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Dental examCareful studies of microscopic scratches in the teeth of Edmontosaurus, a duck-billed dinosaur that lived during the Late Cretaceous, show how the animal’s jaws worked and may shed light on what the dinosaurs ate.Mark Purnell/University of Leicester

Duck-billed dinosaurs may have been the sheep of their ecosystems, instead of the deer.

Patterns of tiny scratches in the fossilized teeth of Edmontosaurus, a type of hadrosaur, suggest the dinos had more complex jaw movements than previously thought, and may have eaten grass-like plants instead of trees, researchers report online June 29 in the Proceedings of the National Academy of Sciences.

Understanding how hadrosaurs chewed their food and what they ate could help scientists better understand how these creatures, the dominant plant-eating vertebrates of the Late Cretaceous period some 75 million to 65 million years ago, fit into their ecosystem, the study suggests.

Microscopic scratches on teeth have been used to identify the eating habits of mammals — especially early human ancestors — but not to probe dinosaurs’ diets until now, says study coauthor Mark Purnell of the University of Leicester in England. Although researchers had noticed the scratches, “what nobody’s done is actually analyze them in a statistical way,” Purnell says.

Plant-eating mammals like cows and humans break down tough plant material by grinding their jaws from side to side and front to back. But dinosaurs had simple hinge jaws that allowed them only to bite, not to chew, which scientists had thought would make Edmontosaurus inefficient herbivores.

Looking at the direction of the most dominant scratches, the researchers concluded that Edmontosaurus probably had hinged upper jaws that allowed the top teeth to flare outward like two trap doors as the dinosaur closed its mouth. The upper teeth would have scraped against the lower ones, breaking down food and creating the micrometer-sized scratches.

The study also suggests what the dinosaurs ate, although Purnell says this conclusion is more tenuous. Previous work in mammals has shown that the teeth of grazers (like sheep) tend to be scratched by abrasive materials in the grasses, while the teeth of twig- and leaf-eating browsers (like deer and giraffes) have tiny pits and chips. If the data from mammals can be extrapolated to dinosaurs, then the absence of pits and chips on the Edmontosaurus teeth implies that they ate grass-like plants instead of tree parts, as was previously thought, the researchers say.

“I'm not sure that I completely accept their conclusions about grazing,” comments paleobiologist David Krauss of the Borough of Manhattan Community College in New York, adding that certain types of trees might have had enough abrasive materials to have caused the scratches. “Still, the confirmation they provide about the jaw mechanics is very interesting, and the approach and implications for dinosaur ecology are a great way to go about paleontological research.”

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