How dinosaurs chewed

Posted: Monday, June 29, 2009 5:00 PM by Alan Boyle

Natural History Museum

This artist's conception shows a hadrosaur eating. An analysis of tooth wear suggests that hadrosaurs were more likely to graze on low-growing, silica-rich plants than on tall bushes. The tooth scratches also reveal how hadrosaurs chewed.

A novel analysis of microscopic scratches on fossilized teeth reveals how plant-eating duck-billed dinosaurs used a now-extinct type of jaw to chew their food. The study also suggests duckbills were more likely to graze on low-lying greenery than to chomp on tree leaves like giraffes (or like the dinosaurs in "Jurassic Park").

The researchers behind the study, published online today in the Proceedings of the National Academy of Sciences, say the technique they used to uncover the tale of the teeth could be applied to other scientific mysteries as well.

"We did it by measuring literally hundreds and hundreds of scratches on these teeth, and then doing a statistical analysis of the directions of the scratches," University of Leicester paleontologist Mark Purnell, who led the research, told me today."The statistical analysis turned out to be quite a tricky business."

Vince Williams / Univ. of Leicester These are teeth from the lower jaw of a hadrosaur, showing its multiple rows of leaf-shaped teeth. The worn, chewing surface of the teeth is toward the top.

The mouth of a hadrosaur has been compared to a "cranial Cuisinart," with hundreds of teeth lined up in rows to chop up the tough plants of the late Cretaceous. But the dinosaurs didn't have the complex jaw joint that mammals have, leaving scientists to puzzle over exactly how hadrosaurs did all that chewing.

Purnell and his colleagues say they found the answer after going through a three-dimensional analysis of the scratches left behind on fossilized hadrosaur teeth from Wyoming. Co-author Paul Barrett, a paleontologist at Britain's Natural History Museum, said the dinosaurs chewed "in a completely different way [compared] to anything alive today."

"Rather than a flexible lower-jaw joint, they had a hinge between the upper jaws and the rest of the skully," Barrett explained in a news release describing the research. "As they bit down on their food, the upper jaws were forced outward, flexing along this hinge so that the tooth surfaces slid sideways across each other, grinding and shredding food in the process."

Vince Williams / Univ. of Leicester These are teeth from the upper jaw of a hadrosaur known as Edmontosaurus. The specimen was molded and coated with gold for examination using a scanning electron microscope to give high-power magnification of the microscopic scratches.

What was eaten?
The study also sheds new light on another plant-eating puzzle: Were duck-billed dinosaurs grazers, eating low to the ground like today's sheep and cows? Or were they browsers, rearing up to eat leaves and twigs like today's deer and giraffes?

The evidence has been mixed: Last year, a different research team reported that the material found inside the fossilized guts of a hadrosaur appeared to consist of sliced-and-diced conifer needles, leaves, bark and twigs. That would suggest that hadrosaurs were tree-browsers, as depicted in "Jurassic Park," the late Michael Crichton's best-known dinosaur novel.

Vince Williams / Univ. of Leicester

This is a highly magnified view of the surface of one of the hadrosaur teeth, showing scratches created about 67 million years ago by tooth movements and feeding. The black boxes show the areas, each less than half a millimeter wide, in which scratches were analyzed. Click on the image for a larger view.

Purnell and his colleagues, however, say that the wear patterns they saw on the dinosaur teeth are more commonly associated with modern-day grass-eaters.

"Although the first grasses had evolved by the late Cretaceous, they were not common, and it most unlikely that grasses formed a major component of hadrosaur diets," today's news release quoted the University of Leicester's Vince Williams as saying. "We can tell from the scratches that the hadrosaur's food either contained small particles of grit, normal for vegetation cropped close to the ground, or, like grass, contained microscopic granules of silica. We know that horsetails were a common plant at the time and have this characteristic; they may well have been an important food for hadrosaurs."

Purnell cautioned that the conclusions about the hadrosaur's diet are "a little less secure than the very good evidence we have for the motions of the teeth relative to each other." But he said that the dental analysis represented "a new strand of evidence ... a new way of saying something, based on real data, about what they were eating."

Whatever they ate, the hadrosaurs proved to be an "incredibly successful" species, taking their place as the dominant plant-eating species in the late Cretaceous period, Purnell said. Unfortunately, their cranial Cuisinarts couldn't save them when a giant asteroid (or was it climate change?) struck the hadrosaurs and their meat-eating cousins down 65 million years ago.

Mark Purnell / Univ. of Leicester

Microscopic scratches on the tooth of Edmontosaurus, a hadrosaur, show up in false color.

So why should we care how (or what) hadrosaurs chewed? First of all, the researchers' method for analyzing dental wear can be used to study a wide array of long-vanished species, and not just hadrosaurs. "We can start to study weird things like extinct groups of fish, or very early mammals. ... Nobody's looked at very early mammals, particularly what they ate rather than how they ate," Purnell said.

More generally, studying how dinosaurs fit into their ancient environment can help us figure out how to avoid becoming "dinosaurs" ourselves.

"These things were the dominant terrestrial herbivores, so they had a major role in structuring ecosystems in the late Cretaceous," Purnell pointed out. "If we don't know how they ate, that's a big gap in our knowledge. The more we understand the ecosystems of the past, and how they were affected by global events like climate change, the better we can understand how changes now are going to pan out in the future."

More about dinosaurs:

• Nine links in the dino-to-bird transition
• T. rex analysis supports dino-bird link
• The world's seven deadliest dinosaurs
• Dino mummy unlocks Jurassic secrets
• Earth's Timeline: An interactive guide
• Look for dinosaurs on msnbc.com

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