Persistence of Gondwanan woods in Myanmar through the Palaeogene

Many plant taxa of the tropical forests of South and Southeast Asia have been proposed to have Gondwanan ancestors. One significant event contributing to this origin is the African-Indian-Southeast Asian Floristic Interchange, with the first events occurring during the late Cretaceous-early Eocene, before the onset of land connections between India and Asia. Given that the timing and mechanisms of these dispersals remain unclear, we aim to document the dynamics of this interchange better. The Burma Terrane of Myanmar, once a separate tectonic block but now part of Southeast Asia, was accreted to India in the early Palaeogene, making it an ideal location to study floristic exchanges.

We section and describe ten fossil wood specimens, analysing their anatomy to determine affinities with extant taxa and their biogeographical history.

We present six taxa from the late middle Eocene and lower Oligocene of Myanmar from the families Fabaceae, Moraceae (Milicioxylon afromoroides n. sp., Ampaloxylon ficoides n. gen. et sp.), Myrtaceae, Meliaceae (Entandrophragminium pacei n. sp.) and Malvaceae. These findings document an extended Gondwanan distribution and biogeographical history of palaeotropical lineages.

These taxa are likely to have dispersed during the early Palaeogene from Africa to India and Myanmar, forming what we refer to as the Indo-Myanmar Palaeogene floristic province. This province, which had Gondwanan affinities, persisted at least until the early Oligocene, the time of suggested land connection with Asia, and served as a 'holding pen' for the further dispersal of some taxa into Southeast Asia. Other taxa, such as those documented here, did not disperse further and eventually disappeared during the Neogene. The mechanisms behind these dispersal barriers and the reasons for the eventual disappearance of these taxa from the region are yet to be investigated and could be related to the late Cenozoic changes in monsoonal climate of South Asia.