"I combined data from fossils and DNA sequences with new statistical methods and showed that frogs lost their teeth on the lower jaw more than 230 million years ago, but that they re-appeared in G. guentheri within the past 20 million years," Dr. John Wiens of Stony Brook University told the BBC.
But Dollo's law stated that complex traits lost cannot come back, evolution is irreversible - well, so what? It's been a long time since 1890 when paleontologist Louis Dollo asserted that and it was never a law anyway, it was just a hypothesis.
Regardless, it's apparently not a Federal law, since it's been violated without any problem at all.
Citation: John J. Wiens, 'RE-EVOLUTION OF LOST MANDIBULAR TEETH IN FROGS AFTER MORE THAN 200 MILLION YEARS, AND RE-EVALUATING DOLLO'S LAW', Evolution, DOI: 10.1111/j.1558-5646.2011.01221.x