Frogs and Teeth

Dollo’s Law, a theory proposed by the scientist Louis Dollo in the 1800s, says that when a particular property is lost in a species, it never comes back. It explains why humans have no tails, birds and turtles have no teeth and snakes have not regained legs.

But a new analysis about frogs, done by a scientist at Stony Brook University, found that they lost teeth in the lower jaw at least 200 million years ago, but a particular type of marsupial tree frog regained those lower teeth about 20 million years ago.

“It’s a very clear-cut case of re-evolution because of the large time span,” said John Wiens, the Stony Brook biologist who wrote the paper in the journal Evolution.

Dr. Wiens analyzed DNA samples of 170 modern and fossilized frogs to approximate the dates of loss and re-evolution of the teeth. Most frogs have teeth on their upper jaws, which may have made the re-evolution in the tree frog, known as Gastrotheca guentheri, easier, Dr. Wiens said. “They already had teeth in the upper jaw, so they had the enamel, dentine and other necessities,” he said. “There was a way to facilitate new teeth after 200 million years.”

The species is the only known modern frog species with lower teeth, though certain other species with upper teeth have structures similar to teeth on the lower jaw. “That’s a big question now: What’s preventing the other frogs from developing real teeth on the lower jaw?” Dr. Wiens said.

By Sindya N. Bhanoo at www.nytimes.com. Picture by Chris Gash.