Hesperornithidae
Hesperornithiformes are an extinct and highly specialized order of Cretaceous toothed birds. They are also the only known marine dinosaurs of the Mesozoic. Hesperornithiforms include Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis.
Although some of the smaller species might have been able to fly, Hesperornis and Baptornis had vestigial wings. Like living foot-propelled diving birds, the femur and metatarsus are short whereas the tibia was long. The legs were also sprawling and set far back on the body, as in loons and grebes. Hesperornithiformes must have been powerful swimmers but ungainly on the land, and probably spent little time ashore except to nest. The joints of the toes resemble those of grebes, suggesting that the foot bore a series of lobes rather than a single web. The dense bones of these birds decreased their bouyancy, making diving easier. The beak was long and bore a series of simple, sharp teeth, and probably functioned to sieze fish, as in the serrated beak of mergansers.
The earliest known Hesperornithiform is the Early Cretaceous Enaliornis; the majority are known from the Late Cretaceous. Hesperornis has been found in Upper Cretaceous marine chalks from Kansas and in marine shales from Canada. Small hesperornithiform bones are known from the freshwater deposits of the Late Cretaceous of the Judith River Group as well as the Hell Creek and Lance Formations. These birds were about the size of a cormorant or a loon. Hesperornithiform birds were the only Mesozoic dinosaurs to colonize the oceans; (the aquatic "reptiles" of the time, such as the ichthyosaurs and plesiosaurs, were not dinosaurs).
The taxon was originally named Odontornithes by Othniel Charles Marsh in 1880.
The largest known hesperornithiform, described in 1999 and named Canadaga arctica, may have reached a maximum adult length of over 1.5 meters (five feet).
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