All animals make sounds. Many sounds are made as an animal moves through its environment, such as the thud or patter of its feet on the ground, the scraping of its body against rocks, a brushing as it passes through vegetation, and a splashing at the surface of water, as well as sounds associated with respiration, of course. Dinosaurs probably made vocalizations too, because many had the same (or similar) kinds of noisemaking structures found in modern reptiles and birds.
Many dinosaurs are thought to have had larynx-like structures or some other transitional organ that allowed them to vocalize. A larynx is a valve that regulates airflow in vertebrates. While many scientists doubt that dinosaurs used it to make roaring sounds, they suggest that air passing through a vocal organ (or even the animal’s esophagus) could have been used to make growls, hisses, or honks. In addition, some dinosaurs, possibly from the theropod group, the group from which birds descended, may have possessed a syrinx, which sits at the base of the trachea. This structure allows songbirds to produce melodious notes, and, if some dinosaurs had them, they could have been used for vocalizations. However, evidence of a syrinx in dinosaurs stalls out between 66 million and 68 million years ago, which is fairly close to the time when dinosaurs died out, so scientists aren’t sure that dinosaurs had this structure.
Other dinosaurs possessed other sound-making structures. Some had the ability to trap air in inflatable cavities, and they may have made murmuring sounds similar to those produced by reptiles or birds with esophageal pouches. In addition, hadrosaurs such as Lambeosaurus and Parasaurolophus are noted for the peculiar crests and projections on the top of their heads. These structures were hollow expansions of the skull composed almost entirely of nasal bones, and scientists think that they may have been used to make honking noises.