Notes on the hindlimb myology and syndesmology of the Mesozoic toothed bird Hesperornis regalis (Aves: Hesperornithiformes)

Abstract

Partial reconstruction of hindlimb myology and syndesmology of Hesperornis regalis, based on fossil and extant comparative anatomical material, is provided. Possessing a combination of morphological features characteristic of modern loons and grebes, H. regalis was somewhat closer to loons in overall hindlimb anatomy. The acetabular foramen indicates that the femoral head of Hesperornis had a more restricted excursion in the acetabulum compared with modern Aves. The development of femoral retractors m. iliofemoralis and m. caudofemoralis exceeded that of modern foot-propelled diving birds, showing greater protractive forces applied to the femur during the propulsive stroke. The origin of intermediate and lateral portions of m. gastrocnemius reached its maximum in Hesperornis, extending proximally beyond the greater trochanter and migrating onto the cranial surface of the femur. Ligaments of the knee joint, which show typical avian morphology, allowed a high degree of inward–outward rotation of the tibiotarsus, which was held close to the body and was probably enclosed in the thickly feathered skin of the body wall. Flexion–extension movements in the knee joint were highly reduced by the action of well-developed mm. flexores cruris, m. iliofibularis, the postacetabular portion of m. iliotibialis lateralis and m. femorotibialis medialis. The intertarsal joint of Hesperornis possessed a high degree of rotational freedom. In this way it resembled that of loons, showing a loon-like manner of tarsometatarsal movements. This was, however, combined with grebe-like movements of the toes, which undoubtedly had asymmetrical lobes. Hesperornis regalis shows its own independent way of mastering underwater locomotion, which led to the appearance of what became the most specialized avian foot-propelled diver ever known.