In 1951 scientists thought they had found a new invader on American soil. Ectobius, a cockroach found throughout Europe and Africa, had begun turning up in homes in Massachusetts. Three decades later another Ectobius species, commonly known as the dusky cockroach, made an appearance in New Hampshire and then in Vermont. Eventually four Ectobius species were being tallied in the northeastern U.S.

But Ectobius, it turns out, is not really a stranger to North America at all. It has just returned home after an absence of 49 million years.

Entomologists got the mistaken impression that Ectobius was an Old World species back in 1856, when they found the first specimens in 44-million-year-old Baltic amber. That's how the matter sat for more than 150 years. Then, in 2010, Conrad Labandeira, a research scientist and curator of fossil arthropods at the Smithsonian National Museum of Natural History, invited Peter Vrsansky, a cockroach specialist at the Slovak Academy of Sciences, to examine some fossils that had been gathering dust for years in the Smithsonian Institution's collection. They had been taken from the Green River Formation in Colorado. “Lo and behold, [Vrsansky] said, ‘This is Ectobius!’” Labandeira recalls.

The 21 fossils had revealed four species of Ectobius dating back to the Eocene—predating the European specimens by some five million years. “It's amazing,” Labandeira says, “how one little discovery can change the entire understanding of the history of this particular lineage of cockroaches.”

Most likely, Ectobius went extinct in North America as a result of increasingly harsh conditions as the glaciers crept south. When that extinction occurred, exactly, is unknown, but before it happened, Labandeira says, some of the insects made a break for Europe, either traveling through Greenland or else scuttling across the Bering land bridge.

Recently announcing the news in the Annals of the Entomological Society of America, Labandeira and Vrsansky named one of the new cockroaches Ectobius kohlsi—a hat tip to the fossils' original discoverer, David Kohls, an amateur collector in Colorado who has amassed hundreds of thousands of fossil insects. The three other new Ectobius species, though preliminarily identified, did not possess enough detail for the researchers to name them.

“There are so many amazing, beautiful fossils, and so few people have had the chance to work on them,” says Dena Smith, an associate professor of geological sciences at the University of Colorado Boulder. “I think there will be a lot more of this kind of exciting work to come.”