Parasaurolophus was a large plant eating dinosaur that walked both as a biped and a quadruped. Three species are recognized: P. Walkeri, P. Tubicen, and the short-crested P. Cyrtocristatus. The most noticeable feature on Parasaurolophus is the crest which may have been used as visual recognition of both species and sex, thermoregulation, or to make sound. Skin impressions of P. Walkeri show uniform scales but no large bumps. Parasaurolophus is sometimes restored with a flap of skin from the crest to the neck. The crest was hollow, with passages leading from each nostril to the end of the crest before reversing direction and heading back down the crest and into the skull.