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  National Geographic News 3 Jan 05
Thai Rice Field Yields Treasure Trove of Mammal Fossils
 Stefan Lovgren in Bangkok for National Geographic News

Yaowalak Chaimanee, a government paleontologist, regularly receives scores of tips about new fossil discoveries around Thailand.

But when geologists at Suranaree University of Technology called her last March about an ancient jawbone found in a province three hours from Bangkok, Yaowalak knew the discovery was special. The fossil belonged to a stegodon, a prehistoric elephant-like animal that was once common in the region.

But the location of the findin--a rice field--was what really intrigued her. Most important fossils in Thailand have been found in caves.

The next morning Yaowalak and her Bangkok-based team from the Thailand Department of Mineral Resources traveled to the site. The scientists soon began unearthing a treasure trove of fossils: skulls of a gavial (a crocodile-like reptile) and a spotted hyena, deer antlers, and a buffalo horn.

"I've never seen such a community of large mammals in one excavation," said Yaowalak, who has since been studying the remains. The discovery may shed new light on the distribution of prehistoric species, including human ancestors, in the region.

The site documents a period when Thailand had a more temperate climate, attracting animals from India and China. "These southward distribution shifts are important to understand and date, because they may also explain the southward spread of Homo erectus populations and later--Homo sapiens," Yaowalak said.

Exceptional Preservation

Thailand once had a high diversity of prehistoric fauna, and researchers have unearthed many important fossils, including higher primates, in the country. Most discoveries in the past have been made in cave deposits.

The latest fossils, however, were found at a construction site for a new rice mill. An abundance of fossils were found deposited in a thin layer of fine gravel and sand mixed with organic matter about 16 feet (5 meters) below the surface.

"The preservation [of the fossils] is exceptional," said Jean-Jacques Jaeger, a professor of paleontology at Montpellier II University in France. Jaeger, who has announced past discoveries of primate fossils in Thailand, is working with Yaowalak on the latest excavation.

The scientists have not yet established the exact age of the fossils, but they estimate the remains are about 400,000 years old. "Most are [from] extinct animals in Thailand today," Jaeger said.

Southward Migration

The location where the bones were found used to be a river, and researchers believe the area was once a tropical forest that attracted animals from the north.

"During the cold periods of the Middle Pleistocene era [1.8 million years ago to 11,500 years ago], mammalian communities shifted southwards in Asia in a similar way to Europe and North America," Jaeger said.

As the climate changed and the forested area turned into floodplains, the animal species may have gone extinct. "This site serves as a link between the past and the present," Yaowalak said.

Studying what happened to the mammals found in the ancient riverbed, she said, could help researchers understand migration patterns for prehistoric animals and early humans.

Scientists believe that, similar to the southward migration of mammals such as the stegodon, populations of Homo erectus may also have moved south in Asia at one point. Researchers have been focusing on cave deposits as they look for remains of Homo erectus in Thailand.

The latest discovery, however, suggests they may have to broaden their search. "We didn't expect to find these [mammal] remains in a river deposit," Yaowalak said. "Maybe now we will change our mind as to where to look for human ancestors."
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