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 Hesperorn is regalis and Ichthyornis victor. Thin sections of representative ele ments of both these ornithurine birds show a rapid, sustained bone dep osition without any pauses or interruptions in bone formation. This gr owth pattern contrasts sharply with the cyclical pattern of bone depos ition previously reported for the Cretaceous non-ornithurine birds Pat agopteryx and representatives of the enantiornithines. These findings suggest physiological advancement in Cretaceous ornithurine birds. The bone microstructure of the diving Hesperornis shows similarities to t he bone structure of modem penguins, and to that of a loon from the Cr etaceous of Antarctica. (C) 1998 Academic Press.