The Tenontosaurus, a large plant-eating dinosaur, was to the predatory Deinonychus 110 million years ago what the wildebeest is to the lion: lunch. Paleontologists have been turning up bones of tenontosaurs for years in Montana and Oklahoma. But until now they had failed to find an intact skeleton with a well-preserved skull. Scientists at the University of Oklahoma reported yesterday that they had just excavated a nearly complete fossilized skeleton of a tenontosaurs almost 25 feet long.