A forest turned to stone
In the dry zone of central Myanmar, the landscape holds evidence of a much wetter, ancient world. The fossil wood of Yesagyo Township is preserved within a geological unit known as the Irrawaddy Formation. This formation consists of thick layers of sandstones and conglomerates deposited by ancient river systems during the late Miocene and Pliocene epochs. The specific fossil-bearing layers in this region date to the Miocene, approximately 20 million years ago. These sediments, laid down by powerful meandering rivers, buried entire forests of trees, initiating their long transformation into stone.
The process that preserved these ancient trees is permineralization, specifically silicification. Over millions of years, groundwater rich in dissolved silica (silicon dioxide) permeated the buried logs. As the original organic material, the cellulose and lignin of the wood, slowly decayed, it was replaced molecule-by-molecule with silica minerals like quartz, chalcedony, or opal. This replacement process was so precise that it preserved the wood's internal structure in microscopic detail. Anatomical features like vessels, rays, and even individual cells are visible under a microscope, allowing paleobotanists to identify the types of trees that grew here.
Echoes of a Miocene ecosystem
Analysis of the wood anatomy reveals that many of the fossil logs belong to the family Dipterocarpaceae. This family of hardwood trees dominates modern Southeast Asian rainforests, and these fossils confirm their ancestry in the region. Scientists have identified multiple species of fossil dipterocarps, including those belonging to the genus Dipterocarpoxylon. The presence of these trees, along with other identified families like Fabaceae (legumes) and Moraceae (figs), indicates that the Miocene climate of central Myanmar was warm and monsoonal.
This ancient environment was drastically different from the semi-arid conditions found in the region today. The fossil flora suggests a mix of forest types, from wet evergreen to mixed deciduous forests, showing a diverse and thriving ecosystem. Alongside the petrified wood, the Irrawaddy Formation also contains the fossilized bones of contemporary animals. Remains of extinct elephants (Stegolophodon), three-toed horses (Hipparion), rhinoceroses, and crocodiles have been found, further confirming a well-watered, forested habitat. These fossils record the paleoenvironment of Southeast Asia, documenting a period of major climatic and biological evolution.