Occurrence of Shorea Roxburgh ex C. F. Gaertner (Dipterocarpaceae) in the Neogene Siwalik forests of eastern Himalaya and its biogeography during the Cenozoic of Southeast Asia

Abstract

We report the occurrence of two leaf impressions and one leaf compression along with one winged fruit and two fruiting calyx lobes, resembling those of Shorea Roxburgh ex C.F. Gaertner, in the lower Siwalik (Dafla Formation, middle to upper Miocene), middle Siwalik (Subansiri Formation, Pliocene) and upper Siwalik (Kimin Formation, upper Pliocene to lower Pleistocene) sediments exposed in Arunachal Pradesh, eastern Himalaya. We determine their taxonomic positions based on morphological comparison with similar extant and fossil specimens and discuss their phytogeographic and paleoclimatic implications in terms of the distribution and habitat of fossil and modern populations. The Miocene winged fruit is recognized as Shorea pinjoliensis Khan, R.A. Spicer et Bera, sp. nov. while the Pliocene and Plio-Pleistocene fruiting calyx lobes are recognized as Shorea bhalukpongensis Khan, R.A. Spicer et Bera, sp. nov. and Shorea chandernagarensis Khan, R.A. Spicer et Bera, sp. nov. respectively. Based on leaf architecture the Miocene, Pliocene and Plio-Pleistocene leaves are recognized as Shorea mioobtusa Khan, R.A. Spicer et Bera, sp. nov., Shorea pliotumbuggaia Khan, R.A. Spicer et Bera, sp. nov. and Shorea nepalensis Konomatsu et Awasthi respectively. The discovery of members of Shorea indicates that they had arrived in a tropical, warm and humid eastern Himalaya by the Mio-Pleistocene. Present and earlier records Of Shorea suggest that this genus was a common forest element during Neogene (Miocene time) as well as in the Siwalik forests including Arunachal sub-Himalaya. In this paper, we also review in detail the biogeographic history and suggest possible migration routes of the genus. (C) 2016 Elsevier B.V. All rights reserved