: any of a genus (Hadrosaurus) or family (Hadrosauridae) of mainly bipedal dinosaurs of the Late Cretaceous that have the forward part of the jaws toothless and flattened into a beak-like snout and the back part of the jaws filled with rows of numerous small, grinding teeth and that often have a solid or hollow bony crest on the skull : duck-billed dinosaur… the hadrosaurs evolved into some 21 different species during their 20 million-year stint on earth …— Virginia Morell