Teeth reveal changing times in the Pleistocene. (Bone Crushers).

Many carnivores enjoy a good piece of meat, a slab of fat, a liver, perhaps a kidney from a fresh kill, but they tend to leave the bones behind. When the pickings are slim, however, they'll chomp the bones and suck out the marrow, a practice that can break the diner's teeth.

Tooth-fracture incidence among carnivores in the fossil record can indicate how much bone the animals crunched and, therefore, something about the ecology of their time. A new study suggests, surprisingly, that dire wolves, Canis dirus, experienced less tooth breakage as they neared extinction.

The study's authors examined fossilized jaws from two distinct dire wolf populations preserved in the Rancho La Brea tar pits in Los Angeles. Dire wolves from 15,000 years ago broke three times as many teeth as dire wolves from 12,000 years ago did, the scientists report in the July 8 Journal of Vertebrate Paleontology. They speculate that competition for food was fiercer in the earlier period.

Animals that live longer wear out more teeth. Therefore, a high proportion of broken teeth might instead reflect an elderly population rather than a bony diet. To distinguish between these alternatives, the researchers compared the age distributions of the two wolf populations using a new approach.

Biologists typically estimate the age of a dead animal by removing and sectioning its teeth. This approach is too destructive to use on these precious fossils. Instead, the Los Angeles researchers measured the width of the area inside the tooth, from X rays of the wolves' jaws. This pulp cavity closes as an animal ages.

The scientists found that the age distributions of the two wolf populations were similar, leaving diet to explain the lower tooth-fracture rate in the 12,000-year-old fossils.

"It's the opposite of what I thought," says study coauthor Blaire Van Valkenburgh of the University of California, Los Angeles (UCLA). Van Valkenburgh and her colleagues had predicted that predators would be picking at dwindling carcasses and thus breaking more teeth closer to the late Pleistocene extinction that eliminated many animals 11,500 years ago (SN: 7/31/93, p. 71).

The higher tooth breakage 15,000 years ago could be explained by high densities of carnivores and brutal competition for prey, says Van Valkenburgh.

"We always want to think of the Pleistocene as this glorious paradise," she says. "What these data say is: No, it was not an easy life."

As extinction approached, predators might have waned, reducing competition for carcasses and thus reducing tooth breakage, the authors speculate. The scientists will next try to confirm the finding in saber-toothed cats, which are also well represented in the tar pits, says coauthor Wendy J. Binder, also of UCLA.

"I thought it was very clever the way they were using pulp cavities to estimate age groups," says Larry Martin of the University of Kansas in Lawrence. However, while the data do suggest a change in dire wolves' feeding habits, they don't necessarily imply that times were easy for the animals living 12,000 years ago, he adds. After all, the dire wolf did go extinct.

Perhaps, Martin speculates, it was harder for the wolves to find large prey as the extinction event approached, so the animals shifted to eating smaller animals, which have less-tooth-damaging bones.