ICHTHYORNIS

a genus of about six species of extinct birds found as fossils in the Upper Cretaceous deposits (about 66,400,000 to 97,500,000 years old) in the Great Plains area of the United States (Wyoming, Kansas, and Texas). The sole genus of the order Ichthyornithiformes, these flying birds, with strongly developed wings, were one of the notable discoveries of the American paleontologist Othniel Charles Marsh. The two best known species were about the size of the domestic pigeon. Indications are that Ichthyornis (meaning fish bird), like its modern relatives, lacked teeth. Because it was formerly believed to have teeth, Ichthyornis was grouped with another extinct sea bird, Hesperornis. The brain of Ichthyornis showed greater development than that of Hesperornis, but it still was smaller than that of modern birds. Another primitive feature was the vertebrae, which were like those in primitive reptiles. The breastbone (sternum) is large, with a strong keel; and the wing bones are long and well developed, with the shoulder girdle similar to that of living birds of strong flight. The legs were strong, with short shanks (tarsus), long anterior toes, and a small, slightly elevated hind toe. The tail had a well-developed terminal plate of bone (pygostyle). The indication is that these were birds of active flight, probably with webbed feet and resembling in general form modern gulls and terns. The resemblance is considered superficial, as the species of Ichthyornis differ as indicated by the form of the vertebrae. They are, however, recognized as members of the subclass Neognathae, the modern birds. The two groups of birds from Cretaceous time that have had most widespread attention because of their strange form are the divers, such as Hesperornis, and the strong-winged Ichthyornis, also probably of aquatic habit. Because of representatives of modern families found among the fossil species from the Paleocene and Eocene of early Tertiary time, it has seemed evident that groups other than Hesperornis and Ichthyornis must have existed in the preceding Cretaceous. Knowledge of these, based on fragments of fossil bone, has slowly come to light until there is fairly definite record from Cretaceous rock strata of other ancestral kinds of birds related to the living groups of loons, grebes, flamingos, cranes, and shorebirds, indicative of early diversity in the avian class.