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Seldom is it possible to use direct evidence of the physiology of extinct marine organisms as a means of investigating the phylogenetic relationships at any taxonomic level. Hydrodynamic mechanisms of orthoconic cephalopods are an exception because the form and structure of aragonite deposits used as hydrodynamic devices reflect the genetically controlled physiology of the animal. Data on cameral and siphuncular deposits (hydrodynamic devices) of Pennsylvanian (Desmoinesian/Westphalian C) orthocones from the Boggy Formation (=Buckhorn asphalt) of southern Oklahoma provide a test of existing phylogenetic relationships established by standard paleontological methods. The analysis reveals that early growth stages of many taxa considered to be related at the family level have similar to identical morphologies of cameral deposits, while some do not. In all cases, the form of the cameral deposits changes among taxa during later growth stages. In one case, congeneric taxa are shown to belong to different genera on the basis of gross differences in deposits designed to function hydrodynamically.
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