In the early 1990s, biologists in southeast Asia discovered a species of turtle unknown to science. But there was a catch: They didn’t know where it came from. Cuora picturata, as the new turtle was formally named, was found only in markets where it was sold for food.

Nearly two decades later, a clever piece of biological sleuthing has found the native home of C. picturata. The discovery offers hope not just to C. picturata, but to other turtle species known only from markets.

“It’s frustrating. You see this animal being exploited, and likely facing extinction,” said Bryan Stuart, a biologist at...