HESPERORNIS

extinct genus of birds found as fossils in Cretaceous deposits (between 66.4 and 144 million years old). The best known of the birds of the Cretaceous Period, Hesperornis combined primitive and advanced characteristics. It was primitive in that teeth were present in the lower jaw; in the upper jaw, however, teeth were restricted to the back portions. It has been suggested that the horny beak that has come to be characteristic of birds was in the process of formation in the front part of the jaw. The wings were small and were useless for flight; the wing bones were mere splints. The chest was not characteristic of most birds; it lacked the prominent keel that serves as an anchor for powerful flying muscles. The legs were powerfully developed and clearly adapted for rapid diving and swimming through water. The neck was long and slender and the head smallish; both were probably capable of rapid side-to-side movements. Hesperornis was clearly an actively swimming bird that probably chased and caught fish.