Gondwanan fossil woods from the Paleogene of southeast Asia
Bois fossiles gondwaniens du Paléocène d'Asie du Sud-Est
Résumé
Gondwanan elements have long been recognized in the fossil flora of southeast Asia, but their history and dispersal route remain highly debated. Myanmar sits at the crossroads for biotic exchanges between India and Laurasia during the Cenozoic. Burmese fossil woods thus provide an opportunity to document these exchanges. Various angiosperm families have so far been documented in the Paleogene of Myanmar in the mid-Paleocene Paunggyi Formation and the mid-Eocene Pondaung Formation. The first geo- logical unit has yielded remains that are hardly identifiable, while the second has already yielded about 40 wood specimens from 16 different taxa that are all related to extant Asian or nearly pantropical species or genera. This study explores 36 new specimens from the Pondaung Formation and 15 specimens from a new locality belonging to the early Oligocene Shwezetaw Formation. Although not all specimens can be identified with certainty, many morphotaxa are related to the families Fabaceae, Malvaceae, Anacardiaceae, Combretaceae, Dipterocarpaceae, Moraceae, Meliaceae and Lauraceae. Among them, some are related to extant genera with a Gondwanan repartition: Cola, Detarium, Eucalyptus or Entandrophragma. Their presence, sometimes in both Eocene and Oligocene formations (those with an affinity to Cola and Eucalyptus) highlights the complex evolution of forested ecosystems in the region and the biogeographical history of these taxa. The collision between the Indian Plate and the Burma Terrane might have favoured the dispersion of many Gondwanan taxa to Asia and beyond, whereas the dramatic global climate changes of the Cenozoic as well as the rise of the Himalaya and the setup of a monsoon regime shaped their distribution that previously extended outside the Gondwanan area.