In 1871, paleontologist Othniel C. Marsh was leading a group of Yale University students on a fossil-collecting expedition through the western United States. That year, in the Cretaceous chalks of the prairies and badlands of western Kansas, Marsh found bones of previously unknown birds: slender, several feet in length, with powerful legs but very small, stubby wings. On a later expedition, Marsh found the skull of one of these birds, and discovered that it had teeth -- a trait missing from all modern birds, but present in the fossil Archaeopteryx, described only a few years earlier, and then as now the oldest and most primitive bird known. Marsh named his discoveries Hesperornis -- "Western bird" -- and designated several species: Hesperornis regalis, H. crassipes, and H. gracilis (now designated Parahesperornis gracilis. Together with another of Marsh's discoveries in Kansas, the flying toothed bird Ichthyornis, Hesperornis filled a large gap in the fossil history of birds.
Today, several more genera are known that are similar to Hesperornis, including Baptornis (also described by Marsh), Parahesperornis, Enaliornis, and others. Most have been found in western North America, such as the mounted Hesperornis depicted above (on display at the Sternberg Museum of Natural History in Hays, Kansas). However, species are also known from Cretaceous deposits of Europe, Mongolia, and Kazakhstan. These species are grouped in the Hesperornithiformes.
The largest known hesperornithiform, described in 1999 and named Canadaga arctica, may have reached a maximum adult length of over five feet (1.5 meters). Canadaga is the northernmost hesperornithiform yet found, coming from what is now the Northwest Territories of Canada. It is also the latest, dating almost to the end of the Cretaceous.
Despite the retention of certain very primitive characters (such as teeth), the Hesperornithiformes was a highly specialized taxon of Cretaceous birds. Hesperornithiform birds all had highly reduced, probably non-functional wings. The legs were powerful, but were set so far back that walking on land was probably awkward -- hesperornithiform birds probably spent almost all their time in the water, except, presumably, the breeding and egg-laying season. One specimen of Parahesperornis has been preserved with imprints of thick, hairy feathers, useless for flight, but efficient insulators. Like modern-day cormorants, hesperornithiform birds appear to have led an aquatic lifestyle, probably diving to catch fish. Although at least one species has been described from ancient freshwater deposits, most hesperornithiforms were marine. In fact, hesperornithiform birds were the only true dinosaurs to colonize the oceans; the aquatic "reptiles" of the time, such as the ichthyosaurs and plesiosaurs, were not true dinosaurs.
More images and information about Hesperornis are available at the Oceans of Kansas Paleontology, and at the Sternberg Museum of Natural History at Fort Hays State University, Kansas. The Dinosauricon includes a cladogram and listing of known hesperornithiforms.