(Figs. 1,2, Late Cretaceous, Campanian, Marsh 1872, 1.8m long) was a toothed, flightless marine bird with vestigial wings and asymmetrical feet. Although not related to living loons, Hesperornis is often compared to loons, which have no teeth and retain the ability to fly. Both swim with powerful hind limbs. Hesperornis can also be compared to another flightless bird clade, the penguins, with the proviso that penguins swim with powerful forelimbs and their skeletons (Fig. 1) reflect this.
“In terms of limb length, shape of the hip bones, and position of the hip socket, Hesperornis is particularly similar to the common loon (Gavia immer), probably exhibiting a very similar manner of locomotion on land and in water. Like loons, Hesperornis were probably excellent foot-propelled divers, but ungainly on land. Like loons, the legs were probably encased inside the body wall up to the ankle, causing the feet to jut out to the sides near the tail. This would have prevented them from bringing the legs underneath the body to stand, or under the center of gravity to walk (Reynaud 2006). Instead, they likely moved on land by pushing themselves along on their bellies, like modern [loons].”
It was not difficult
to animate a bipedal Hesperornis (Fig. 2). It appears fully capable of doing so penguin-style. But the comparison to loons is indeed compelling.
Loons are ungainly
on the beach. See a YouTube video here. Yes, it does look wounded, unable to walk like a normal bird. It would probably fly if it was in a hurry. Hesperornis shares many traits by convergence with loons, but, if anything, loon hind limbs are more extreme in their proportions, including a proportionately larger projecting patella (Figs. 3, 4).
Just added after publication: The axis of the acetabulum is further foreword in Hesperornis, at the 51% mark on the torso (measured from the posterior pelvis) versus the 43% mark on the loon. That big butt makes Hesperornis less top heavy.
The loon femur is a little shorter and the patella is a little larger
(Figs. 3, 4) than on Hesperornis (Figs. 1,2). It’s up to our imaginations whether or not that would enable a more penguin-like locomotion in Hesperornis. Note that penguins do have a patella (knee bone) but it does not extend above the femur as it does in Hesperornis and loons.
According to Marsh:
“The clavicles are separate, but meet on the median line, as in some very young existing birds.The coracoids are short, and much expanded where they join the sternum. The latter has no distinct manubrium, and is entirely without a keel. The wings were represented by the humerus only, which is long and slender, and without any trace of articulation at its distal end.”
believe the humerus would have been hidden beneath the skin and appressed to the ribs. As is typical for Kansas fossils, Hesperornis specimens are typically crushed flat. In the large reptile tree Hesperornis nests with its volant contemporary, Ichthyornis.
Marsh OC 1872. Discovery of a remarkable fossil bird. American Journal of Science, Series 3, 3(13): 56-57.
Marsh OC 1872. Preliminary description of Hesperornis regalis, with notices of four other new species of Cretaceous birds. American Journal of Science 3(17):360-365.
Marsh, OC 1880. Odontornithes, a Monograph on the Extinct Toothed Birds of North America. Government Printing Office, Washington DC.
Reynaud F 2006. Hind limb and pelvis proportions of Hesperornis regalis: A comparison with extant diving birds. Journal of Vertebrate Paleontology 26 (3): 115A. doi:10.1080/02724634.2006.10010069.
Wilson L. and Chin K 2014. Comparative osteohistology of Hesperornis with reference to pygoscelid penguins: the effects of climate and behaviour on avian bone microstructure. Royal Society Open Science. 1: 140245. doi: 10.1098/rsos.140245