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Hesperornithes is an extinct and highly specialised clade of Cretaceous toothed birds. Hesperornithean birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera such as ''Hesperornis'', ''Parahesperornis'', ''Baptornis'', ''Enaliornis'', and ''Potamornis'', all strong-swimming predatory waterbirds. Many, if not all, species were completely flightless. The largest known hesperornithine, described in 1999 and named ''Canadaga arctica'', may have reached a maximum adult length of over 1.5 metres (5 ft). Hesperornithean birds were the only Mesozoic birds to colonize the oceans. They became extinct at the Cretaceous–Paleogene boundary, along with enantiornithine birds, all non-avian dinosaurs, and many other mostly reptilian life forms. ==Anatomy and ecology== Most of what is known about this group rests on analyses of single species, as few provide sufficiently diverse fossils for analysis. In most cases, what holds true for one hesperornithine also applies to the others, as these birds were quite stereotypical and extreme in their autapomorphies. Although some of the smaller species might have been able to fly, ''Hesperornis'' and ''Baptornis'' had only vestigial wings. As in the case of living foot-propelled diving birds, the femur and metatarsus were short, whereas the tibia was long. The legs were also set far back on the body, as in loons, grebes or penguins. Hesperornithids must have been powerful swimmers and divers but extremely ungainly on the land, and probably spent little time ashore except to nest. They were rather long-bodied, and measured about long. Some researchers think that on land they had to slide on their bellies and push with their legs; the hip and knee joints were shaped such that these birds could not move them dorsoventrally, and in a resting position the feet projected sideways from the body, which would have prevented them from walking upright.〔 Whereas ''Hesperornis'' probably had lobes of skin on its toes similar to the grebes of today, the toes of ''Baptornis'' could not rotate (necessary in lobed feet to reduce drag) and thus were more likely webbed, as in loons. The dense bones of these birds decreased their buoyancy, making diving easier. The beak was long, probably hooked at the tip, and bore a series of simple, sharp teeth which were set into a longitudinal groove. These probably helped to seize fish, as does the serrated beak of mergansers today; unlike the reptilian teeth of more primitive toothed birds, those of the hesperornithids were unique.〔Marsh, Othniel Charles (1880): ''Odontornithes, a Monograph on the Extinct Toothed Birds of North America''. Government Printing Office, Washington DC.〕 They also retained a dinosaur-like joint between the lower jaw bones. It is believed that this allowed them to rotate the back portion of the mandible independently of the front, thus allowing the lower teeth to disengage.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hesperornithes」の詳細全文を読む スポンサード リンク
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