Bone microstructure of the diving Hesperornis and the volant Ichthyornis from the Niobrara Chalk of western Kansas

We report on the bone microstructure of the Cretaceous birds Hesperornis regalis and Ichthyornis 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 birds Patagopteryx and representatives of the enantiornithines. These findings suggest physiological advancement in Cretaceous ornithurine birds. The bone microstructure of the diving Hesperornis shows similarities to the bone structure of modem penguins, and to that of a loon from the Cretaceous of Antarctica. (C) 1998 Academic Press.