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A Parasaurolophus at the Natural History Museum of Utah. Photo by the author.

Of all the crested hadrosaurs, Parasaurolophus is one of my favorites. The long, slightly-curved tube that projects from the back of the dinosaur’s head is a wonderful ornament. But why did this peculiar dinosaur decoration evolve?

Parasaurolophus was initially described by paleontologist William Parks  in 1922 on the basis of a skeleton found in the vicinity of Alberta’s Red Deer River. This dinosaur was clearly different from other ornamented hadrosaurs–such as Corythosaurus and Saurolophus–that had been found before, and especially perplexing was the makeup of the dinosaur’s crest. The structure was not solid–a break in this part of the skull revealed a series of internal tubes separated by thin walls of bone.

No one was exactly sure why Parasaurolophus had a hollow crest, but the supposed hadrosaur lifestyle generated a number of speculative answers. Hadrosaurs were supposed to be amphibious dinosaurs who acted like giant, dabbling ducks. After all, their broadened snouts gave them the popular moniker “duckbill dinosaurs.” Paleontologists therefore considered the dinosaur’s crest in reference to a life spent foraging for soft plants in Cretaceous swamps.

Paleontologist James Hopson reviewed these ideas in a 1975 Paleobiology paper about the role hadrosaur crests may have played in display. In 1933 Alfred Sherwood Romer speculated that the crest might have been used as a snorkel or an air storage chamber. While there was no hole in the crest to allow air to come in–the snorkel idea was scuttled–the air tank hypothesis was popular. As a young dinosaur fan, I remember encountering an image of a submerged Parasaurolophus in Edwin Colbert’s The Dinosaur Book with a solid black line running through the crest to indicate the amount of stored air. Another book, Rudolph Zallinger’s Dinosaurs and Other Prehistoric Reptiles, featured an even more detailed vision of Corythosaurus and Parasaurolophus paddling around beneath the surface of a prehistoric lake. But this notion didn’t last either. The anatomy of hadrosaurs has undeniably cast them as terrestrial animals, not expert swimmers, and the amount of air these dinosaurs were able to store in their crests would have been miniscule compared to their lung volume–the supposed air tanks would not have done them much good.

Charles Mortram Sternberg, son of the celebrated dinosaur collector Charles H. Sternberg, proposed a different variation of the aquatic feeding theme. In 1935 Sternberg wrote a paper on the “hooded” hadrosaurs from the Late Cretaceous of Canada and proposed that a U-shaped bend in the tubular crest passage prevented water from entering the respiratory system while the dinosaur was feeding underwater. Again, this idea is based on the notion that hadrosaurs frequently dipped their heads underwater to feed, and paleontologist John Ostrom later pointed out that, in such a scenario, the water pressure would have overcome the air pressure inside the crest and flooded the passage. Whatever the function of the Parasaurolophus crest, the structure was certainly ill-suited to underwater feeding.

Paleontologists kicked around a few other ideas. In a series of papers published in the late 30s and 40s, Martin Wilfarth suggested that elaborate hadrosaur crests were attachment areas for long, fleshy snouts. No evidence was found to support this. Likewise, Ostrom’s later suggestion that the nasal passages were extended to give the dinosaurs a better sense of smell was refuted–there was no indication that  the convoluted passageways had anything to do with a better sense of smell.

Hopson himself considered the crests to primarily be visual display structures, and hadrosaurs with hollow crests, such as Parasaurolophus, may have also used their crests as resonating chambers to send low-frequency sounds over long distances. This is the view generally taken now, but settling on particular functions for the crests does not necessarily illustrate how those structures evolved. Perhaps the origin of the various hadrosaur crest shapes was driven by pressures associated with species recognition–the need to identify members of one’s own kind, be they parents, rivals, mates, etc. Then again, perhaps some aspect of sexual selection was at play. Exactly what evolutionary factors led to the origin of such strange skull shapes is difficult to ascertain. Much remains unknown about the evolution and social significance of fantastic ornaments in dinosaurs.

References:

Hopson, J. 1975. The Evolution of Cranial Display Structures in Hadrosaurian Dinosaurs. Paleobiology, 1 (1). pp. 21-43

Naish, D. 2009. The Great Dinosaur Discoveries. Berkeley: University of California Press. pp. 72-73