Rock rat's family goes way back

The Laotian rock rat, a rodent that is sold for food in markets and was said last year to represent a new mammalian family, is actually the descendant of creatures thought to have become extinct 11 million years ago, making it kind of a living fossil.

Mark A. Klingler, Science via AP

Scientists say the Laonastes, which was discovered living in central Laos last year, was believed to have been extinct for 11 million years. It is the only living representative of the otherwise extinct Diatomyidae -- a family of rodents living in South Asia and Japan.

Click photo for larger image.

It's a striking example of the "Lazarus effect" partly because, among mammals, that's one of the longest gaps between disappearance from the fossil record to resurrection, said paleontologists led by Mary Dawson of the Carnegie Museum of Natural History in a paper published today in Science.

"I have worked for a long time with fossil rodents, but it never even crossed my mind something like this might happen," Dr. Dawson said. "It was an amazing thing to encounter this."

In most cases of the Lazarus effect among mammals, the gap between disappearance and reappearance is only tens of thousands to just over a million years, she said.

The surprising story began when mammal biologists led by Paulina Jenkins, of London's Natural History Museum, published a paper in April describing carcasses of animals called kha-nyou, or rock rats, that were sold as food in markets in Laos. No Western scientist has seen one alive.

After examining its anatomy and genetics, the Jenkins team concluded the kha-nyou was so unlike any other known rodent that it not only represented a new genus and species, named Laonastes aenigmamus, but also a brand new family, dubbed Laonastidae.

That was big news in the scientific arena, and it got a lot of attention in mainstream news media, too, said Chris Beard, the Carnegie's head of vertebrate paleontology.

So he and Dr. Dawson, an expert on rat fossils, promptly downloaded the Jenkins paper from the Internet to take a closer look.

"We immediately knew what it was," Dr. Beard said. "For the first time, we've found a living example of a group of mammals that we thought were extinct for 11 million years."

To Drs. Beard and Dawson, Laonastes already had a family, called Diatomyidae, which had other ancient Asian rodents as members and was described by paleontologists more than three decades ago. In taxonomy, a family contains genera and species that are closely similar.

In June, Dr. Dawson received a diatomyid fossil that had been uncovered a month earlier in China. The anatomy of the specimen, which was in better shape than previous finds, further bolstered the Carnegie team's contention about Laonastes' origins.

"That was a really lucky stroke to get that new fossil specimen at that time," Dr. Dawson noted.

She and a French colleague then went to the London museum, which has the only Laonastes specimens, to see how the skeletal and dental features compared to fossils of the Asian rodents.

Although she had permission to examine Laonastes from Ms. Jenkins, "I did not in words make it clear to the mammalogists what we were doing," Dr. Dawson said, laughing. "But they're not dumb."

The paleontologists, who rarely get to test theories, predicted that the modern rodent would share some novel anatomical features with its ancestor, such as a certain kind of tooth enamel and molars anchored by four roots. They were right.

"Laonastes is exactly like the fossils," Dr. Beard said. "It's such a slam dunk" that scientists will likely soon consider the families Laonastidae and Diatomyidae to be in synonomy, or the same. The rock rat would still retain the genus and species names given by the London team. He said the modern and ancient rodents appear to be roughly the same size and probably would look similar.

Ms. Jenkins has continued to study the kha-nyou, and the Carnegie's findings open up new research avenues and stimulate even more interest in rodent evolutionary relationships.

She added that it would be surprising if the Carnegie's conclusions remain unchallenged because more researchers will scrutinize Laonastes specimens and the fossil record to learn more about rodent evolution.

Still, "the idea of Laonastes as an example of the 'Lazarus effect' is incredibly exciting and inspiring," said Ms. Jenkins. "This is a fascinating, exciting and important new theory."

In her paper, the fossil rodent record was described as "impoverished." The London team did not include a paleontologist and the few diatomyid fossils that were known at the time were damaged. The Chinese specimen permitted comparisons, she said.

Diatomyids "have a pretty good fossil record" that covers millions of years, Dr. Beard said. "They are known from multiple sites, stretching from Pakistan through Thailand and India, all the way into China and Japan."

The region the kha-nyou hails from is a hot spot of biodiversity, Ms. Jenkins noted. In the past five years, a new species of bat, rodent and a hedgehog-like animal have been discovered there.

First published on March 10, 2006 at 12:00 am