Bivalvia fossils believed to date back 200 million years have been discovered in the Lao River in Phayao province.
The presence of marine fossils indicates the region was an underwater environment in the past, experts say. Staff from the Living River Siam Association collected samples of the fossils after villagers in Phayao's Chiang Kham district told them they had found an unknown group of petrified prehistoric organisms.
Geologists later identified them as bivalves, a class of marine and freshwater molluscs with bodies enclosed by a shell consisting of two hinged parts. Clams, mussels and oysters also belong to the bivalve group.
According to a survey around the Lao River, volcanic rocks are the most common sediment found in the area.
Bivalve seashells fossilised among the sedimentary rocks are categorised in the Hong Hoi Formation group and Lampang Group, said Rattanaporn Fongngern, a professor at the Department of Geological Sciences, Faculty of Science, Chiang Mai University.
She said there are other two sedimentary rocks found in the Lao River: red beds and limestone. They form limestone streams that are parts of the Lao River's tributaries.
The bivalve fossils found in Phayao can also be found among volcanic rocks and rocky shores. Using geological dating, the age of the rocks can now be calculated precisely as their compounds hold indicative characters, she said. Villagers also found fossils of an elephant's molars. Geologists said the teeth resemble those of the surviving genus of elephants we know today.