FUNCTIONAL MORPHOLOGY OF THE HINDLIMBS OF HESPERORNIS REGALIS: A COMPARISON WITH MODERN DIVING BIRDS

Hesperornis regalis was a flightless seabird from the Cretaceous interior seaway of North America. Since its description by O.C. Marsh in his 1880 monograph Odontornithes, Hesperornis has been compared to the common loon (Gavia immer) in both anatomy and presumed life habits. Though the analogy between the flightless Hesperornis and the volant Gavia is widely assumed, it has not previously been quantified or otherwise tested. Comparative measurements of the pelvis and hindlimbs of Hesperornis regalis and several extant waterbirds confirm that Hesperornis is structurally similar to modern loons and grebes in these areas. In loons and grebes, known foot-propelled divers, the tibiotarsus is elongate relative to the femur and the tarsometatarsus (up to 4 times as long in Gavia immer). In addition, the acetabulum is displaced anteroventrally on the pelvis relative to non-divers. Hesperornis also exhibits these characteristics, generally to an even greater extent. If these attributes are characteristic of foot-propelled divers, then the interpretation of Hesperornis as a loon-like diver is supported. Furthermore, comparative morphology suggests that Hesperornis was more structurally specialized for foot-propelled diving than either loons or grebes. Both loons and grebes are volant to varying degrees, while Hesperornis was certainly flightless, so this specialization is consistent with adaptation to an exclusively diving lifestyle. This high degree of specialization belies the primitive phylogenetic status usually assigned to this toothed Cretaceous bird.