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A close relative of its fellow hadrosaur, or duck-billed dinosaur, Corythosaurus--with which it shared the same North American territory during the late Cretaceous period--Parasaurolophus was distinguished by the long, hollow crest of bone atop its head, which gave it a sleek, aerodynamic, sporty profile unmatched by most other dinosaurs of its era. (See 10 Facts About Parasaurolophus and a gallery of Parasaurolophus pictures.)
For a long time, the function of Parasaurolophus' ornate head crest was a mystery--which was only settled when some enterprising paleontologists created a hollow reproduction of this crest and blasted air through it, producing piercing blasts of sound. Clearly, Parasaurolophus used its crest to communicate with others of its species (the crests of the males were longer than those of the females, meaning males probably bellowed loudest when they were eager to mate). This backward-curving crest may also have had other, secondary functions, such as dissipating heat or (when brightly colored) signaling other members of the herd.
By the way, Parasaurolophus was an entirely different dinosaur from two other, similarly named hadrosaurs: Saurolophus, a comparably sized duckbill that roamed both North America and Asia, and Prosaurolophus, which had only a minimal head crest. However, it's possible to view the contemporary Asian Charonosaurus as an even bigger version of Parasaurolophus; this similar-looking but plus-sized hadrosaur was probably able to bellow for even greater distances.