How to read a dinosaur's menu.

Sometimes, deducing an ancient creature's diet is as simple as looking at its teeth. tyrannosaurus rex, for example, brandished long, serrated fangs -- ideal utensils for tearing meat. But many animals had less obvious dentition, and some had no teeth at all. In these cases, chemical clues can help scientists infer ancient eating habits.

Peggy H. Ostrom of Michigan State University in East Lansing has examined nitrogen isotopes to determine whether long-extinct creatures dined on flesh or vegetation. Scientists know that leaf-eating animals, compared with meat eaters, have relatively little nitrogen-15 in their bodies. And among the carnivores, those that eat herbivores, such as deer, have less nitrogen-15 than do those that eat other carnivores, such as wolves. Thus, the ratio of nitrogen-15 to nitrogen-14 indicates where an animal resides on the food chain.

Ostrom used this principle to study 22 animals that lived 75 million years ago. After isolating organic matter from bones, teeth and shells, she analyzed the amino acids in the material to make sure it came from the fossils and not from some contaminating source, such as bacteria or the hands of a paleontologist. Once convinced the organic material belonged to the fossils, Ostrom sent it through a mass spectrometer to determine its nitrogen isotope ratio.

For some well-known animals, the test resuts fit with previous interpretations. For instance, ancient crocodiles and tyrannosaurs had higher ratios of nitrogen-15 to nitrogen-14 than did turtles or the herbivorous hadrosaurs. Ostrom also used the technique to study enigmatic, ostrich-like dinosaurs, which apparently had toothless beaks. Scientists have debated whether such creatures ate grass, eggs, insects or even small vertebrates. On the basis of the nitrogen isotope ratios, Ostrom concludes that these animals were not strict herbivores. While they may have eaten grass or leaves, the new evidence indicates they also consumed something more protein-rich, she says.