We report on the bone microstructure of the Cretaceous birdsHesperornis regalisandIchthyornis victor. Thin sections of representative elements of both these ornithurine birds show a rapid, sustained bone deposition without any pauses or interruptions in bone formation. This growth pattern contrasts sharply with the cyclical pattern of bone deposition previously reported for the Cretaceous non-ornithurine birdsPatagopteryxand representatives of the enantiornithines. These findings suggest physiological advancement in Cretaceous ornithurine birds. The bone microstructure of the divingHesperornisshows similarities to the bone structure of modern penguins, and to that of a loon from the Cretaceous of Antarctica.
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Chinsamy et al. (1998) studied this question.