Teeth offer a taste of ancient lifestyles.

Teeth can't talk, but they can tell a visual tale about how the dinosaurs and other ancient animals lived, say paleontologists who have devised two ways to study long-extinct creatures by probing their dental details.

By looking inside fossilized reptile teeth, one scientist has developed a process that may help in estimating dinosaur populations. Another researcher has examined the outside of mammal teeth to help decipher the eating habits of ancestral primates. The two described their work last week in separate talks at the annual meeting of the Society of Vertebrate Paleontology, held in San Diego.

Gregory M. Erickson of the Museum of the Rockies in Bozeman, Mont., studied dental growth lines visible inside the fossilized teeth of dinosaurs. Using living alligators as an analog, he demonstrated that each line in the fossil tooth corresponds to one day of growth - enabling researchers to determine how long it took different dinosaurs to develop their teeth.

To make his case, Erickson injected tooth-staining tetracycline into live alligators, which were raised at a commercial farm and later killed for their skins. Examining a cross section of each tooth under a scanning electron microscope, he then located the tetracycline stain and counted the growth rings that had developed since the injection. The number of rings matched the number of days between injection and slaughter, proving the teeth grew a ring each day. Because dental rings in fossil alligators and dinosaurs resemble those in live reptiles, Erickson reasoned that the rings inside dinosaur teeth also represent daily growth.

Such information can reveal how often a dinosaur shed its teeth, he says. Erickson found that plant-eating hadrosaurs went through their teeth rapidly, replacing each after two to three months, whereas the knife-like teeth of the carnivorous Tyrannosaurus rex lasted 2 1/2 to three years before failing out.

Paleontologist Kevin Padian of the University of California, Berkeley praises Erickson's study for combining several analytical techniques that are making their way into studies of ancient animals.

Teeth, as the hardest part of the body, resist erosion and fossilize much more readily than bone. In the past, experts have tried to estimate dinosaur populations from the numbers of various teeth found at particular sites. But many scientists rejected those studies, in part because investigators had no means of calculating how often the animals replaced old teeth with new ones. Erickson believes paleontologists can now use growth lines to address population questions such as the number of adult dinosaurs versus young ones or the number of predators versus prey.

In separate work, Suzanne G. Strait of Duke University in Durham, N.C., studied microscopic abrasions on tooth enamel to deduce the dining preferences of ancient animals. Other anthropologists and paleontologists have used this technique, but Strait is the first to apply it to small "faunivorous" mammals, which eat insects and/or vertebrates.

Examining living primates and bats, she showed that a diet of hard objects, such as beetles or bone, produces a different pattern of enamel scratches than does a diet of soft objects, such as moths or meat. She then used micro-scratch patterns and the general tooth shape to infer that a group of early primates primarily ate hard insects.

Erickson and Strait will share the Romer Prize, awarded for the best student paper presented at last week's meeting. John J. Flynn of the Field Museum in Chicago says their work "has potential application to a wide variety of [animal] groups."