Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators have recently identified a new fossil oak from northern Thailand. The newly identified fossil species Quercus liensis provides crucial evidence that the region experienced a cooler and wetter subtropical climate during the late Oligocene to Early Miocene (about 25 million years ago).
The study, published in Geobios, is based on exceptionally well-preserved fossil leaves unearthed from the Li Basin in Lamphun Province. Dating to between 23 and 33 million years ago, these leaves are characterized by its distinct leaf architecture, including an acuminate apex, a serrated margin, and a specific pattern of secondary veins, or venation. Microscopic analysis also revealed anomocytic stomata and unicellular trichome bases preserved on the leaf epidermis.
After detailed morphological comparisons with both living and fossil species, the researchers confirmed that the fossil species is new to science. Its closest living relative Quercus shennongii is today restricted to the subtropical belt of southeastern China, a biogeographic link that hints at a once continuous vegetation corridor.
Combined with palynological evidence, the study revealed that during the late Oligocene to early Miocene, subtropical evergreen broad-leaved forests dominated by Fagaceae existed in northern Thailand, significantly different from the modern tropical monsoon forests.
The researchers suggest that this dramatic shift in climate and vegetation was driven by a combination of powerful forces. The global cooling event that occurred during the Oligocene epoch, one of the most significant climate shifts in Earth's history, played a major role. Concurrently, the tectonic southward extrusion of the Southeast Asian landmass, which pushed the region into lower latitudes, contributed to a long-term change in temperature and rainfall patterns.
"Quercus liensis currently represents the southernmost fossil occurrence of its kind in the world. Its presence in Thailand is a clear indicator of a southward expansion of these subtropical plants into Southeast Asia during the Oligocene, a movement likely prompted by the same global cooling and paleogeographic shifts," said HUANG Jian of XTBG.
The study provides crucial evidence for the southward expansion of subtropical plants and helps researchers understand the long-term impacts of climate and tectonic change in the region.
Fossil of Quercus liensis (Image by Napussawan Thongsangtum)
Available online: 6 August 2026
