Crawling through time: fish bones reveal past climate change.

The snakehead fish, best known lately as a threat to U.S. waters, may finally be doing some good. A German scientist asserts that fossils of relatives of the much-feared invader can shed light on major climatic changes of the distant past.

The air-breathing freshwater fish can crawl on its strong pectoral fins over land between lakes and rivers, taking over ecosystems that it moves into. The snakeheads now invading the United States aren't typical. Most of the world's 29 snakehead species in the family Channidae thrive only in warm, wet conditions.

The fish's need for humid air makes the snakehead an indictor of climate, says Madelaine Bohme of the University of Munich in the May Geology. Scientists can deduce that wherever they find snakehead fossils, the climate must have been humid at the time the fish lived there, she says.

"It's a novel approach" says James Kennett, a paleoclimatologist of the University of California, Santa Barbara.

Bohme combed through the fossil record of snakeheads worldwide and found two major movements of the fish. Snakeheads first appeared near Pakistan around 50 million years ago. In their first diaspora, around 17.5 million years ago, they began showing up in Western Europe, Kazakhstan, and southern Siberia. This corresponds to a well-known brief period of warming in the Northern Hemisphere between 25 million and 17 million years ago. It occurred when Antarctica, but not the Arctic, froze, pushing equatorial temperatures northward. Bohme's findings suggest that, along with a Eurasian temperature increase, there was also a boost in summer rainfall there.

Then, 13 million years ago, snakehead fossils disappeared from Europe. About that time, Earth began another long period of cooling, but the change in distribution of snakehead fish indicates that the Eurasian summers were getting dryer as well, Bohme concludes.

The snakehead's second mass exodus came around 8 million years ago. Snakeheads made their way from the Indian subcontinent into China and Africa, where they're found in abundance today. This migration, Bohme found, corresponds with an already recognized intensification of the Asian monsoon system. The warm, humid air from the southern Atlantic and Indian Oceans crashed into the cold, dry air of the then-young Himalayas, causing massive rainfall in Southeast Asia and Africa.

Not all scientists agree with Bohme's conclusions. The climate in Europe could have been humid enough for the fish well before they actually arrived, says Kennett. "[Bohme] doesn't consider other factors that could have led to the distribution of this group; he argues.

One of those factors, says Kennett, may be related to plate tectonics. Scientists don't know if there was a permanent land bridge at that time close to the fish's original home on the Indian subcontinent and near freshwater regions in Eurasia. Without such abridge, the fish couldn't migrate to Europe no matter how ideal the temperature and humidity. Lack of a land bridge may also have prevented the fish from moving from India into subtropical Africa earlier.

"Not everything that happens in the spread of animals is always caused by climate change," notes William J. Sanders, a paleontologist from the University of Michigan in Ann Arbor.

Bohme's critics agree, however, that her work is thought provoking. "Such ideas usually drive debate," says Sanders. Bohme is currently in Cambodia. There, she's filming snakehead fish leaping 2 feet into the air and, she told Science News, "walking over land."