Study of Tertiary fossil leaves from Thailand has received little attention since the work of Endo in 1964 and 1966. He described fossil leaves from the Li Basin in Northern Thailand, which is thought to be Late Oligocene or Early Miocene in age. No published articles about fossil leaves exist since that time. However, palynofloras of several Tertiary basins have been studied, including pollen and spore taxonomy and the use of pollen/spores in the reconstruction of the paleovegetation, the paleoecology and the paleoclimate. Fossil conifer shoots and angiosperm leaves have been recovered from lacustrine sediments at Ban Pu and Ban Pakha subbasin coalmines of the Li basin. These show affinities to several taxa in addition to the ones reported by Endo. Some show affinity to Sequoia and Podocarpoxylon, and the fossil assemblage may represent a mixed forest of angiosperms and gymnosperms, dominated by Sequoia. Various types of anacardiaceous leaf fossils were also found which are similar to leaves of Bouea,Buchanania,Gluta, Mangifera, Melanochyla, Semecarpus and Swintonia of the Anacardiaceae. The assignment of fossil leaves to living taxa was attempted only after the anatomy and morphology of 26 extant species belonging to the above genera were studied in detail. These comparisons were based upon the characters illustrated by Dilcher in 1974. Morphological and venation analyses show that many extant species examined have leaves with extensive variations in morphology, size and venation patterns. These variations may overlap between some species. Naming these fossil morphotypes should reflect the variations observed among all possible living counterparts with equal possibility. The anacardiaceous leaf fossils from this region are limited in the number of specimens collected and studied allowing only tentative paleophytogeographic conclusions to be made at present.

Key words: Anacardiaceae, fossil leaves, Tertiary, Thailand