Dino Rat

Living Fossil: DNA puts rodent in family that's not extinct after all

Susan Milius


The Laotian rock rat, which is very much alive, belongs to a rodent family that scientists had assumed had vanished 11 million years ago, [okay, so the dinosaurs vanished sixty-five million years ago and the title 'Dino Rat' is bad, but it sounds cool] says an international research team that examined DNA evidence. The family resemblance was also suggested from fossil evidence last year.

The Laotian rock rat (Laonastes aenigmamus), or kha-nyou, was new to science in 1996 when a wildlife-survey team bought some specimens in a food market in Laos. Since then, scientists have debated what sort of rodent it is, even proposing that it belongs to a new family of mammals.

Now, researchers in five countries have finished the biggest rock rat–DNA analysis yet. Their study dashes the idea of the new mammal family, says Dorothée Huchon of Tel Aviv University. The team argues for an even more dramatic solution: The rock rat is a member of a supposedly extinct family, the Diatomyidae.

"People think the world is explored, and it's not," comments mammalogist Darrin Lunde of the American Museum of Natural History in New York City.

The Laotians who live near the creature's rocky outcrops know of the animals. But until 1996, mammalogists hadn't encountered the dark, squirrel-size creature, which has a long skull, rounded ears, and a furry tail.

In 2005, researchers at the Natural History Museum in London placed the rock rat in a new family of mammals (SN: 5/21/05, p. 324), the first to be described since the bumblebee bat family in 1974. The Laotian rock rat's family, they claimed, belongs within the Hystricognathi group, which includes guinea pigs, chinchillas, and porcupines.

Huchon says that when she read about the new family, she was unconvinced because the researchers had studied DNA from only one of its sources within cells. She appealed to the London team for tissue samples to expand the genetic analysis. So did other researchers, and an international network was born.

Altogether, the researchers looked at two mitochondrial genes, four stretches of nuclear DNA, and genetic elements that insert themselves randomly into the genome. Overall, the lines of genetic evidence agreed, Huchon says.

The new species doesn't fit easily within the Hystricognathi group. Instead, its closest living relatives are African rodents called gundies, the researchers report in a paper now online for an upcoming Proceedings of the National Academy of Sciences.

Fossil evidence had indicated that the gundies are close relatives to the Diatomyidae family. In 2006, a team of paleontologists based at the Carnegie Museum of Natural History in Pittsburgh had argued that the rock rat is a Diatomyidae, making that family a Lazarus taxon, one that reappeared after seeming to be extinct.

Lawrence Flynn, a paleontologist who described some of the Diatomyidae fossils, says that he's happy to welcome a living member to that family. Flynn, at Harvard University's Peabody Museum in Cambridge, Mass., says that the rock rat has such a strong family resemblance that had independently suggested a connection.

Huchon's new genetic study makes "a very nice molecular confirmation," comments Ronald DeBry of the University of Cincinnati, who uses DNA analysis to examine rodent evolution.