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Cows of the Cretaceous
4 October 2012 2:05 pm
The hadrosaurids, or duck-billed dinosaurs, were among the most successful plant-eating dinos to roam the earth. They ranged widely in North America, Europe, and Asia during the Upper Cretaceous period, about 100 million years ago to 65 million years ago. What was the secret to their success? A new study of hadrosaurid teeth finds that they were among the most sophisticated chompers ever known, capable of grinding and slicing like a prehistoric Veg-O-Matic.
Scientists already knew that duckbill teeth made them formidable chewing machines. Earlier research had shown that the creatures had up to 1400 of the choppers, which they shed and replaced over the course of a year much as sharks do. The surfaces of their teeth were flat, like those of horses and cows, rather than sharp and pointed like the tooth surfaces of most reptiles. Studies of the fossilized stomach contents of hadrosaurids, as well as microscopic wear patterns on their teeth, suggested that the animals ate low-lying grasses, the tough leaves of plants such as horsetails and ferns, and the woody parts of newly evolved conifer trees. All of these features have earned the hadrosaurs the nickname "cows of the Cretaceous."
To get a more incisive look at how duckbill teeth tackled these abrasive plant foods, a team of paleontologists and engineers sliced up some teeth from the well-known hadrosaurid Edmontosaurus, using nearly 70-million-year-old specimens from the collections of the American Museum of Natural History in New York City. The researchers, led by biologist Gregory Erickson of Florida State University in Tallahassee, also used light and electron microscopy to examine the depth and directions of the abrasions on the surfaces of the teeth. The team also determined the hardness of the teeth and tested how quickly the teeth wore out by drawing a diamond-tipped probe across their surfaces at a steady downward pressure intended to mimic the forces of chewing.
Their results, which they publish in tomorrow's issue of Science, go a long way toward explaining the duckbills' success. A typical reptile has only two kinds of dental tissues—hard enamel and soft dentine—and a typical mammal has four, including cementum and a second kind of dentine. But Edmontosaurus had six different kinds of dental tissues (see illustration). Moreover, the arrangement of these tissues varied within and across the tooth, allowing different parts of the tooth to be used for either grinding or slicing tough plant material.
"These guys were like walking pulp mills," Erickson says. This presumably gave them a competitive advantage over other dinosaurs, he says, and the capacity to feed on new types of vegetation like flowering plants, which became widespread at the beginning of the Upper Cretaceous.
The new study represents a "very novel approach," says Mark Purnell, a paleobiologist at the University of Leicester in the United Kingdom, whose earlier work on the jaw mechanics of Edmontosaurus had shown that it could chew in several different directions. He points out that since the ancestors of the duckbills did not have such sophisticated teeth, the evolution of the six-tissue structure may have allowed hadrosaurids to diversify and "explode" across the landscape.
Moreover, Purnell says, the new work may help explain why the duckbills survived until the very end of the age of the dinosaurs about 65 million years ago. "They were able to broaden their niche and grind up food that other species struggled to process. In times of real resource shortages, they had something to fall back on."