance of certain characters, such as presence or absence of the adipose fin, seems not to have provided any breakthroughs in scientists’ understanding ...(100 of 5249 words)