Study of fossil wood from the Middle–Late Miocene sediments of Dhemaji and Lakhimpur districts of Assam, India and its palaeoecological and palaeophytogeographical implications

Abstract

In order to reconstruct the palaeoclimate, a number of fossil wood pieces were collected and investigated from two new fossil localities situated in the Dhemaji and Lakhimpur districts of Assam. They belong to the Tipam Group considered to be of Middle-Late Miocene in age and show affinities with Gluta (Anacardiaceae), Bischofia (Euphorbiaceae), Bauhinia, Cynometra, Copaifera-Detarium-Sindora, Millettia-Pongamia, and Afzelia-Intsia (Fabaceae). The flora also records a new species of Bauhinia named Bauhinia miocenica sp. nov. The assemblage indicates a warm and humid climate in the region during the deposition of the sediments. The occurrence of some southeast Asian elements in the fossil flora indicates that an exchange of floral elements took place between India and southeast Asia during the Miocene.

Biotic interchange between the Indian subcontinent and mainland Asia through time

“…This coincides with the global Mid Miocene Climatic Optimum 31 32 (17–14 Ma), as well as with the establishment of a rain forest belt along the lower Himalayan foothills from Burma to Bhutan in the course of the Himalayan uplift 21 31 32 (represented today by a narrow belt of seasonal evergreen dipterocarp forests 33 34 ). During the Middle Miocene the monsoon system was probably weaker 35 and with globally warmer climates this allowed the establishment of widespread evergreen rain forests across northern India, reflected by abundant macrofossils, many of which are of SE Asian origin 14 36 37 38 . We propose that during the Middle Miocene, with both northern India and SE Asia being again characterized by more perhumid climates 13 14 , a second continuous dispersal corridor formed from mainland SE Asia to northern India along the Himalayan foothills.…”

Late middle Eocene fossil wood of Myanmar: Implications for the landscape and the climate of the Eocene Bengal Bay

“…Interestingly, G. burmense, together with Cynometroxylon holdenii, Bauhinium paleomalabaricum, Shoreoxylon deomaliense and Terminalioxylon coriaceum were also described in the Miocene Tipam Group of Assam (Prakash and Awasthi, 1969a,b;Mehrotra et al, 2011), thus suggesting a close resemblance between the Miocene forests of the nearby Assam and the Eocene forests of Myanmar and the long-term persistence of similar forest elements in the Assam-Myanmar region.…”

Fossil wood flora from the Siwalik Group of Arunachal Pradesh, India and its climatic and phytogeographic significance

“…The Nearest Living Relative (NLR) approach is used for the reconstruction of palaeoecology and palaeoclimatology. This method considers that the climatic requirements of a fossil taxon are similar to the ones of its NLR and is confidently used by the palaeobotanists in the palaeoclimate reconstruction (Bande and Chandra 1990;Prakash et al 1994;Estrada-Ruiz et al 2007;Jeong et al 2009;Feng et al 2010;Bamford 2011;Mehrotra et al 2011Mehrotra et al , 2013Mehrotra et al , 2016Tiwari et al 2012;Licht et al 2014Licht et al , 2015. The modern analogues of the fossil assemblage are: Miliusa velutina (Annonaceae), Calophyllum tomentosum and Kayea (Calophyllaceae), Lophopetalum littorale (Celastraceae), Diospyros melanoxylon (Ebenaceae), Intia bijuga and Sindora siamensis (Fabaceae).…”

“…The modification in architectural structures, regardless of their taxonomic affinities, can be viewed either externally Sinnott 1915, 1916;Wolfe and Upchurch 1987;Spicer et al 2011) or internally (Carlquist 1975(Carlquist , 1977(Carlquist , 1988Baas 1976Baas , 1986). These modified architectural features are often used in the reconstruction of palaeoclimate Sinnott 1915, 1916;Wolfe 1979;Baas 1991, 1993;Wiemann et al 1998;Poole 2000;Mehrotra et al 2011;Spicer et al 2011;Srivastava et al 2012;Tiwari et al 2012). In case of woods, several anatomical features have been identified which directly or indirectly represent their corresponding climatic conditions such as density and diameter of vessels, presence or absence of growth rings, diffuse porosity, vessel grouping, structure of perforation plate, intervessel pitting, extent of parenchyma and nature of ray (Wolfe and Upchurch 1987;Wheeler and Baas 1993;Woodcock and Ignas 1994).…”