A mouth that only a dentist could love.

In the world of fossils, teeth are often the most expressive part of the body. The serrated daggers of Tyrannosaurus rex recount a life spent ripping through flesh. The broad molars of an ice age mammoth describe a steady diet of plants. But no mouth speaks as clearly as the bizarre dental battery of Pterodaustro-a South American flying reptile from the Cretaceous period.

With 1,000 needle-thin bristles packed along its lower jaw, Pterodaustro had a sieve for a smile. Although the purpose of these bristles was evident-they were ideal for filtering tiny animals and plants from water-paleontologists had long wondered whether they were actually teeth or a hornlike material similar to the baleen of blue whales.

Now, detailed analysis of Pterodaustro's mouth reveals it had the real thing.

"These are true teeth. They have enamel and dentine and the structure of teeth. To some extent, they were flexible, which is interesting because enamel is typically rigid," says Luis M. Chiappe, a paleontologist at the American Museum of Natural History in New York City. Chiappe and colleague Anusuya Chinsamy of the South African Museum in Cape Town describe their study in the Jan. 18 Nature.

Specimens of Pterodaustro were first discovered during the late 1960s by paleontologist Jose F. Bonaparte, working in the province of San Luis, Argentina. The pelican-sized pterosaur has a long beak that curves upward at the tip. The upper jaw has small, rounded teeth that may have helped crush animals caught in the basket of teeth in the lower jaw.

Fossils show the long teeth of the lower jaw leaning in various directions, indicating that they were pliable. Paleontologists therefore wondered whether the teeth were made of keratin-the same protein found in fingernails and baleen, but nobody had sliced open a fossil to analyze it.

During excavations in 1994, Chiappe collected new specimens of Pterodaustro and sacrificed one for microscopic analysis. Thin sections of the teeth revealed a structure typical of vertebrate dentition.

Other pterosaurs also evolved comblike dental arrangements useful for straining water, but neither they nor any other vertebrate developed as many teeth as Pterodaustro. "This is certainly one of the most peculiar dental specializations that we know of," says Chiappe.

The Pterodaustro of San Luis lived along a large inland lake and may have flocked together like birds, judging from the number of individuals found in the same deposit. Chiappe suggests that the flying reptiles may have behaved somewhat like flamingos, whose mouths have a different type of filtering structure, not made of teeth.

Kevin Padian, a paleontologist at the University of California, Berkeley, agrees with the analogy to flocking birds. Last year, he and C. Michael Bell of Cheltenham and Gloucester College of Higher Education in England reported finding thousands of scattered pterosaur bones at a site in Chile. They suggested that the bone bed formed when flood waters swept over a pterosaur rookery.