Teeth scratches reveal dinosaur menu
30 June 2009, by Sara Coelho
Show me your teeth and I'll tell you how you eat: tiny scratches in dinosaur teeth reveal that the ancient reptiles chew their greens in a different way than modern herbivores, scientists have found.
Hadrosaurids - dubbed the 'cows of the Cretaceous' - were the dominant herbivores for millions of years before the mass extinction that killed off the dinosaurs.
They are known from many complete skeletons and even mummified pieces of skin. But scientists were not sure what they ate or how their massive jaws worked.
'They might have been grazers, cropping vegetation close to the ground - like today's cows and sheep - or browsers, eating leaves and twigs - more like deer or giraffes,' says Dr Mark Purnell, from the University of Leicester. It's not an easy question to answer because dinosaurs disappeared many millions of years ago and no modern species is close enough to hadrosaurids to be used as a living example.
'It seems that these dinosaurs invented their own way of chewing.'
Dr Mark Purnell, University of Leicester
But Purnell, along with graduate student Vince Williams and Dr Paul Barrett from the Natural History Museum, found an overlooked piece of evidence: tiny scratches on the dinosaur's teeth.
Chewing on solid food always leaves tiny scratches on the teeth's surface. By looking at the size and orientation of these markings, scientists can learn about the movement of jaws. Purnell realised the method was 'very useful' when he applied the technique to pre-historic fishes and conodonts, a group of ancient jawless animals, but this was his first study with dinosaurs.
The team looked for tooth scratches in Edmontosaurus jaws, collected from Late Cretaceous rocks in the United States of America. The fossils were carefully cleaned and used to make detailed replicas of the tooth surfaces, which were then scanned under the microscope. Williams - Purnell's PhD student - measured the length and orientation of all the scratches, most just a fraction of a millimetre long.
The results, published yesterday in the Proceedings of the National Academy of Sciences show that the Edmontosaurus jaws have four different sets of parallel scratches. The scratches can be used to learn how the dinosaur ate because, 'we can relate each set to a specific jaw movement,' says Purnell.
Instead of moving the lower jaw like modern mammals, the duck-billed hadrosaurid dinosaurs moved them upwards, pushing the upper jaws sideways and outwards as the lower teeth slid against the upper teeth. The scratches record a complicated sequence of up and down, sideways and front to back motions. The upper and lower jaws were aligned, so 'it was not a scissor-like movement - it seems that these dinosaurs invented their own way of chewing,' he adds.
Horsetail on the menu?
But what did the Edmontosaurus eat? The scratches are remarkably equal in all teeth. In fact, write the authors in the report, measuring an area of just one square millimetre is enough to sample a whole jaw. The evenness of the scratches suggests that the dinosaurs were using the same series of jaw motions repeated over and over again. The diet was probably made of leaves, without bulkier items such as twigs or stems that might have required a different chewing method and created different wear patterns, suggests Purnell.
Modern grasses might have been on the menu, but back then when dinosaurs roamed the Earth, grasses were not as abundant as they are today and it's unlikely that they provided the bulk of dinosaur's diet. Purnell and his team suggest that the Edmontosaurus were probably eating horsetails - a plant which is now considered a living fossil.
The study left Purnell convinced that looking at teeth scratch marks can provide reliable information about dinosaur's diet and chewing mechanism. The method 'gives sound scientific results, grounded in solid statistical analyses,' he adds.
Vincent S. Williams, Paul M. Barrett and Mark A. Purnell. Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding. PNAS, published online before print June 29, 2009, doi:10.1073/pnas.0812631106
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