An international team of paleontologists are digging for fossils in southwest China, hoping to unravel the mystery of how some marine reptiles had probably migrated from China to Europe before they became extinct more than 200 million years ago.

Scientists from the United States, Britain, Switzerland and Italy are carrying out the project with Beijing University and authorities in Guanling, Xingyi and Panxian county of Guizhou Province and Luoping in the neighboring Yunnan Province this week.

The field trips followed an international symposium on maritime vertebrates in Guanling on Wednesday.

Scientists are hoping to find more evidence in Guizhou to prove the hypothesis that Triassic era marine reptiles, including ichthyosaurs and placodonts, migrated from east to west, said Prof. Olivier Rieppel from the Field Museum in Chicago.

Prior to the China field trip, scientists had found the Triassic reptile fossils unearthed in Guizhou were similar to, though more primitive than, some Tethys Ocean genus fossils found in today's Mediterranean.

Experts believe China's southwest and central Europe were located on the western and eastern banks of the Tethys Ocean respectively during the Triassic Period 200 million to 250 million years ago, both were habitats of maritime reptiles.

Sudden changes of the maritime environment, however, probably forced the creatures to migrate from China to Europe, and eventually caused their extinction in southwest China, said Jiang Dayong, an expert from Beijing University.

"Such changes had either caused a lack of oxygen in the water, or changed the sea into land," said Prof. Jiang.

Andrea Tintori, an Italian paleontologist, said the discovery of Triassic fossils in southwest China had changed traditional theories on the evolution of maritime reptiles and filled a gap in ichthyosaur evolution.

A scientist with the Chinese Academy of Sciences discovered the 230-million-year-old fossilized remains of a reptile in Triassic era limestone in Guizhou Province in 2002.

A huge variety of fossils have been found, including ichthyosaurs at least 10 meters long, and thalattosaurs and armored placodonts.

Among the most recent discoveries was a Triassic turtle species that had teeth, but no complete carapace.