Why Did Some Dinosaurs Have Crests? Parasaurolophus and the Trombone on Its Head
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The duck-billed dinosaur Parasaurolophus wore on the back of its skull a curved tube of bone up to a metre long, and inside the tube the nasal passages ran from the nostrils up to the tip and back down to the throat, a loop of airway three metres long folded into a crest. Its relatives had a helmet, a fan, a hatchet and a bump, each hollow and each with the nose routed through it, and for a century palaeontologists proposed snorkels, air tanks, salt glands and weapons before settling on the answer that the anatomy had been pointing at, which is that the crest was for making a noise.
The crested duckbills
The hadrosaurs, or duck-billed dinosaurs, were the commonest large plant-eaters of the Late Cretaceous in North America and Asia, eight to twelve metres long, walking on two legs or four, with broad toothless beaks and batteries of hundreds of grinding teeth. They came in two families: the flat-headed ones, such as Edmontosaurus, with solid crests or none, and the lambeosaurines, whose skulls carried hollow crests of every shape. Parasaurolophus, described in 1922 from Alberta, is the most extreme; Corythosaurus had a rounded helmet like a Corinthian soldier's, Lambeosaurus a hatchet with a spike behind it, and Hypacrosaurus a fan, and in every one the bones of the snout were extended up into the crest and the airway went with them.
The wrong answers
The crests were explained in sequence by ideas that the anatomy eventually ruled out:
- •A snorkel: the tube had no opening at the tip, so an animal underwater could not have breathed through it
- •An air reservoir for diving: the volume was too small for a breath
- •A salt gland or a chamber to improve the sense of smell: possible in part, since the passages did pass olfactory tissue, but not the reason for the size
- •A weapon or a device for pushing through vegetation: the bone was thin and the crest fragile
- •A sexual display alone: true, but it did not explain why the display was hollow
The resonator
The idea that the crest was a resonating chamber was proposed in 1931 and became the consensus in the 1990s, when the crests were scanned. The internal passages of Parasaurolophus form a set of tubes whose lengths, like an organ pipe's, set the frequencies they resonate at, and in 1997 a team at the New Mexico Museum of Natural History used a CT scan of a skull and a computer model to reconstruct the sound: a low, penetrating note around 30 hertz, felt as much as heard, like a foghorn or the lowest note of a pipe organ, with overtones that would have varied with the shape of the crest and the size of the animal. Different species had differently shaped passages and would have sounded different, and juveniles, whose crests were small, would have piped higher, so that a herd of duckbills was a chorus in which each species and each age had its pitch.
Why sound
The duckbills lived in herds of hundreds or thousands, as their bone beds show, on floodplains thick with vegetation, and a low-frequency call travels far through cover, as elephants' rumbles do today. A crest that identified the species and the individual by sight, since it was a large coloured structure, and by sound, since it made a distinctive note, would have served to keep herds together, to find mates and to tell one species from another among several that shared the same plains, which is what the horns and antlers of modern herd animals do. The crest also grew with the animal, small in hatchlings and elaborate in adults, which is the pattern of a display that advertises maturity; whether the sexes differed is argued, since the two crest shapes once assigned to male and female Lambeosaurus may be two species or two ages.
What it tells
The crests are the best evidence that dinosaurs communicated by sound, since sound leaves no fossil and the resonating chamber is the next best thing; the inner ears of hadrosaurs, also reconstructed from scans, were tuned to the low frequencies the crests produced, which is the sort of match that evolution makes between a signal and its receiver. The animals that were once pictured as silent, dim reptiles were, on this evidence, loud, social and listening to each other across a Cretaceous plain, and the trombone on the head of Parasaurolophus was played.
The takeaway
The hollow crests of the lambeosaurine duck-billed dinosaurs, of which Parasaurolophus's metre-long tube is the largest, contained their nasal passages, and after snorkels, air tanks and weapons had been ruled out, scans and acoustic models showed them to be resonators that produced low, far-carrying calls, differing by species and age. Matched to inner ears tuned to the same frequencies and worn by animals that lived in herds of thousands, the crests were signals for sight and sound, and the strongest evidence that dinosaurs called to each other.